WoodPreservation

OTHER TITLES FROM E & FN SPON Defects and Deterioration in BuildingsB.A.Richardson

Building FailuresW.H.Ransom

Building Services EngineeringD.V.Chadderton

Clays Handbook of Environmental HealthSixteenth editionW.H.Bassett

Practical Timber FormworkJ.B.Peters

Timber StructuresE.C.Harris and J.J Stalnaker

Timber EngineeringPractical Design StudiesE.N.Carmichael

The Maintenance of Brick and Stone Masonry StructuresA.M.Sowden

For more information about these and other titles please contact:The Promotion Department, E & FN Spon, 26 Boundary Row, London, SE1 8HN

WoodPreservationSecond edition

Barry A.RichardsonConsulting and Research ScientistDirector, Penarth Research InternationalLimited

E & FN SPONAn Imprint of Chapman & Hall

London Glasgow New York Tokyo Melbourne Madras

Published byE & FN Spon, an imprint of Chapman & Hall, 26 Boundary Row,London SE1 8HN

Chapman & Hall, 26 Boundary Row, London SE1 8HN, UK

Blackie Academic & Professional, Wester Cleddens Road, Bishopbriggs,Glasgow G64 2NZ, UK

Chapman & Hall Inc., 29 West 35th Street, New York NY10001,USA

Chapman & Hall Japan, Thomson Publishing Japan, HirakawachoNemoto Building, 6F, 1711 Hirakawa-cho, Chiyoda-ku, Tokyo 102,Japan

Chapman & Hall Australia, Thomas Nelson Australia, 102 DoddsStreet, South Melbourne, Victoria 3205, Australia

Chapman & Hall India, R.Seshadri, 32 Second Main Road, CIT East,Madras 600 035, India

First edition 1978

This edition published in the Taylor & Francis e-Library, 2003.

Second edition 1993

1978, 1993 B.A.Richardson

ISBN 0-203-47403-1 Master e-book ISBN

ISBN 0-203-78227-5 (Adobe eReader Format)ISBN 0 419 17490 7

Apart from any fair dealing for the purposes of research or privatestudy, or criticism or review, as permitted under the UK CopyrightDesigns and Patents Act, 1988, this publication may not be reproduced,stored, or transmitted, in any form or by any means, without the priorpermission in writing of the publishers, or in the case of reprographicreproduction only in accordance with the terms of the licences issued bythe Copyright Licensing Agency in the UK, or in accordance with theterms of licences issued by the appropriate Reproduction RightsOrganization outside the UK. Enquiries concerning reproduction outsidethe terms stated here should be sent to the publishers at the Londonaddress printed on this page.

The publisher makes no representation, express or implied, withregard to the accuracy of the information contained in this book andcannot accept and legal responsibility or liability for any errors oromissions that may be made.

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication data

Richardson, Barry A., 1937Wood preservation/Barry A.Richardson.2nd ed.

p. cm.Includes bibliographical references (p. ) and index.ISBN 0-419-17490-7 (alk. paper)1. WoodPreservation. I. Title.

TA422.R53 1993674.386dc20 9230664

C I P

To my friend John F.Levy whoseadvice and encouragement have

been highly valued for manyyears by all of us who are involved

in studies of wood deterioration andpreservation

Preface ixPreface to the first edition xi

1. Preservation technology 11.1 Introduction 11.2 Preservation principles 121.3 Wood structure 13

2. Wood degradation 232.1 Introduction 232.2 Biodegradation 232.3 Moisture content fluctuations 332.4 Fire 41

3. Preservation systems 433.1 Preservation mechanisms 433.2 Application techniques 663.3 Evaluating preservative systems 93

4. Preservation chemicals 974.1 Preservative types 974.2 Tar-oils 984.3 Inorganic compounds 1054.4 Organic compounds 1274.5 Organometal compounds 1354.6 Carrier systems 1404.7 Water repellents, stabilizers and decorative systems 1424.8 Fire retardants 1464.9 Stain control 1474.10 Remedial treatments 149

5. Practical preservation 1535.1 General principles 1535.2 Uses of preserved wood 1605.3 Health and the environment 171

Further reading 177

Contents

vii

Appendix A. Selection of a preservation system 179Table A.1 Typical preservative retentions for Baltic redwood 180Table A.2 Properties of principal construction woods used in northern

and southern hemisphere temperate zones 182

Appendix B. Wood-borers 185Table B.1 Wood-destroying termites 204

Appendix C. Wood-destroying fungi 211

Index 219

viii

Contents

Preparing a second edition of a technical book isalways interesting because the alterations thatare necessary indicate the amount of progressthat has been made since the issue of the firstedition. In this case I believe that very littleprogress has been made since I wrote my Prefaceto the first edition in December 1977, andcertainly much less progress than was made inthe previous 15 years, which were probably oneof the most exciting periods in modern woodpreservation development.

There are several reasons for this limited anddisappointing progress in recent years. It is easyto forget that the world suddenly became awarethat we were rapidly exhausting our reserves ofhydrocarbon fuels, and shortages and escalatingfuel prices then affected our lives andparticularly our industrial operations. Whilst theuse of petroleum solvents in preservatives wasobviously discouraged, the shortage of energyaffected the manufacture and application of allwood preservatives at a time when it was alsobeing recognized that forest resources were beingharvested faster than natural and plantationrenewal, and when wood preservatives weretherefore becoming particularly attractive as ameans to reduce unnecessary deterioration andconsumption. The energy crisis triggered aneconomic recession throughout the world andthe wood preservation industry was affected inthe same way as most other industries, andresearch and development expenditure has beendrastically reduced. At the same time the

development of new preservative systems hasbecome increasingly difficult due to theintroduction of more stringent health andenvironmental controls. As a result only verylarge companies and consortia can now afford todevelop new preservatives, and small companiesare suffering serious difficulties as theirestablished products become subject toincreasing restriction or even prohibition.

This increasing awareness of health andenvironmental dangers has not necessarilyresulted in the introduction of safer products,but instead the continuing use of products whichhave been widely accepted for many years. Forexample, widely used preservatives such ascreosote and the copper-chromium-arsenicsystems could not be introduced today, yet thedevelopment of safer alternatives is virtuallyimpossible because of the enormous costsinvolved, even if a new preservative is based onestablished knowledge and experience. Thepresent system is therefore actively discouragingthe development of new preservatives which aremore efficient and safer, and is insteadencouraging, through economic necessity, theextended use of established and less safe systems.

I believe that, when we look back at thisperiod in 15 or 20 years time, we will consider itto be perhaps the most depressing period in thehistory of wood preservation, but I also hopethat our health and environmental controlsystems will become more realistic, activelyencouraging the development of safer systems

Preface

ix

but equally actively discouraging hazardoussystems. One of the most serious problems withthe present system is the lack of understandingof the hazards; controls are based so often onparticular groups of toxic compounds withoutrecognizing that some individual compounds aremuch less toxic than others, and without ap-

preciating that the toxicity of a preservative doesnot depend on the presence of a toxic ingredialone, but also on the concentration at which itis used. Perhaps I can help a little by including,in this completely revised second edition of thisbook, an entirely new section in which I discussthese health and environmental problems.

Barry A.RichardsonLatchmere

Lainston CloseWinchester

Hampshire SO22 5HJ

x

Preface

Perhaps the most difficult task facing an authoris to decide upon the type of person for whom heis writing. This book is an attempt to provide areasonably comprehensive and non-controversialaccount of wood preservation of value to aperson approaching a study of this subject forthe first time, yet it is likely to be of equal valuefor reference purposes to the person who isalready involved in commercial woodpreservation or related research. Indeed, those inindustry would naturally tend to specialize,perhaps concentrating upon certain preservationprocesses involving the use of a particular typeof preservative applied by a particular method.Those involved in research are likely to beconcerned with a limited geographical area andtheir interest will normally be confined withintheir own scientific discipline, such asentomology or mycology. To all these personsthis book attempts to provide information on theother areas of wood preservation beyond theirdaily experience.

Wood Preservation is primarily an account ofthe situation in the principal temperate areas ofEurope, North America, South Africa andAustralasia, but the text refers in many respectsinitially to the situation in the British Isles,where wood preservation is most advanced.Wood has been imported into the British Isles forseveral centuries, so that it is widely acceptedthat it is valuable and preservation has long beeneconomically justified. Wood preservation wasfirst introduced as an industrial process in

England and it has continued to be used insituations where decay is otherwise inevitable,such as for railway sleepers (ties) andtransmission poles. However, it is not sufficientto confine this account to the British Isles alone,for even British readers require information onmany other areas. As modern trade hasexpanded preservatives and preserved woodproducts have been exported to an increasingextent to countries with substantially differentdecay hazards. In addition new borers and fungihave been introduced on imported materials.

