the rapid preparation of structural images from undisturbed, non-cohesive material (milled peat)

Ž .Geoderma 86 1998 159–168

The rapid preparation of structural images fromž /undisturbed, non-cohesive material milled peat

S.J. Mooney a,), N.M. Holden a, S.M. Ward a, J.F. Collins b

a ( )Agricultural and Food Engineering Department Peat Technology Centre , UniÕersity CollegeDublin, Earlsfort Terrace, Dublin 2, Ireland

b Faculty of Agriculture, UniÕersity College Dublin, Belfield, Dublin 4, Ireland

Received 22 September 1997; accepted 20 April 1998

Abstract

ŽA new method for the acquisition of undisturbed in situ samples of milled peat non-cohesive.particulate material suitable for image analysis was developed. Samples were obtained in the field

Ž .by dripping Endura epoxy resin onto a stockpile surface. Blocks of material 160 mm=120 mmwith original pore structure up to 140 mm deep were removed to the laboratory and re-impreg-nated with Crystic resin from which polished blocks and thin sections were produced. All resinswere mixed with fluorescent dye so that when viewed under ultraviolet light, a resolution of ca. 20mm was possible from only partially polished faces. Careful observation in the field indicated that

Ž .few if any particles were not bound by the initial impregnation. When prepared as thin sectionsŽ .without a dehydration period , the two impregnants could be distinguished within the pore spaceunder crossed polarised light. It can be concluded that the method permits observation of

Žundisturbed macro, meso and some micro pores )75 mm, 75–30 mm and 30–5 mm diameter,wrespectively Soil Science Society of America, 1997. Glossary of Soil Science Terms. Soil Science

x.Society of America, WI, USA, 134 pp. . q 1998 Elsevier Science B.V. All rights reserved.

Keywords: peat; image analysis; micromorphology; soil structure; porosity

1. Introduction

The economic value of milled peat as an energy source in Ireland is to a largeextent determined by the water content at point of sale. Milled peat from Irish

) Corresponding author. Tel.: q353-1-7067318; Fax: q353-1-7067481; E-mail:[email protected]

0016-7061r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 98 00047-0

( )S.J. Mooney et al.rGeoderma 86 1998 159–168160

production bogs is created by scarifying the drained bog surface to produceŽ .particulate peat Ward, 1986 that has a particle size ranging from 5 mm to 50

mm. The particulate layer is dried atmospherically for up to 96 h and thenmoved to stockpiles to maintain a buffer against consumption demand. About

Ž .85% of production uses the PECO system Bord na Mona, 1984 which stores´Ž .peat in a series of stockpiles ca. 4 m high, 12 m wide and up to 1000 m long

distributed fairly evenly over a large bog area. An alternative system, theHAKU, stores the milled peat in large stockpiles at specific points on the bog

Ž .area as opposed to spread across the area . The storage period can sometimesexceed 18 months, therefore milled peat is subjected to many periods of wettingunless covered.

The structure of a porous medium is the most important property affectingŽwater movement. Laboratory and field evidence Holden, 1998; Holden and

.Ward, 1997a indicate that lines of preferential water movement form in astockpile resulting in rapid channelling of water from the surface into the centreof the pile. A macroscale description of the physical properties of milled peatthat is not structured as it is on the bog would probably yield little informationabout why flow paths occur where they do. However, a morphological investiga-tion which preserves pore architecturergeometry could be of use in this regard.Examples in the literature where direct structure quantification has been used tosupport the physical analysis of soils, particularly with reference to studying

Ž .soilrwater interactions, include the works of Bouma et al. 1977 and VepraskasŽ .et al. 1991 . A detailed understanding of pore and particle size distributions on

macro, meso and micro scales can be used to support observations based on themeasurement of physical properties. The use of polished blocks and thinsections allows the application of established micromorphological techniques forcharacterising pore structures, potential water movement and water storage. Inorder to obtain representative images of milled peat structure it was necessary todevise a method of taking undisturbed samples from the field.