Wood Preservation is a book on a science (oris it an art or technology?) that is steadilydeveloping, a fact that may be overlooked byscientists using this book, who will almostcertainly criticize the lack of a bibliography andreferences to specific statements in the text. Thiscannot be accepted as a serious criticism asanyone with such an advanced interest in woodpreservation will already be aware of the paperspublished in, for example, the Records of theAnnual Conventions of the British WoodPreserving Association and the Proceedings ofthe American Wood Preservers Association,which have extensive bibliographies and whichprovide a far more up-to-date source of furtherinformation than can be provided in any book.Other readers may criticize the failure to quotespecifications for test methods, preservativeformulations and treatment requirements, butagain these are continuously revised and vary ineach country so that the function of this book is

Preface to thefirst edition

xi

simply to establish the principles involved,leaving the individual reader to obtain copies ofappropriate specifications when required directfrom the issuing authorities, such as the BritishStandards Institution, the Nordic WoodPreservation Council and the American Societyfor Testing and Materials. No doubt a furthercriticism will be the failure to comprehensivelylist proprietary preservatives but again these aresubject to frequent changes; some are mentionedby name and a few are described in detail whenit is considered that they are particularlyimportant, but the enormous space required tolist and describe the several thousandpreservatives that are now available cannot bejustified.

Wood Preservation is concerned with woodpreservation, not with wood deterioration.Whilst it is obviously necessary for anyoneinvolved in preservation to possess at least abasic knowledge of the deteriorating agenciesthat require to be controlled, the identificationof deterioration is of limited importance. Areasonably detailed account of deterioratingorganisms is given in the appendices, but theselack the diagnostic tables that are so often afeature of such descriptions; the identification ofdeterioration is the speciality of those whoinspect structures and prepare specifications forremedial treatment, a subject that is consideredin far greater detail in Remedial Treatments inBuildings. Although a section on wood structureis included in Chapter 1, it is assumedthroughout that the reader has a basicknowledge of the properties and uses of woods.

If this creates a difficulty for any reader, he canrefer to Wood in Construction or to Appendix A,which not only summarizes the most importantpreservation treatments but also includes asummary of the properties of the more importantstructural woods.

I was introduced to wood preservation by myfather, Stanley A Richardson, and it is a subjectthat has always proved interesting to me. Themore I know about wood preservation the moreI become aware of our lack of knowledge andthe need for further observations andinvestigations. Wood preservation is aremarkably complex subject, involving so manydifferent disciplines, and my first impressionupon completing the writing of this book was ofthe enormous amount of information that it hadbeen necessary to omit and thus the very limitedamount that could be included. I have alwaysbeen encouraged in my studies of woodpreservation by my many friends in industry andthe related academic and research institutions,many of whom have kindly provided theillustrations that I have used. There are toomany of them to list, but I would like to mentionDr John F. Levy of the Imperial College ofScience and Technology, London, whosethoughts on deterioration and preservation are astimulation to so many of us. We must give himcredit for the very profound statement that asfar as a fungus is concerned, wood consists of alarge number of conveniently orientated holessurrounded by food, surely the most impressivestatement ever in support of the need forscientific wood preservation.

Barry A.Richardson

Preface to the first edition

xii

11.1 Introduction

It must be accepted that wood decay is inevitable.Indeed, if this were not the case our forests wouldsoon become cluttered with the giant skeletons ofdead trees. Natural durability is simply anindication of the rate of decay, but there is afurther factor of fundamental importancewhilstdecay may be inevitable in the forest it is notnecessarily inevitable in wood in service. Forexample, fungal decay is dependent on anadequate moisture content, so that a structuredesigned to maintain wood in a dry condition issufficient to ensure freedom from fungal decay,whatever the species of wood. In areas wherewood borers exist which are capable of destroyingdry wood, these structural precautions areinsufficient and it becomes essential to selectwood species which, whilst they may besusceptible to ultimate destruction from fungaldecay, possess good natural resistance to thewood-borer concerned. If wood with adequatenatural durability cannot be obtained it becomesnecessary to adopt preservation processes,although these cannot be applied universally butonly to those woods which are sufficientlypermeable to permit the required penetration andretention of preservative.

Need for preservation

Preservation involves additional cost and mustclearly be justified. The environmentalist maysee preservation as a means for reducing our

demand for replacement wood, thus conservingour forests. The economist may wish toconserve our forests for rather different reasonsbut the principle remains the same. Indeed,wood-importing countries will wish to preservein order to conserve foreign currency byreducing wood imports, whilst wood-exportingcountries will adopt preservation in order toreduce home demand for replacement wood,thus leaving the maximum possible volumeavailable for export. Even in the most primitivetropical jungle village wood preservation haseconomic importance, for in these conditionsthe ravages of fungi, termites and other wood-destroying organisms ensure that anunacceptable amount of time and effort isdevoted to replacing wooden structures such ashomes and bridges. If preservation is practised,either by the selection of more durable speciesor by the adoption of a simple preservationprocess, structures may double their life. In thisway more time and effort is available toimprove the quality of life in the community,perhaps by growing extra crops for sale. In suchprimitive communities wood has no value as itis freely available, but the labour for repair andreconstruction represents a substantial burdenon the community which is just as significant inmore sophisticated countries. For example, intemperate climates a normal transmission polepressure-treated with creosote will have atypical life of 4560 years, whereas an identicaluntreated pole will last only 612 years. Asimilarly treated railway sleeper (tie) can be

Preservationtechnology

1

Preservation technology

2

expected to last more than 35 years incomparison with only 810 years for untreatedwood. In these conditions preservation has nowbeen universally adopted and, as a result, thereis a tendency to forget the basic economics; ifuntreated structural wood deteriorates theexpense incurred is not confined to the cost ofits replacement, or even this and the additionalcost of labour required, but it also involves theperhaps much higher cost arising throughstructural failure. It can always be argued thatfailure can be avoided through regularinspection, but this cannot reduce the amountof disruption caused whilst services areinterrupted during repair and replacement.

Preserved wood must be regarded as anentirely new structural material and must not beconsidered as just an improved form of wood, asit can be used in entirely different circumstancesand certainly in more severe exposure situations.The most obvious advantage of preserved woodis that it can be used with impunity in situationswhere normal untreated species would inevitablydecay, but it may be argued that, in manysituations, this is a property that it enjoystogether with many competitive materials. Infact, the use of wood has many advantages. It isextremely simple to fabricate structures fromwood and, even in the most sophisticatedproduction processes, the tooling costs arerelatively low compared with those forcompetitive materials. Wood is ideal if it isnecessary to erect an individual structure for aparticular purpose but it is equally suitable forsmall batch or mass production. When theseworking properties are combined with the otheradvantages of wood, such as high strength toweight ratio, its excellent thermal insulation andfire resistance, and the unique aestheticproperties of finished wood, it sometimesbecomes difficult to understand why alternativematerials have ever been considered! However,there is one feature of wood which is uniqueamongst all structural materials; it is a cropwhich can be farmed, whereas its competitors

such as stone, brick, metal and plastic are allderived from exhaustible mineral sources.

With all these various advantages wood haslittle to fear from competitive materials,provided it is efficiently utilized and eitherselected or preserved to ensure that it iscompletely durable in service. The need fordurability is obvious, yet traditions are difficultto displace and in many countries the progressivedeterioration of wood in service is generallyaccepted. It is unlikely that all owners ofbuildings and other wooden structuresthroughout the world can be educated toappreciate the actual costs of the material andlabour involved in repairing decay damage, butthe authorities in many countries are becomingincreasingly conscious of the way in which thesecosts can affect prosperity. In this connectionone current problem is the demand for woodpulp which directly competes with structuralwood for the available forest resources. A highpulp yield can be achieved after short growingperiods so that there is a tendency to fell forestswhilst they are very immature to give rapidreturn on the invested capital. This has resultedin rapid increases in the cost of wood and afurther justification for its efficient utilizationand its preservation to avoid decay.

History of preservationPreservation is not, in fact, new. The ancientsworried little at first about decay as theirbuildings were seldom very permanent andreplacement wood was easily obtained.Probably the person earliest recorded as usingwood preservative was Noah who, whenbuilding the ark, was instructed by God topitch it within and without with pitch. In fact,various oils, tars and pitches were used fromtimes of the most remote antiquity. Herodotus(c. 484424 BC), a Greek whose monumentalwork earned him the title of Father of History,writes of the art of extracting oils, tars andresins. Healso draws attention to a much older

Introduction

3

system of preserving organic matter, the ancientEgyptian art of mummifying or embalmingbodies. This is probably the most efficientmethod of preserving organic matter that hasever been devised. The Egyptian mummies arenow at least 4000 years old and many are aswell preserved as when originally entombed.Herodotus and Diodorus Siculus (1st centuryBC) indicate that the body was steeped innatrum (or natron) for 70 days and then in anoily or bituminous substance for a similar time.Natrum, the production and use of which was astate monopoly in Ptolemaic times, from c. 320BC, was a mixed solution of sodiumsesquicarbonate, chloride and sulphate. It wasobtained from three centres fed during the floodseason by seepage from the River Nile. Themost important centre was an oasis in theWestern Desert still known as Wadi Natrum. Itis not possible that mummifying was practisedin the simple way described as the bituminoussubstance would scarcely penetrate, yet it hasbeen found that even the interior of the boneshas been penetrated. It is probable that thebody, after steeping in natrum, was placed inthe bituminous substance which was heated totemperature above the boiling point of water sothat the water within the body volatilized andwas then replaced by the oil. Boulton carriedthis out on a piece of wood in the middle of the19th century; his results indicated thecorrectness of the theory and were also theorigin of the Boultonizing treatment which isstill in use today.