Methods for the morphological examination and description of pore structuresŽhave been established over many years Brewer, 1964; FitzPatrick, 1984;

.Bullock et al., 1985 . Most methods documenting the quantification of soilstructure require the soil to be free of water but to retain its original character.

Ž .Methods such as freeze drying Blevins et al., 1968 and acetone replacementŽ .Miedema et al., 1974; FitzPatrick and Gudmundsson, 1978 can create sampledisruption and increase sample preparation time by several weeks. The idealpreparation for milled peat samples would involve no interference, particularlybecause milled peat is non-cohesive and any attempt to transport samples to thelaboratory is likely to cause significant disruption to the particle arrangementwithin the sample. One solution to this problem is to impregnate the samples in

Ž .the field at field water content such that all particles are held firmly in placeprior to transportation. Further impregnation can then be undertaken in thelaboratory to produce high quality images of pore structure, using impregnated

( )S.J. Mooney et al.rGeoderma 86 1998 159–168 161

blocks which can then be prepared for microscopic investigation as thinsections. Previously, impregnations to preserve structure have been done using

Ž . Ž .Carbowax Holden, 1994 , epoxy resin Moran et al., 1989 , paraffin waxŽ . Ž .Dexter, 1976 and plaster of Paris FitzPatrick et al., 1985 . Following an

Ž .impregnant suitability study Mooney et al., 1997 , Endura epoxy resin wasselected as most appropriate for milled peat. The objective of this research wasto devise a method for the sampling of undisturbed, non-cohesive particulatematerial, which could be used with other micromorphological techniques.

2. Materials and methods

Peat samples were collected in situ from stockpiles on two different bog typesŽ X X.located at Noggusboy Irish grid reference: N1122; Lat. 53815 , Long. 785 and

Ž X X.West Boora Irish grid reference: N1520; Lat. 53812 , Long. 7848 , CountyOffaly, Ireland. The milled peat types were medium-low density and high

Ž .density, respectively Holden and Ward, 1997b .ŽAt a given sampling location on the stockpile position along and height from

.the base , a representative area was selected for sample isolation. A rectangularŽ .template 160 mm=120 mm was used to mark out the area of impregnation to

ensure a uniform sample size. The sample size chosen was the largest possiblefor use with the available laboratory equipment. In the authors’ opinion thesesamples are large enough to obtain a useful estimate of the population pore sizedistribution and structure and are larger than standard Kubiena micromorpholog-

Ž .ical thin sections 70 mm=50 mm . Approximately 200 ml of Endura epoxyŽ .resin Mooney et al., 1997 was gradually introduced by dripping the resin from

a glass rod evenly over the area so that it gently infiltrated into the milled peatwithout causing any disturbance. The resin mixture was two parts Endura epoxyto one part hardener component and about 0.5 g of Uvitex OB fluorescent dyeŽ .Ciba-Geigy . As soon as all the resin had been introduced onto the entiresample area, the site was covered to prevent possible interference by precipita-tion and left overnight to harden. The following day the sample was extracted bypulling it gently from the stockpile. It was then labelled, carefully packed andtransported to the laboratory. Sampling was restricted to periods without rainfallon the day of impregnation because Endura Epoxy can suffer restricted curingwhen there is free water in its immediate vicinity, although successful impregna-

Žtions were achieved with sample water contents of up to 55% wet weight.basis . Depth of resin percolation was typically between 60 and 140 mm.

In the laboratory the samples were sliced once in the vertical plane by an oillubricated, continuous rim diamond saw giving two roughly equal blocks of 120mm=80 mm= impregnation depth which were a more suitable size for thinsection preparation. The blocks were randomly assigned for vertical sectioning

Ž .or sectioning aligned parallel to the stockpile surface referred to as horizontal .


The blocks were placed in aluminium foil moulds for a second impregnationunder vacuum for 3 h to fill any remaining voids. The impregnant used at this

Ž . Žstage was a fast curing 48–72 h mixture of Crystic resin Crystic resin 200 ml,.Catalyst 6 ml, Accelerator 0.8 ml, Uvitex OB dye 0.5 g following a procedure

Žwhich is well documented for use in soil micromorphology FitzPatrick, 1984;.Murphy, 1986 . The solid blocks produced were then removed from the moulds

and cut in the designated plane at approximately 15 mm intervals typicallyyielding 3–4 cuts per block. A slow cutting rate was adopted to minimisedamage to the cut surface. The selected face of the individual block was thenmarked by pencil to identify the surface prior to a third impregnation. The same

ŽCrystic resin mixture was applied by paintbrush in very small quantities -5.ml to fill any remaining voids in the block face. Each block was then placed in

a dessicator under vacuum for 5 min and examined using a stereo microscope toidentify any unfilled voids. The procedure was repeated as necessary until all thevoids had been filled. The blocks, once fully hardened, were then ground bymachine and lapped by hand using fixed grit Silicon Carbide abrasive papersŽ .grade 600, 800 and 1000 until a smooth surface had been produced.

The samples were placed on an adjustable stage below two high intensityŽ .longwave Ultra-Violet lamps 100 W Blak Ray fitted with visible light filters.

This procedure caused the ultraviolet dye in the resin to fluoresce and reveal theŽ .pore space in contrast to the solid material Ringrose-Voase, 1996 . Ensuring a

standard focal length, images were captured in a dark room using a solid stateŽ .video camera Panasonic BL200 with a 16 mm lens attached to a frame

grabber. Reflected UV light was filtered out using a UV filter attached to theŽ .camera lens. An image analysis package SEMPER was used to classify and

quantify soil and pore characteristics.

3. Results and discussion

The main concerns when evaluating the efficacy of the method presented arewhether it permitted the production of high quality images and if the samplesproduced were truly undisturbed. Blocks of milled peat after initial impregnationŽ .Fig. 1 were seen to have had much unfilled pore space; therefore it wasappropriate to assume a secondary impregnation would yield a good final result.It was accepted that some pore spaces were not accessible to the secondimpregnation due to blockage by the first impregnation. For this reason a final‘fine-tuning’ of the block face before lapping was undertaken. It should be notedthat the depth of lapping possible was restricted before the surface touch-up hadto be repeated. A field sampling regime that involved collecting 72 blocksyielded no failed samples at any stage during the procedure and resulted in over300 images of pore structure. From this it can be concluded that the method wasboth workable and efficient. The major potential limitation to the method was


Fig. 1. A partially impregnated milled peat sample as removed from the field.

the presence of free water in the pore space being impregnated. In the summerthis was not a problem in most milled peat stockpiles because they areinherently dry due to the production process. However, the method might not beso effective for winter sampling when a wet surface layer can exist. In the resin

Ž .trials Mooney et al., 1997 the Endura epoxy resin was found to function withwet milled peat, but only to a limited extent. Such conditions should perhaps beavoided.

The issue of sample disturbance is of significance because there is no purposein quantifying a structure that is an artefact of the sampling method. The most

Žlikely artefacts are the transport of fine particles in the resin viscositys500–800.mPa s at 258C , and the loss of ‘non-holding’ of particles when the sample is

extracted as a hardened block. Quantified evidence to refute the existence ofeach possibility is difficult to obtain. With respect to the movement of fineparticles, it would be expected that images with accumulations of fines inparticular areas would result. Surface samples could not be used to evaluate thispossibility because there is a distinct surface layering on mature stockpiles but

Žsamples taken from within the body of the stockpile show no such effect Fig..2 .

The non-holding of particles was a simpler issue to resolve. When extractingsamples from the stockpile after initial impregnation great care was taken toobserve whether particles fell away from the block. After wrapping and trans-


Fig. 2. A milled peat image captured from a polished peat block under UV light.

porting to the laboratory the samples were carefully examined for the presenceof significant loose particles within the wrapping material. Little if any was everfound. As a safeguard against surface loss or damage, the edges of the sampleswere discarded prior to image analysis; this was the area most likely to havepartial impregnation and which was the most susceptible to particle removal.Any suspect images were disregarded when carrying out quantified analysis.