The Egyptians were not the only people touse metallic salts as preservatives. The Chinesewere immersing wood in sea water or the waterof salt lakes prior to use as a building materialbefore 100 BC. Well preserved props have beenremoved from old Roman mines in Cyprus andexamination has shown them to containmetallic copper, well distributed throughoutboth the heart and sapwood. Various theorieshave been advanced to explain its presence as aRoman attempt at preservation, but is seems

more likely that the true explanation involvesthe copper found in the soil in this area. It ispossible that the process was electrolytic, oneend of the prop being in one type of soilcontaining copper and the other end being inanother type of soil so that the damp woodformed a rather complex cell.

Marcus Porcius Cato (234149 BC), a Romanwhose condemnation of the luxury of his timesearned him the nickname of Cato Censorious,commented on wood preservation, but by far themost informative writer was the renownedRoman naturalist Pliny. Pliny the Elder (AD 2379), who perished at Pompeii during the eruptionof Vesuvius, mentioned that Amurca, the oil-lessby-product in the manufacture of olive oil, andalso oils of cedar, larch, juniper and nard-bush(Valeriana spp.) were used to preserve articles ofvalue from decay. He claimed that wood wellrubbed with oil of cedar was proof againstwoodworm and decay, and in his writingsdescribed the preparation of 48 different kinds ofoil for wood preserving. He also observed that themore odoriferous or resinous the wood the moreresistant it was to decay. Because the statue ofZeus (Jupiter) by Phidias was erected in a dampgrove at Olympia its wooden platform wasimbued with oil. The statue of Diana at Ephesuswas made of wood and was believed to have beenof miraculous origin. Pliny, quoting an eye-witness Musicians, notes that it was still thoughtnecessary to saturate it with oil of nard throughsmall orifices bored in the woodwork. TheRoman use of olive oil was copied fromAlexander the Great (356323 BC), the king ofMacedon who conquered a large area of theknown world. He is said to have ordered piles andother bridge timbers to be covered with olive oilas a precaution against decay.

The previously mentioned statue of Diana atEphesus was underpinned with charred piles. Thiswas not a new idea as a prehistoric race, theBeakermen, applied charring to wood. Theaborigines called the Tiwi who live on MelvilleIsland near Darwin, Australia, and whose

Preservation technology

4

civilization is said to be 50 000 years behind ourown, mark their graves with brightly painted poleslike American Indian totem poles, which are madefrom Bloodwood, a hard red-sapped wood whichthey have found to be resistant to termites andfungal decay. Prior to painting, the wood is charredand covered with beeswax, orchid sap or white ofturtle egg. This may be the continuation of someprimitive knowledge of wood preservation.Alternatively it may be solely a method of forminga suitable background for painting.

As soon as man began using wood as abuilding material it was only a matter of timebefore decay became domesticated. The fungusthat probably causes the greatest damage inbuildings is the common Dry rot fungus Serpulalacrymans. It has never been recorded asoccurring in nature and appears to be associatedonly with man-made structures. The name isderived from the Latin word lacrima, a tear, forSerpula lacrymans, (formerly known as Meruliuslacrymans is the weeping fungus as fresh growthcan often be observed covered with drops ofwater. It is this weeping or fretting which enablesit to be identified as the fretting leprosy of thehouse in the Old Testament Book of Leviticus.Until comparatively recent times the priest wasthe person who was called in to deal with anykind of trouble or pestilence and consequentlyLeviticus contains full instructions on how todeal with the leprosy of the house. The priest,when carrying out his inspection, was to look forhollow strakes, greenish or reddish, on thewalls. If they were present the house was to beshut up for 7 days, and if after that time theplague be spread in the house, it is a frettingleprosy and Dry rot rather than one of the Wetrots, and he was to command that they takeaway the stones in which the plague is, andcast them into an unclean place without thecity,the house to be scraped within roundabout, andpour out the dustwithout the cityinto an unclean place. This may appear ratherruthless but even today an affected area must bestripped to the bare masonry to ensure the

successful application of fungicide, the Dry rotfungus spreading through masonry in the searchfor wood. It is in these verses of Leviticus, whichalso describe leprosy in man, that we may readof early ideas of contagion, and people enteringthe house were required to wash themselves andtheir garments thoroughly on leaving. If thepriest found that the fungus had not developedafter replastering the affected area he was toapply final fungicidal treatment and take tocleanse the house two birds, and cedar wood,and scarlet, and hyssop. He was instructed tosacrifice one bird and sprinkle the house seventimes with the blood. The other bird, after beingdipped in the blood, was freed and flew away,presumably taking the pestilence or infectionwith it!

It was the belief that the words of the Bibleand several other books were completelyirreproachable that severely discouraged thedevelopment of science and technology up to theearly 16th century. Ideas not in agreement withthese standard works were considered heretical,and often people were put to death forexpressing them. Other than those alreadyquoted, references to timber preservation beforethe 18th century appear to be negligible,although timber decay was frequently describedand appears to have been a serious problem.

Early problemsIn the reign of Elizabeth I, Britains greatest assetwas her navy. When Elizabeth came to thethrone she found that ten out of the 32 Royalships were suffering from decay. There was,apparently, no accepted method of woodpreservation, and the condition of the navy wasso bad a few years later during the reign ofJames I that a commission of inquiry wasappointed. The findings emphasized theimportance of constructing ships of seasonedwood, but little notice appears to have beentaken of this. James I was succeeded by his sonCharles I, who was beheaded, but whose son

Introduction

5

Charles II, was crowned on the restoration of themonarchy. A period of rearmament commenced,the navys programme of shipbuilding being inthe control of Samuel Pepys, the Secretary to theAdmiralty. In his famous diary Pepys remarkedon the shortage of wood being such that a largeamount of green and unseasoned wood was usedin shipbuilding. The result was that many shipsbegan to decay before being commissioned.Pepys very wisely suggested that this would notbe so if the ships were better ventilated. Thesetroubles were not confined to Royal Navy ships,and the merchantmen of the East India Companyseldom made more than four, and sometimesonly three, voyages to India before becominguseless through decay.

The shortage of wood that Pepys referred towas becoming serious. The almost continuousarming and rearming occurring throughout thesetimes meant that extremely large quantities ofwood were required for shipbuilding.Concurrently the rise in the use of iron saw theprogressive destruction of the Wealden and otherforests to provide smelting charcoal. DanielDefoe in 1724 wrote that the Sussex ironworkswere carried on at such a prodigious expense ofwood, that even in a country almost overrunwith timber, they began to complain ofconsuming of it for the furnaces and leaving thenext age to want for timber for building. Hesaw no justification for the complaint, for Kent,Sussex and Hampshire were one inexhaustiblestorehouse of timber. Defoe was wrong, ofcourse, for wood was being used at such analarming rate that there was already a noticeableshortage in the 16th century. Various Acts werepassed through Parliament in attempts to limitconsumption and obtain supplies from abroad.There were also attempts to transfer the ironsmelting industry to North America but thisnever came about because of the introduction ofcoal for smelting. Later a further significanteconomy occurred when prejudices against theuse of coal as a domestic fuel were finallyovercome. Softwood began to arrive in

increasing quantities from the Baltic and Canadabut, despite this, the general shortage soon raisedprices and the need for preservation becamemore apparent.

The wastage of ships in the Royal Navy onaccount of decay was becoming an extremelygrave problem. Little seems to have been doneagainst Dry rot, for in 1771 Lord Sandwich hadthe fungus dug out of the reserve ships so that hemight inspect the timbers. Dry rot was not theonly problem the navy had to contend with, forthere were also the marine-borers such asshipworm and other fouling organisms thatattacked the outsides of ships. At the time ofVasco de Gama (14691524) the Portuguese areknown to have charred the outsides of their shipsas a protection against shipworm, and in 1720the Royal Navy built a ship the Royal Williamentirely from charred wood. It appears that theexperiment failed for it was never repeated, butcharred wood is still used for shipbuilding by theSolomon Islanders, who apparently learned thepractice from the Portuguese. Covering the hullwith sheet metal as a protection againstshipworm was a system used as early as Romantimes. Lead was the metal that was most easilyworked into sheets but it was so heavy that itpulled away from its fixings. Copper was triedby the British Navy but this coincided with theintroduction on a large scale of iron fittings forships. Electrolytic action occurred, seriouslydamaging rudder bearings, so that the use ofmetal sheeting was discontinued. In 1782 theprevalence of Dry rot in Royal Navy ships wasmade very apparent by the tragic loss of theRoyal George at Portsmouth. At the courtmartial investigating the deaths of the 800 menon board it was disclosed that previously thebottom had fallen out while the ship was beingheeled over for slight repairs.

Early preservationIn 1784 the Royal Society of Arts offered a goldmedal for the discovery of the various causes of

Preservation technology

6

Dry rot in timber and the certain method of itsprevention. It was awarded in 1794 to Batsonwho, in 1778, had treated an outbreak of Dry rotin a house by removing sub-floor soil andreplacing it with anchorsmiths ashes. In the early19th century Britain was successfully negotiatinga most difficult period of history, highlighted bythe wars against Napoleon and in North America.The Royal Navy was more important than everbefore but, although the shortage of availablewood brought the decay of ships daily into clearerperspective, it was not until 1821 that theAdmiralty asked James Sowerby to investigate theproblem. He reported on the state of the QueenCharlotte, a first rater of 110 guns that waslaunched in 1810 at a cost of 88 534. Only 14months after launching she had to be re-built at acost of 94 499 before she could becommissioned. By 1859, when her name waschanged to Excellent, the total cost of repairs hadrisen to 287 837. Sowerby reported the cause asfungal rot and identified a score or more of fungi.

The Royal Navy was not the only suffererfrom Dry rot. In 1807 James Randell spoke tothe Royal Society of Arts on the subject of Dryrot and mentioned that it had destroyed thegreat dome that Robert Taylor had built on theBank of England. By this time people werebeginning to take serious interest in chemicalpreservatives. 1812 saw the first attempt atfumigation when Lukin experimented inWoolwich dockyard with the injection ofresinous vapours; the attempt was abandonedafter the apparatus exploded with fatalconsequences to the workmen.

Salt preservativesThe 19th century produced further interest inwood decay on account of the widespreadexpansion of the railways. The stone blocks firstused for supporting the rails were found to betoo rigid and wooden sleepers were substituted.These rapidly decayed and obviously a goodchemical preservative was required. The first list

of established preservatives, published in 1770by Sir John Pringle, was followed shortlyafterwards by a second list drawn up by DrMacbride. The age of chemical woodpreservation had arrived and these lists bothappeared in the 1810 edition of EncyclopaediaBritannica. By 1842 five processes wereestablished using mercuric chloride, coppersulphate, zinc chloride, ferrous sulphate with asulphide, and creosote respectively.

Mercuric chloride, first used by the Frenchscientist Homberg in 1705 to preserve woodfrom insect attack, was later recommended byDe Boissieu (1767) and Sir Humphrey Davy(1824), and in 1832 Kyan took out his patentfor this process which became known asKyanizing. Kyans first success was thepreservation of the Duke of Devonshiresconservatories. The British Admiralty tested itand found it failed against marine organisms.This was more than a century after the DutchGovernment had found exactly the same result.Mercuric chloride, also known as corrosivesublimate, is soluble in water, volatile atordinary temperatures and poisonous. Its usecontinued for some time in the United Statesand Germany but probably the last large-scaleinstance of Kyanizing in Britain was in 1863.

The use of copper sulphate was recommendedas early as 1767 by De Boissieu and Bordenave.Thomas Wade recommended it in 1815, and in1837 Margary took out a patent for its use inwood preservation. Copper sulphate was by farthe most successful metallic salt and long after itdied out of use in England its popularitycontinued in France, where it was applied by amost ingenious method known as the Boucherieprocess, by which a newly felled tree isimpregnated by replacement of the sap. TheBoucherie copper sulphate process was verypopular in France from about 1860 until it wentout of favour in 1910 because of some failureson alkaline soil. It was used for preservingtelegraph poles in Britain prior tonationalization in 1870, and continued in use in

Introduction

7

the South of France and Switzerland until a fewyears ago. The Boucherie process was revived in1935 by the Deutsche Reichpost but with amultisalt preservative.

The antiseptic properties of copper sulphatehave never been questioned and it is only thesolubility in water of this and other metallic saltsthat makes them unsuitable as woodpreservatives in wet situations. It will be recalledthat the ancient Egyptians had a very effectivemethod for the preservation of bodies, whichBoulton suggested had involved steeping innatron followed by pickling in a bituminoussubstance. Boulton impregnated a piece of woodwith natron and afterwards placed it in creosoteat a temperature above the boiling point ofwater. He found that water evaporated,depositing the natron salts in the wood, and thecreosote then penetrated well into the driedwood. This process formed the basis of a patentby Boulton in 1879 except that copper sulphatewas used in place of natron. It was successful butwas little used because of the success of creosotealone, the addition of the copper sulphateserving only to increase the cost. Boultonsuggested, however, that an oil with nopreservative properties could be used in place ofthe creosote, its purpose being solely to preventleaching of the copper sulphate. ModernBoultonizing involves the use of high-temperature creosote and vacuum, but simply toboil off moisture within the wood so that thecreosote is able to penetrate.

Zinc chloride was recommended as a woodpreservative in 1815 and 1837 by Thomas Wadeand Boucherie respectively. In 1838 Sir WilliamBurnett patented its use but throughout its woodpreserving history it has suffered because of itsextreme solubility in water. Despite this it wasmuch used by the Royal Navy. Because of theshortage of creosote in the United States its usecontinued there long after it was forgotten inBritain. Even there, however, its use graduallydiminished because of the expansion of the coalgas industry and the increased imports of

creosote from Britain as return cargo inpetroleum tankers. Although extremely soluble itwas found to retain its power of preservation toa small extent. This was thought to be due to theformation of zinc oxychloride, insoluble in waterbut poisonous to fungi because of its solubility intheir enzyme secretions. This hypothesissuggested the idea of the formation of insolublepreservatives within wood by the application oftwo or more solutions, a process that has beeneffectively applied using various materials sincethe beginning of the 20th century. However, thefirst precipitation process was a complete failure.It was in 1841 that Payne was granted a patentfor his two-stage process which involvedimpregnation of wood with ferrous sulphatefollowed by calcium sulphide. It was said that adouble decomposition occurred within the poresof the wood, forming ferrous sulphide andcalcium sulphate, both only sparingly soluble inwater, but the treatment was found to havenegligible preservative effect.

Coal-tar productsCreosote is certainly the most successful of thepreservatives developed during the 19th century.The process, known as creosoting, is basedessentially on a patent granted to John Bethell in1838. Bethells patent lists 18 substances,mixtures or solutions, including metallic salts,oleaginous and bituminous substances. Althoughthe word creosote is not used, mention is madeof a mixture of dead oil with two or three partsof coal-tar, and this is the origin of creosoting bypressure impregnation. It was not the firstattempt at the use of tars, for as early as 1756experiments were carried out in Great Britainand America (Knowles) on the impregnation ofwood with vegetable tars and extracts. Boultonsuggested that the term creosote originated in apatent granted to Franz Moll in 1836. Thispatent was concerned with injecting wood inclosed iron vessels with extracts of coal-tar, firstas vapour and then as oil in a liquid state. Moll

Preservation technology

8

termed oils lighter than water Eupion and thoseheavier Kreosot, the latter being said to haveantiseptic qualities. The process was notpractical as the light oils immediately evaporatedon the application of the heated kreosot, and itwas left to Bethell to patent the moderncreosoting process two years later. Franz Mollprobably derived his term kreosot from theGreek words kreas for flesh and soter forpreserver but, although the term kreosot wasapplied to the heavy coal-tar oils, the termcreosote was not derived directly from it. Evenin Franz Molls time the term creosote wasapplied to a product of the dry distillation ofwood, and the term was applied to the heavy tar-oils in the belief that true creosote was identicalto the carbolic acid contained in coal-tar.Boulton mentions that Tidy compared these twosubstances and showed them to be dissim-ilar.He also demonstrated that coal-tar contained notrue creosote but the term creosote is nowuniversally applied to the heavy oils producedduring the distillation of coal-tar.

By 1853 creosote had established itself as amost reliable and persistent wood preservative,and most other processes were abandoned. InFrance, however, creosote established laterbecause of the popularity of copper sulphateallied by the Boucherie process. In 1867Forestier, working for the French Governmentand also Dutch Government investigators,showed that creosote of a suitable grade,efficiently applied, rendered wood resistant toshipworm. At about the same time, Crepin,working for the Belgian Government, showedthat this applied also to other marine animals.However, there were distinct failures resultingfrom the use of the wrong type of creosote andthese focused attention on studies of thecomposition and effectiveness of various grades.

In 1834 the German chemist Rungediscovered phenol (carbolic acid) in coal-tar, andin 1860 Letheby attributed the preservativeproperties of coal-tars to this component.Carbolic acid was recognized as an effective

fungicide and it was due to investigations intothe wood-preservative properties of coal-tar thatthe world gained the benefit of a most importantmedical development. A young surgeon, JosephLister, was discussing with a railway engineer hismethod for preserving sleepers (ties) and wasinformed that it was the carbolic acid whichprevented decay. Lister, worried by the highdeath rate due to infection during operations,immediately saw that carbolic acid might beused to prevent it. He started operating under acontinuous carbolic acid spray, all hisinstruments and his own hands having beenwashed in the acid. Conditions were mostunpleasant but he thought it worthwhile if hecould save a few lives. He was surprised anddelighted to find that, instead of the slightimprovement that he had hoped for, he hadachieved almost complete success. There wasopposition to his idea at first, but his dramaticresults eventually gained him universalrecognition and it was not long before carbolicacid was described in medical textbooks as theaerial disinfectant par excellence.