The polished blocks produced by the sampling method were of high quality.Slight blemishes on the faces were not important as they were not visible underUV illumination. The quality of images was consistently very high with aminimum discernible pore size of -20 mm. This was significantly better thanhad been expected at the outset and is a marked improvement over wax

Ž .impregnants Dexter, 1976; Holden, 1994 . It can be concluded that the methodis highly suitable for polished block production.

Initially it was assumed that the sampling method would only provideŽ .samples suitable for macroscopic investigations the polished block stage .

However the quality of the impregnation enabled the production of good qualitythin sections with few production artefacts. The greatest concern when viewingthe thin sections was the optical properties of the epoxy resin. Crystic resin isroutinely used for thin sections because it does not have interference coloursunder plane polarised or crossed polarised light; however when viewed undercrossed polarised light Endura epoxy exhibits first order pale grey, white, andpale yellow interference colours. Fig. 3a and b are photomicrographs taken from


Ž . Ž .Fig. 3. Milled peat as viewed in thin section a in plane polarised light and b crossed polarisedlight to highlight the Endura and Crystic resin distributions.


a thin section of West Boora high density milled peat. Normal thin sectionthickness is 25 mm however the thin section in question is approximately 100mm thick since the contrast between resins is highlighted best at this thickness.Fig. 3a was taken in plane polarised light, where as Fig. 3b was taken in crossedpolarised light and has been over-illuminated and over-exposed to show the twovisible impregnants. From Fig. 3b, milled peat particles can be seen to be

Ž .encapsulated by a bright, white medium Endura epoxy set in a darkrlight greyŽ .background Crystic resin . For a material such as milled peat with little or no

mineral content, micromorphological description in plane polarised light andporosity analysis with UV illumination was not compromised.

The images that were obtained from polished blocks consisted of 750=550pixels each of which is attributed a grey level between 0 and 255 depending onbrightness. The images were processed by establishing a ‘threshold’ to yield

Ž .1-bit images with white pore space and black particles Fig. 4 . Total pore spaceŽ .)ca. 20 mm was obtained by expressing the number of white pixels as a

Žproportion of the total image pixels, and solid volume also including pore space.-ca. 20 mm could be obtained by a similar procedure with the black pixels.

More complex descriptions of porous structure are possible such as particle andpore area, perimeter, shape and other morphological parameters. The quantifica-tion of pore space and its distribution is of particular importance for highlyhydrophobic milled peat where the water characteristic is difficult to measure

Ž .Fig. 4. A thresholded milled peat image as Fig. 2 .


Table 1Porosity and solid space determined by image analysis and saturated volumetric water content

Ž .Black pixels Solid space by subtraction White pixels PorosityŽ . Ž . Ž . Ž .% % % %

NoggusboyStockpile Base 43 54 57 46Stockpile Top 55 66 45 34

West BooraStockpile Base 48 44 52 66Stockpile Top 69 71 31 29

using a sand-table, hence laboratory-derived porosity measurements were gener-Ž .ally less than those derived by image analysis Table 1 .

4. Conclusions

It can be concluded that a field impregnation method that allowed undisturbedsampling of non-cohesive particulate material was satisfactorily developed. Thetechnique permitted the acquisition of good quality digital images within 7 daysof sampling without requiring a dehydration period during sample preparation.Furthermore, after macroscale examinations, the same samples could be pro-cessed into thin sections enabling microscale descriptions and quantificationsŽ .-30 mm within another 7 days. There is no evidence to suggest that anysampling or production artefacts interfered with the quality of results. One useof such images might be to improve understanding of water movement in peatstockpiles.

Acknowledgements

S.J.M. wishes to acknowledge the financial assistance of Bord na Mona.´N.M.H. is the Bord na Mona Newman Scholar in Peat Technology.´

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the rapid preparation of structural images from undisturbed, non-cohesive material (milled peat)

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