Letheby, appreciating the antiseptic propertiesof carbolic acid, specified that naphthalene andpara-naphthalene should be excluded as far aspossible from creosote as he considered them tohave no preservative value. However, in 1862Rottier concluded that carbolic acid, althoughan energetic antiseptic, had little persistent effectdue to its volatility and solubility in water. Heattributed the durable success of creosote to theheavier and less volatile components of coal-tar.Despite the interest in wood preservationresulting from the expansion of the railways andlater of the telegraphs, it was not until 1863 thatreally instructive experiments were carried out.These experiments, which were conducted byCoisne on behalf of the Belgian Government,were repeated in 1866. Coisne treated woodshavings with various grades of creosote andplaced them in a putrefying pit for 4 years. Theresults were entirely in favour of the use of theheavier oils, tar acids by themselves having no

Introduction

9

persistent effect. These results were confirmed bylong-term experience and the BelgianGovernment adopted the recommendations forits successful creosoting specifications.

About this time there were two theories on thecause of decay. The accepted theory was that ofthe great scientist Liebig, enunciated in greatdetail in 1847 and 1851, which claimed thatputrefaction or decay of an organic material wasa form of slow combustion which he termedEramacausis and that it was initiated oncontact with bodies which were already affected.He discovered that it could be prevented by lackof moisture and atmospheric air, and from thishe deduced (and later showed) that it wasprovoked by oxygen. Daltons atomic theory,proposed in 18081810, was by this time one ofthe foundation stones of science and Liebigclaimed that the method of transfer was thecommunication of motion from atoms of theinfected matter to atoms in the contactingmaterial. He denied that fermentation,putrefaction or decomposition was caused byany fungi, animalcules, parasites or infusoriathat might be present, their presence beingcoincidental or due to a preference for feedingon decaying matter.

The parts of animals and plants which decaymost rapidly are the blood and the sap. It wassuggested that decay could be prevented bycoagulants of albumin, such as mercuricchloride, copper sulphate, zinc chloride and thetar-oils. In 1854 Louis Pasteur was appointedProfessor and Dean of the Faculty of Science atLille. Here he concentrated on his study offermentation in the production of beer and wine.Three years later he moved to the Ecole Normaleat Paris as Director of Scientific Studies, andwhile there he proclaimed that fermentation wasthe result of the action of minute organisms. Iffermentation failed to occur it meant that theorganism was absent or unable to developproperly.

Liebig had observed that decay requiredatmospheric air and deduced that this was

because oxygen was necessary. He confirmedthis theory by showing that oxygen accelerateddecay. Pasteur repeated the experiments and in1864 announced that decay was caused byminute organisms that were not spontaneouslygenerated but were instead present inatmosphere, and this was one reason whyatmospheric air was necessary. It was only a yearafter this that Lister appreciated the significanceof infection in surgery and, as previouslymentioned, initiated the use of carbolic acid as asurgical disinfectant. Pasteurs theory wasconfirmed by Koch and soon gained supportamong authorities such as Tyndall. This wasimmediately thought to be a simplification of thetheory of preservation as the only problem wasto discover substances toxic to the decay-causingorganisms.

Pasteur was not the first to declare decay tobe caused by living organisms. As far as woodwas concerned there was the obvious damage byborers and also the presence of fungi. In 1803Benjamin Jonson had declared Dry rot to be theresult of a visit from a plant and is and ever wasso but he left it to Theodore Hartig in 1833 torecognize the general association between fungiand decay. This association had been noticedbefore but it was Theodores son Robert who in1878 showed fungi to be directly responsible fordecay. He continued his studies in 1885 andmade a thorough investigation into the Dry rotfungus and its effect on wood.

By 1884 the wood-preserving industry hadbeen established long enough for serious interestto be taken in long-term effectiveness. Themetallic salts had broken down completely andtheir use had been largely discontinued in favourof creosote. It was many years before interest insalt preservatives revived with the developmentof the multisalt preservatives, described in detailin Chapter 4. Creosote had been known to failbut because of the careful records of treatmentkept by some of the impregnating companies,coupled with the work of people like Coisne,Boulton and Tidy, specifications had been

Preservation technology

10

developed which could be relied on to give goodprotection. Boulton carried out tests in 1884 ona 29-year-old fence in London Docks, apparentlyas sound as when it was erected. He detected notar acids but found the semi-solid constituents oftar-oils, including naphthalene, to be present. Hefound very little distilling below 232C (450F)and 6070% distilled above 316C (600F). Hemanaged to detect acridine solidified in the poresof some of the specimens. This is an acrid andpungent substance, neither volatile nor soluble inwater, that had been discovered by Graebe andCaro. Greville Williams also examined samplesfrom the fence and, although he managed todetect traces of tar-acids, the indication was veryslight and was probably due to the heaviest tar-acids trapped within solidified portions of theoil. In nearly all of the specimens he detectednaphthalene and in all he detected acridine andbasic substances. He concluded that thepreservative action was due more to the latterthan to the tar-acids. Tidy experimented onnaphthalene, finding that it remained in thepores of the wood. Although not as powerful anantiseptic as the tar-acids it was far morepersistent. He decided that the para-naphthaleneor anthracene contained in tar-oils was probablywithout wood-preserving properties and drew uphis creosote specification accordingly. Thisstandard, introduced in 1883, was the basis ofnearly all British specifications until the BESA(now BSI) specification was introduced in 1921.

In 1824 Hennell had synthesized alcohol and 2years later Wohler was responsible for thesynthesis of urea. These achievements opened thedoor to tremendous developments in industrialsynthesis of organic compounds. Coal is averitable treasure chest of raw materials for theseprocesses, and it was not long before coal-tarbegan to suffer from the extraction of some of itscomponents. Typical of this was the use made ofanthracene. From the earliest times the roots ofmadder (Rubia tinctoria) had been used as adyestuff in India and Egypt. The principal dyeinvolved is alizarin which is present in the root as

a glucoside, ruberythric acid. This can behydrolysed to glucose and alizarin, and wasextensively employed until towards the end of the19th century in the production of Turkey Red dyefor dyers and printers. However, in 1868 Graebeand Lieberman found that alizarin could bereduced to anthracene by heating with zinc dust.They suggested a rather expensive process forsynthesizing alizarin from anthracene, which wassoon relinquished in favour of an alternativeprocess they discovered simultaneously withPerkin, the Father of Dyeing.

Whilst the increasing sophistication of thechemical industry threatened to reduce theeffectiveness of creosote, it was also ultimatelyresponsible for the development of compoundssuch as pentachlorophenol and the organo-chlorine insecticides which made the formulationof organic solvent-based preservatives possible,as described in Chapter 4. Fortunately, Tidy hadalready shown that anthracene had only weakwood-preserving properties, so that there was noconflict between dye manufacturers and creosoteusers. Other changes in the composition ofcreosote were caused by the different methods ofcoking and the varying grades of coal. All thismade it more important that the principal wood-preserving components in creosote should beidentified. Work has continued to the presentday, but despite improved methods thepreservative action of creosote is still imperfectlyunderstood. In 1951 Mayfield concluded thatthe toxicity of creosote is not due to one or avery few highly effective materials but is due tothe many and varied compounds which occurthroughout the boiling range. The value ofcreosote as a wood preservative depends largelyon whether or not it remains in the wood underthe conditions and throughout the period ofservice. Essentially this means that a particulargrade of creosote cannot be said to be efficienton the merits of its chemical composition alone.The only true test is to use it and see how itperforms in normal service, but the difficulty isthe length of life expected of creosote; the fence

Introduction

11

tested by Boulton in 1884 lasted about 70 years.Even then it was demolished only to make wayfor another structure and was still reasonablypreserved. Any field test would take as long, sothat evaluation of new preservatives is oftenbased on laboratory comparisons of preservativetoxicity.

Application methodsLittle has been said of the methods used forapplying preservatives. An effective preservativecan be a complete failure if inefficiently applied,and this is the explanation of the early failures ofcreosote in the United States. Vacuum andpressure methods of impregnation undoubtedlygive the greatest certainty of lastingpreservation. Breant is said to have been theinventor of this process when he took out apatent in 1831, but in Great Britain Bethell wasgranted a patent in 1838 which includedamongst other substances creosote applied bythis means. The method soon became known asthe full-cell or Bethell process, although it wasmodified to its present commercial form, whichwill be described in detail in Chapter 3, by Burt,who was granted a patent for his improvementsto the method. With creosote the method isineffective when applied to unseasoned or wetwood, so that extensive storage facilities arerequired for drying and seasoning. In 1879Boulton was granted a patent for his Boilingunder Vacuum process, using hot creosote to boiloff the water in the wood. This process may befollowed by the full-cell process or an empty-cellprocess such as the Rping process. Steamingand steaming-and-vacuum processes were triedas alternatives to the Boulton process but withno great success.

There are several difficulties encountered withthe full-cell process. Creosote bleeding is likelyto occur, an annoying factor with fences andpoles that pedestrians and animals are likely toencounter. Another aspect is the quantity ofpreservative used, a very important point in

countries where preservatives, especiallycreosote, are scarce and expensive. The empty-cell processes are a great improvement asbleeding is less likely to occur and there is a 4060% reduction in the use of preservative. Thelatter is especially important in the case ofparticularly permeable woods and those with ahigh proportion of sapwood. The empty-cellmethods in common use, the Rping and Lowryprocesses, will be decribed in Chapter 3.

The Rping process was initially patented byWasserman in Germany in 1902, althoughRping applied the process commercially andAmerican patents were subsequently granted inhis name. The process is commenced by theapplication of an initial air pressure. When theentire process is complete the pressure isreleased, the compressed air in the cells drivesout some of the preservative and a short periodof vacuum recovers more preservative, so thatthe net retention in the wood is only about 40%of the gross absorption, a saving in preservativeof 60%. The Lowry process, which was patentedin America in 1906, differs only in that it relieson compression of air at atmospheric pressurefor return of excess preservative, so that there isno initial compression stage. The recovery ofpreservative is about 40%.

Other similar processes due to Hlsbert,including the Nordheim process of 1907, havebeen entirely superseded by the Rping process.In 1912 Rtgerswerke AG were granted a patentfor treatment of insufficiently dry timber by theRping process. It is identical with Boultonspatent except that an oil, used for evaporating thewater, is drawn off before the Rping process isapplied. The vacuum and pressure methods arethe most important and most effective methodsused for the application of wood preservatives.They suffer, however, from the great disadvantagethat special plant is required and it is oftenimpossible or uneconomical to send wood to theplant for treatment. Numerous non-pressuremethods are available but are suitable for use onlywith specially developed preservatives such as the

Preservation technology

12

low-viscosity organic solvent products for sprayand dip treatment of dry wood, and theconcentrated borate solutions which can be usedfor diffusion treatment of freshly felled woodwith high moisture content. Preservationprocesses are discussed in detail in Chapter 3.

1.2 Preservation principles

The simplest method to avoid deterioration is touse only naturally durable wood. Durability isan embarrassment in nature as it delays thedisposal of dead trees, and it can therefore beappreciated that only a limited number of woodspecies are truly durable. This durability isalways confined to the heartwood but theelimination of sapwood, coupled with selectionfrom a very limited range of species, isunrealistic unless very high costs can betolerated. It is far more realistic to select a woodspecies for its physical properties and then totake suitable precautions to ensure thatdeterioration is avoided. This does notnecessarily mean the use of preservativetreatments. For example, the most efficientmethod to avoid fungal decay is to keep wooddry, and this is most simply achieved bystructural design, such as the incorporation ofoverhanging eaves and gutters to dispose ofrainfall and damp-proof membranes to isolatestructural wood from dampness in the soil orsupporting structure. However, there are somesituations, usually termed severe hazardconditions, where deterioration is una-voidableunless naturally durable or adequately preservedwood is used. The most important severe hazardrisk is the ground contact condition which arisesin transmission poles, fence posts and railwaysleepers (ties). In some areas insect-borer attackis virtually inevitable whatever the serviceconditions, such as in areas subject to the DryWood termites. In some parts of Europe theHouse Longhorn beetle, sometimes known as theHouse borer, represents a severe hazard to

softwood. In other situations deterioration maynot be inevitable, yet it may be possible or evenprobable, representing a moderate hazard. Thusthe Common Furniture beetle is a particularlywidespread cause of damage, yet it seldomresults in structural collapse. Similarly, fungaldecay may not normally present a risk, yet itmay be able to develop if structural woodworkbecomes wet through accident or neglect.

It is obviously important to identify thedeterioration hazard before deciding on theprecautions that are necessary. However, thehazard does not vary only with the conditions towhich the wood is exposed but also with thewood species. For example, a group ofBasidiomycetes are responsible for the fungaldecay that is commonly known as Wet rot. TheCellar rot Coniophore puteana occurs inpersistently damp conditions, such as when adamp-proof mem-brane is omitted and whenplates under floor joists are in direct contact withdamp supporting walls. If the moisture contenttends to fluctuate, as in wood affected by aperiodic roof leak, the White Pore fungus Poriavaporaria is far more common in softwoods and,for example, the Stringy Oak rot Phellinusmegaloporus in oak. Coriolus versicolorsometimes develops when non-durable tropicalhardwoods are used as drips or sills on windowand door frames, and Paxillus panuoidesgenerally occurs where the conditions are too wetfor these other fungi. A knowledge of the basicnature of various wood species, and perhaps evenof the principles for their identification, istherefore essential for a proper understanding ofthe decay hazard. The reliability of preservationprocesses also varies widely with the woodspecies. The most important requirement is toachieve an adequate retention of the preservativewithin the wood. In many species this can beachieved relatively easily in the sapwood but theheartwood may be completely impermeable. InBaltic redwood (Scots pine) the treatment of thesapwood may be all that is necessary as theheartwood possesses significant natural

Wood structure

13

durability. In other species such a Balticwhitewood (spruce) even the heartwood is non-durable, yet neither heartwood or sapwood issufficiently permeable to permit adequatepreservative penetration. Preservative efficacyalso varies with the microscopic structure of thewood. Thus the usually reliable copper-chromium-arsenic water-borne preservatives aremuch less efficient in hardwoods than insoftwoods, apparently through the inadequatemicro-distribution of the preservative within thecell walls. Clearly a detailed knowledge of the finestructure of wood is necessary if these variousproblems are to be fully understood.

1.3 Wood structure

The tree

The basic structure of wood, the variationbetween softwoods and hardwoods, thedifferences between species and the significanceof various features ar all described in greaterdetail in the book Wood in Construction by thepresent author. Many of the features are ofimportance in wood decay and preservation.Wood is the natural supporting skeleton of largerplants and it is important to understand theorigin of the skeletal parts in order to fullyappreciate their properties. A plant consists of acrown of leaves, a supporting stem and the rootsthat anchor it within the soil. A tree is specialonly in regard to the scale of its development,and thus the need for a supporting skeletonwhich ultimately becomes the wood ofcommerce. However, the trunk does not performsolely this passive supporting function but alsoacts as a storage area, and the outer zones arethe conducting routes between the crown andthe roots. In addition, the growth of the crownmust be accompanied by similar growth in theroots, and an appropriate enlargement in thetrunk to enable it to continue to perform itssupporting function.

The growth arrangement of a tree comprising acrown of leaves connected to the roots by usuallya single main stem, trunk or bowl is known as thedendroid habit. The sole purpose of this veryelaborate structure is simply to survive and tosupply the cells within the plants. This is achievedfirstly by the roots, which absorb watercontaining dissolved mineral salts which is thenconveyed by the trunk, branches and twigs to thecrown and the individual leaves. The function ofthese leaves is to absorb atmospheric carbondioxide, which is combined with the water fromthe roots to form simple sugars by the processknown as photosynthesis; the chlorophyll in greenplants is the essential catalyst which enables thisprocess to proceed whenever adequate ultravioletradiation is received from the sun. The sugars arethen conveyed throughout the plant to the leaves,twigs, branches, trunk and roots. The primaryfunction of the sugars is to provide an energysource or food for the individual living cellswithin all these components of the tree, but asecondary function is to provide the basic unitsfrom which the skeletal structure of the tree isformed. Whenever there is sufficient sunlight thesimple sugars will be produced by the leaves andwill be found distributed throughout the livingtissue of the tree. Some of this sugar will beformed into starch and deposited within the livingtissue as a reserve energy source which can beutilized by the cells whenever sugar is unavailablefrom the leaves, such as at night or during thewinter months when deciduous trees shed theirleaves. Finally the sugar units are joined intocellulose chains which are then assembled into themain skeleton of the woody parts of the tree.

Wood formation

A tree is continuously increasing in size (Fig.1.1) and this is the function of the embryonictissue distributed around the whole plant. Theincrease in the overall size of the crown is theresult of the activity of the apical meristem or

Preservation technology

14

bud at the end of each twig which achieves theprogressive extension in length. The detailedstructure of this bud has little significance on thestructure of wood but the meristem tissueactually extends over the entire surface of thetree, just beneath the bark of the twigs, branchesand trunk but extending similarly over the entireroot system. The purpose of this lateral meristemis to enable all the structural components of thetree to increase in girth so that they are capableof supporting the enlarged crown. Each twig asit lengthens consists initially only of pith formedby the apical meristem, but it is coveredexternally by the lateral meristem to permitsubsequent increase in girth, although it alsoprovides a protective covering to the new shootto control water loss and to prevent diseasedamage. As this meristem tissue generates newcells which increase the girth the protectivecovering splits, exposing inner tissue, but themeristem tissue then generates a new protectivelayer which becomes the rough outer bark of thebranches and trunk (Fig. 1.2).

A trunk in its simplest form could thusconsist of single twig, progressively increasing in

FIGURE 1.1 Diagrammatic trunk showing annualrings.

FIGURE 1.2 Wood zones in a trunk.

Wood structure

15

length or height as it is extended by the bud atits apex, the lateral meristem also increasingthe girth, so that the trunk possess a steepconical shape which is essentially the portionof the tree which is the wood of commerce.The trunk consists of a central pith enclosed,in effect, by a series of cones, each conerepresenting the annual growth increment orannual ring. The wood tissue around the pithis the heartwood and consists of dead cells.The heartwood is surrounded by the livingcells and the sapwood or xylem which iscovered by a thin layer of phloem cells and theprotective bark. The interface between thexylem and phloem cells is known as thecambium, the term used by wood technologistsfor the actively dividing cells which aredescribed by botanists as the lateral meristem.These dividing cambium cells are known asfusiform initials, the cells splitting off on theinner side of the cambium forming becomingxylem or wood tissue and those on the outerside forming phloem or bark tissue. Inconiferous trees the xylem cells are termedtracheids whilst in dicotyledons or broad-leaftrees they are termed fibres. The cambium alsopossess additional active cells known as rayinitials which generate horizontal radial bandsof cells known as rays or parenchyma tissuewithin the xylem.

This description is not very complex, yet itexplains the development of wood within thetrunk of a tree. Xylem can be formed only whenthe entire tree structure is active. The xylemdeposits thus tend to be seasonal, particularly intemperate areas, so that a trunk will increaseeach year by the addition of an outer cone oftissue representing the growth for a season. Thisgrowth varies from a wide band of large butthin-walled cells known as the early or springgrowth, to much smaller thick-walled cells whichrepresent the summer or autumn growth. Thislate growth terminates sharply as cell formationdiscontinues with the onset of winter and thissharp terminal line is then followed by the large

cells of the early growth for the followingseason.

These details explain the basic structure ofwood but they have additional significance. Thexylem or sapwood is the tissue that conductswater and dissolved salts from the roots to thecrown of the tree, whilst the phloem is the tissuethat conducts sugars from the crown to thevarious growing cells throughout the structure ofthe tree. When a xylem cell, a tracheid or fibre, isfirst formed it consists of a thin wall of sugarswhich have polymerized into cellulose. Thesugars from the phloem continue to be suppliedto xylem cells, the rays perhaps providing routesfor this transfer, so that successive secondarylayers of cellulose are formed within the originalprimary wall. Once the cell structure is complete,a relatively rapid process occurring within theoriginal growth season, the much slower processof lignification commences. This consists of theprogressive deposition of lignin, initially withinthe middle lamella, an amorphousundifferentiated region between the cells, butthen within the cellulose cell walls. Thislignification serves to stiffen and strengthen thecells, occurring progressively whilst the cellsremain alive. The sapwood or living xylem cellsconsist of a reasonably constant number of ringsor depth of xylem, presumably controlled by theavailability of food and oxygen to maintain theliving cell processes, so that further annualgrowth at the cambium is accompanied at theinner surface of the sapwood by the death ofcells and their conversion into heartwood. Theonly significant difference between sapwood andheartwood is the large amount of material that isdeposited in the latter, apparently waste arisingfrom the living processes of the tree. Thesedeposits in the heartwood cells reduce theirporosity and are often significantly toxic, so thatheartwood is usually more resistant to insect andfungal attack than sapwood. These deposits alsotend to make heartwood more stable, so that it ismuch more resistant to swelling and shrinkagewith changes in moisture content.

Preservation technology

16

Softwoods and hardwoodsThere are fundamental differences in the natureof the wood of the conifers or softwoods andthe dicotyledons or hardwoods. In softwoodsthe principal longitudinal cells are known astracheids and serve both conducting andmechanical support functions. In transversesection a piece of wood appears as ahoneycomb, with the annual rings arisingthrough the change in density of appearancebetween the early wood with its large thin-walled cells and the late wood with its smallthick-walled cell. The transverse section mayalso show occasional vertical resin canals whilstthe radial and tangential sections may showhorizontal features such as rays. In contrast thehardwoods possess fibres, similar to softwoodtracheids, to provide structural support but theconducting cells are termed vessels or pores.These vessels are distributed singularly or inclusters throughout the wood, or in radially ortangentially orientated groups. For example,tangential distribution results in the ringporouswoods, a term that results from the observationthat a distinct ring can be seen with the nakedeye on a transverse section of species of thistype such as oak, ash or elm. Whilst thetransverse and radial sections of bothhardwoods and softwoods show annual ringswhich may appear to be superficially similar,they actually result from rather differentvariations in the structure.

In order to examine these microscopicfeatures of a piece of wood it is necessaryeither to macerate the sample or to preparethin sections. For maceration the sample isfirst treated with chromic acid in order todissolve the middle lamella and thus releasethe individual components. Thin sections areprepared by soaking the wood until it is softand then cutting the sections with amicrotome. In both cases stains can be usedwhich have an affinity for various individualcomponents so that they are more readily

visible under the microscope. Maceration hasthe advantage that it is possible to examineindividual components in their entirety, but thedisadvantage that their relative positions arecompletely unknown. Thin sections give anindication of the relative positions but it isnecessary to prepare a large number of serialsections in order to encounter individualcomponents and thus construct a three-dimensional picture of the entire wood.

This description is not intended to be acomprehensive account of the microscopicfeatures of wood but simply a contribution tothe understanding of wood structure. As anexample Scots pine, Pinus sylvestris, has beenselected to represent softwoods with Europeanoak, Quercus robur, to represent hardwoods(Fig. 1.3).

The maceration of Scots pine gives a largenumber of thin needlelike units about 0.8 mm(1/32 in) long. These principal longitudinalstructural elements are hollow, four-sided andpointed at each end. Pits are scattered alongopposite sides of the tracheids; the bordered pitsare circular whilst the simple pits arerectangular. It is also possible to distinguishparenchyma cells in the macerated sample, eachoblong, box-shaped and about 0.1 mm (1/200in) long. Examination of sections shows thatScots pine is composed predominantly oftracheids which were laid down in regular rowsas they were formed by the cambium at the outerlimit of the sapwood. The tracheids are fittedend-to-end with an overlap to give both strengthand continuity through the pits in thelongitudinal conduction of fluids. The springwood tracheids are large in cross-section andthin-walled compared with the summer woodtracheids which are distinguishable by their verythick walls. The only other longitudinal featuresare vertical resin canals which appear as spotsin cross-section and as fine white hair-lines inthe radial and tangential sections. These resincanals are narrow tunnels lined with smallrectangular cells. The other principal features are

Wood structure

17

the medullary rays running as horizontal ribbonsin the radial direction; with luck they might bevisible in a radial section but the ends of the rayswill certainly appear in a tangential section. Indepth they consist of three to ten small oblongcells with their length in the horizontal direction.The top and bottom rows consist of ray tracheidswith walls of irregular thickness whilst the middlerows are parenchyma cells which are connected tothe vertical tracheids via the simple pits. The raymay also incorporate horizontal resin canals.

The structure of a hardwood such asEuropean oak is entirely different. In mosthardwoods the vessels are the dominant featuresand in cross-section they appear in the oak aslarge pores. These vessels run for a distance ofseveral metres vertically within the tree andconsist of the many squat tubular cells that canbe seen in a macerated preparation. These cellspossess thin walls so that increasing porositynecessarily leads to decreasing strength in ahardwood. The tubular cells also possessnumerous pits connecting with the adjacent

tracheids and fibres. The tracheids are rather likethose in softwoods but less regular in form.Fibres occur in clumps and are responsible forthe principal longitudinal strength of the wood.Each fibre is spindle-shaped, long, thin andtough with a thick wall and only a small cavity.The fibres are interlocked or cemented to eachother to give a hard tough wood. In additionthere are small vertical rays and very largehorizontal medullary rays, usually composedentirely of regular-sized parenchyma cellswithout the ray tracheids or resin canalsassociated with medullary rays in softwoods. Inoak there are two sizes of medullary ray, onebroad and the other narrow and barely visible tothe naked eye. These medullary rays are a sourceof weakness as the vertical tracheids and fibresare deflected around them so that shrinkage inoak and other hardwoods is often associatedwith splitting through the medullary rays. Insome hardwoods the medullary rays give aregular pattern, the ripple marks or silver figurefrequently seen in quarter-sawn oak.

One of the features of the hardwood vessels is

FIGURE 1.3 Reconstruction of 3 mm cubes of wood, (a) A softwood, Scots pine Pinus sylvestris.(b) A hardwood, European oak Quercus robur.

Preservation technology

18

the formation of tyloses as the sapwood isconverted into heartwood. Living cells bulgethrough the pits to give the appearance initiallyof small balloons in the vessel cavities. Thesegrow until eventually the vessel is completelyblocked. Tyloses occur only in certain species;white oak possesses tyloses which block thevessels so that it is particularly suitable for barrelmaking, whereas red oak has no tyloses andsmoke can actually be blown through the vessels.

Annual ringsThe annual rings represent the amount of woodformed each year but the structure in softwood isentirely different from that in hardwoods. In thesoftwoods a wide ring of thin-walled spring cellsis formed, followed as the season progresses by anarrower ring of thick-walled summer or late-wood cells. In hardwoods the spring wood isformed with very large vessels but as the seasonprogresses the vessels become smaller with anincreasing density of fibres and a smaller numberof tracheids. In fast-grown softwoods the woodis generally of low density and inferior quality,whereas in fast-grown hardwoods the woodtends to have a high density and superior quality.This is, of course, only a generalization; veryslow-grown softwoods have excellent work-ability but inferior strength due to theircomparatively short tracheids, whilst very fast-grown hardwoods tend to lack the durabilityassociated with slower grown wood.

Wood structureThese are the various microscopic features whichare visible under a normal light microscope butit is also necessary to consider the sub-microscopic features which can only be seenwith an elec-tron microscope, as well as theultimate chemical structure, in order tounderstand some of the features of wood decayand the explanations for the action of the moresophisticated wood preservative. The

microscopic fibres or tracheids consist oforientated microfibrils and it has been deducedthat these in turn consist of bundles of cellulosechains. Crystalline and amorphous cellulosestructures have been identified, as well as relatedcarbohydrates such as hemicellulose and starch,all these components being assembled fromsugar molecules. All the characteristic features ofwood are found to be derived from these sugar-based structures and it is therefore hardlysurprising that the entire purpose of the tree is tosupport and supply the leaves where sugars aregenerated through photosynthesis.

The trunk or stem of the tree is, of course, thewood of commerce. The initial twig isrepresented by the central pith which issurrounded by the heartwood consisting of cellswhich are so far removed from the bark thatthey have died and become filled with variousextractives. Around the heartwood is thesapwood of living cells, the inner zoneconducting water upwards to the leaves and theouter or phloem conveying sugars from theleaves to the living cells throughout the tree,providing them with energy to sustain life andsugar components to form cellulose,hemicellulose and starch. The outer sapwoodcells immediately beneath the phloem are knownas the cambium and are distinctive as they candivide to form new cells. The cambium consistsessentially of two types of dividing cell, thefusiform initials and the ray initials. Thefusiform initials adjacent to the sapwood giverise to xylem or wood cells which ultimatelybecome the tracheids of conifers or softwoodsand the fibres of dicotyledons or hardwoods.The outer fusiform initials give rise to the bark.The ray initials also produce xylem but in theform of parenchyma or ray cells. In this way thewood is formed so that the vertical tracheids andfibres give longitudinal strength, as well asvertical transport routes within the tracheids inconifers and within the pores surrounded byfibres in dicotyledons. In contrast, the ray cellsare orientated along horizontal radial paths in

Wood structure

19

order to provide conducting tissue between thephloem and the cells deep within the sapwoodand heartwood, apparently conductingextractives towards the heartwood and sugars tothe living cells in the sapwood, and also oftenproviding tissue for the storage of starch,particularly in deciduous hardwoods which shedtheir leaves in winter and thus require a sourceof stored energy to enable them to survive.

The strength properties of wood can beattributed to the principal components,particularly the basic longitudinal cellularelements which are the tracheids in softwoodsand the fibres in hardwoods. When a fusiforminitial divides the resultant new xylem cell issurrounded by an amorphous undifferentiatedsubstance which subsequently becomes themiddle lamella. The cell rapidly achieves itsultimate length and rectangular cross-section,squeezing the middle lamella to form a thin layerbetween adjacent rectangular cells. The initialor primary cell wall (P layer, Fig. 1.4) consistsof loose and irregularly orientated microfibrils,an important feature as this thin P layer mustbe capable of extension as the cell grows to itsultimate dimensions. The microfibrils areorientated in a predominantly shallow spiral,perhaps at about 60 to the vertical axis of thecell. Once the P layer has achieved its ultimatedimensions the secondary wall (S layer) iscommenced and is formed typically in threeseparate stages. The first secondary wall (S1layer) is thin with a predominantly shallow

microfibril spiral, perhaps at about 50 to thelongitudinal axis of the cell. The S1 layer sometimesconsists of two or more lamellae spiralling inopposite directions and is morphologically andstructurally intermediate between the P and S2layers. The second secondary wall (S2 layer)consists of very regular and closely packedmicrofibrils at a very steep spiral angle, perhapsonly 1020 relative to the longitudinal axis, and itis also it is also very thick and the dominant cellwall. Finally a third secondary wall (S3 layer) issometimes formed consisting of a thin shalloworientated layer of microfibrils, perhaps at about50 to the longitudinal axis. In all cases thesecondary walls are more regularly orientated thanthe primary wall.

These cell walls account for the cellulosewhich comprises about 60% of the woodsubstance. The S2 layer is always dominant andit is therefore hardly surprising to find that thebasic longitudinal orientation of the microfibrils,and thus the cellulose chains, within this layeraccount for many of the basic physical propertiesof wood. This S2 layer is also rather moremassive in late wood than in early wood, againaccounting for the different properties betweenthese zones of the annual rings.

The rectangular fibres and tracheids that areso readily observed when a cross-section of apiece of wood is examined under the microscopeare comparatively large but their componentmicrofibrils are quite small. It is difficult toimagine microfibrils without a knowledge of theAngstrom (), the unit of dimension that mustbe used at this scale. By definition an Angstromis 110-8 cm, or 0.0000001 mm. Once thedefinition of the Angstrom is appreciated it canbe said that a microfibril consists of elementaryfibrils having a diameter of about 35 , so thatmicrofibrils have typical diameters of 35, 70,105, 140, etc. , although some microfibrils areflattened with dimensions such as 10050 . Ifthese measurements are now converted tosomething familiar, such as a fraction of amillimetre, it will be appreciated that theFIGURE 1.4 Cell wall layers in a softwood tracheid.

Preservation technology

20

elementary fibrils are very small, yet eachconsists of a bunch of about 40 cellulose chains,and thus the basic structure of the wood cell hasbeen reduced to its ultimate chemicalcomponents.

It is usually considered that the cellulosechains consist of between 200 and 2000 glucoseunits, although it is sometimes reported that asmany as 15 000 units may be involved. As eachglucose unit has a length of about 5 the chainsare relatively long, perhaps 10 000 (0.001mm) or perhaps far longer. As wood is a naturalmaterial some discontinuity of the structureoccurs but the chains lie parallel for considerablelengths, perhaps over 120 units (600 ) or more,and it is these essentially parallel cellulose chainsin the dominant S2 layer that account for theprincipal properties of the wood.

Figure 1.5 illustrates the way in which glucoseunits are assembled to form cellulose and thusthe principal structural elements of wood. Thisstructure has considerable importance indetermining the properties of wood. Firstly thesugar units are formed from water and carbondioxide within the leaves by the process knownas photosynthesis:

6H2O + 6CO2 C6H12O6 + 6O2water cabon dioxide glucose oxygen

The water is obtained from the surrounding soilby the roots and is conveyed up the tree throughthe living xylem cells in the inner sapwood to theleaves. Carbon dioxide is then absorbed from theatmosphere by the leaves, glucose is formed byphotosynthesis and conveyed down the tree inthe phloem between the xylem and the bark.

Production of glucose by photosynthesis canoccur only in the presence of the catalystchlorophyll, the green pigment in leaves, and isdependent upon the absorption by the leaves oflight energy, particularly ultraviolet light.

Energy in woodThe glucose product thus has a far higherenergy level than the water and carbon dioxideconstituents, and this energy can be released ina variety of ways. For example, animals eatglucose or other sugars, converting them backto the original water and carbon dioxide bythe addition of oxygen and releasing energy inthe process. This energy is used formaintaining life processes or for generatingheat, and the burning of sugars is perhaps themost obvious illus-tration of the way in whichoxygen can be combined with glucose toreverse photosynthesis and release energy. Inthe formation of cellulose chains a smallproportion of the energy is lost but aconsiderable amount remains, and thisexplains why wood burns and why it isattractive to some insects and fungi as a sourceof nourishment.

The glucose produced by a tree is largely usedfor formation of structural cellulose but theenormous mass of living cells in the roots,sapwood and leaves all requires energy tomaintain life, this being obtained from theglucose and associated sugars such as xylosewhich are also produced by the leaves. Duringdarkness or the winter months the leaves are notproducing sugars, yet the cells still requireenergy and obtain this from glucose accumulated

FIGURE 1.5 Glucose and the formation of cellulose.

Wood structure

21

within them or, for longer periods such as thewinter, from deposits of starch which is formedfrom glucose and fairly readily reconverted whenrequired. Some attacking insects are unable toutilize cellulose as a source of nourishment butthey will attack wood just for the starch orsimple sugar content; even mammals such asrabbits and squirrels will strip the bark fromtrees to gain access to the sugary sap in thephloem.

Water and woodIt has already been explained that the cellulosechains are assembled into microfibrils whichare orientated in a predominantly longitudinaldirection within the cell walls of softwoodtracheids and hardwood fibres. The physicalproperties of wood largely result from thislongitudinal orientation of the cellulosechains, and this is particularly the case in therelationship between wood and water. Eachglucose unit in a cellulose chain possesses threehydroxyl groups which have an affinity forwater. This ensures firstly that cellulose chainswill wet easily but in addition water will beheld between the chains, pushing them apartas illustrated in Fig. 1.6. As the chains becomeseparated by water the bond between thembecomes weaker, so that a high moisturecontent in wood is associated with loss ofstrength, particularly a loss of sheer strengthand rigidity, so that a beam is more flexibleand wood will cleave more readily when wet.If this separation of the cellulose chains werepermitted to continue indefinitely the wood

would eventually break down into individualcellulose chains and thus disintegrate, butmovement with moisture change can be largelyattributed to the predominantly longitudinalorientation of the microfibrils and thecellulose chains in the S2 layer which accountsfor most of the mass of the cell wall, butseparation of these chains is limited by thechains in the P and S1 layers that are wrappedaround them.

As orientation of the microfibrils in the S2layer steepens from the pith to the bark of thetree the cross-sectional movement with changeof moisture content increases approximately inproportion to the cosine of the an