analytical pyrolysis as diagnostic tool in the

Analytical Pyrolysis as Diagnostic Toolin the Investigation of Works of Art

G. Chiavari1* / S. Prati2

1 Chemistry Department G.Ciamician, University of Bologna, via Selmi 2, 48126 Bologna, Italy; E-Mail: [email protected] Laboratory of Chemistry, CIRSA, University of Bologna, via Marconi 48100 Ravenna, Italy

Key Words

Gas chromatography-mass spectrometryPyrolysisWork of artReview

Summary

This paper is a review of the activity of our research group in the last decade. A discussionabout the application of analytical pyrolysis for the examination of objects of art during thisperiod is presented.

Introduction

Cultural Heritage can be defined as

‘‘Every material evidence of civilisation’’

and for this reason its safeguard is a

‘‘must’’ for our society.

The commitment of scientists is cru-

cial for the protection, the restoration

and the exploitation of Cultural Heritage.

Scientists are asked to transfer to this

field technologies developed in different

areas and to develop new scientific tools

suitable for specific areas of Cultural

Heritage. Art conservation is a new dis-

cipline which utilizes modern analytical

techniques in the study and preservation

of works of art: in the last few years new

materials and technologies have been

developed which are continually being

evaluated by conservators for their

potential application to preservation and

restoration [1].

Several analytical techniques have

been developed for the characterisation

of samples in the artistic field [1–3].

Spectroscopic techniques are widely

applied due to the fact that they are

mainly conservative non-destructive

techniques and the sample can be exam-

ined with other complementary tech-

niques. Whenever possible non-

destructive methods are used because

they can be applied directly to the objects

under investigation. Moreover portable

instruments have been produced to avoid

sampling even for frescoes or other large

objects. These techniques are useful to

obtain preliminary information that of-

ten must be confirmed or clarified by

other analyses.

The examination of a work of art

generally begins with a visual inspection

by using special lighting to facilitate the

distinction of differences in colour, gloss

and texture. At this stage photographic

techniques can be used [1]. Optical tech-

niques are important for studying the

morphology of the samples and give

information about the state of conserva-

tion or the technique used in the pro-

duction. FTIR, Raman and X ray

techniques are particularly useful for the

identification of the inorganic compo-

nents (pigments, salts and composition of

mortars or stone). The first two tech-

niques can also give information about

the organic components, but generally

the samples are complex mixtures and

identification can be difficult. In the field

of organic characterisation the separation

techniques such as HPLC and GC (in

particular coupled with mass spectrome-

try) are mainly applied. They are

destructive techniques, but pre-treatment

procedures which use very small quan-

tities of material have been developed

[4–32].

HPLC is mainly used for the charac-

terisation of proteins, by quantification

of the amino acids [4–7] or for the anal-

ysis of organic colorants [8–14]. GC is

used for less polar compounds such as

natural binders and protectives [15–32].

However artistic samples are mixtures of

organic and inorganic substances which

have been chosen for their chemical sta-

bility. Some artistic materials decrease

in solubility and volatility with ageing

DOI: 10.1365/s10337-003-0094-7

2003, 58, 543–554

0009-5893/03/11 $03.00/0 � 2003 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH

Review Chromatographia 2003, 58, November (No. 9/10) 543

and the pre-treatments can be difficult

to apply.

Analytical pyrolysis can be useful for

complex, high molecular weight and polar

compounds which can hardly be analysed

by traditional gas chromatography. It

consists of a thermal pre-treatment of the

sample in an inert atmosphere. The com-

pounds are fragmented by the high tem-

perature (600–800 �C) to produce more

volatile molecules. The pyrogram displays

the decomposition fragments and is a

fingerprint of the original matrix, because

the fragmentation pathway under the

same conditions (pyrolysis temperature,

heating rate, time of pyrolysis, interface

temperature) is reproducible [33].

Pyrolysis has been used since the end

of XIXth century, but only reached its

full potential with the introduction of gas

chromatography-mass spectrometry.

Pyrolysis has been used to characterise

artistic materials for thirty years [34–67].

Several studies have been performed to

build up a data base for the detection of

the organic compounds used in the

artistic field, for the evaluation of the

degradation status and for authentication

purposes [34–36].

Pyrolysis coupled to GC-MS allows

the characterisation of materials such as

waxes, resins, oils, proteinaceous binders,

amber, synthetic polymers and organic

colorants. The characterisation of the

organic composition of a paint layer can

give information about the best restora-

tion operations to be employed or about

the authentication of the work under

examination. For instance the presence of

a synthetic resin in some paintings

attributed to Pieter De Hooghs and to

Vermeer, indicated that the paintings

were fakes [37–38].

Pyrolysis is particularly useful in the

artistic field because even if it is a

destructive technique, it needs only a

small quantity of sample (<1 mg). Three

different types of pyrolyzers are com-

mercially available: filament, Curie point

and furnace pyrolyzers.

Filament and Curie point units are

used mainly for flash pyrolysis with the

sample kept at high temperature for a few

seconds.

In the first type, pyrolysis is performed

on a platinum resistance coil which can

reach temperatures of 600–800 �C in a

few microseconds. The sample is intro-

duced in a quarz capillary which fits in

the platinum coil. With this system

pyrolysis temperature can be set at any

desired temperature in a large range.

However as the resistance may modify in

time, the correspondence between the set

operating temperature and the actual

temperature will change during the life of

the filament. Moreover the filament may

be not uniformly heated over its length

and this aspect can lead to reproducibility

problems.

In the Curie point apparatus the

thermal treatment is produced by a fer-

romagnetic conductor which can be rap-

idly heated by interaction with a high

radio frequency. The increase in temper-

ature is limited to the Curie point tem-

perature, a temperature specific for each

material where the transition from ferro-

magnetic to paramagnetic properties oc-

curs. The end temperature is, for this

reason, well defined and it depends on the

composition of the ferromagnetic metal

or alloy. The sample can be placed in

close contact with the conductor that can

be shaped in different forms such as a

wire, ribbon, cylinder to properly hold

the sample. Commonly the sample is

holded in a ribbon shaped ferromagnetic

alloy which is subsequently inserted into

the middle of the RF coil. With the Curie

point pyrolyzers the contact between the

sample and the heated alloy assures that

the heat transfer to the sample is rapid

and uniform and the reproducibility of

the pyrolysis temperature is more accu-

rate, but it is not possible to change this

temperature without changing the com-

position of the wire [68].

In the furnace pyrolyzer the sample

is inserted in the pyrolysis chamber via a

steel tube. The unit is kept at the pyrolysis

temperature and the degradation process

generally lasts a few minutes. However

flash pyrolysis can be performed at the

same way. In this system the condensa-

tion processes are reduced as well as the

memory effects. In some systems a cryo-

genic trap is used to focus the decompo-

sition fragments. After the focussing

phase, the trap is rapidly heated to

desorbe the products into the GC system.

The major problem of the system is

the poor contact between the sample and

the hot source: the sample, in fact, may

reach a lower temperature than that of

the furnace wall. Moreover generally the

furnaces can reach lower temperature

respect to the other apparatus [68].

The gas chromatographic behaviour

of the thermal decomposition fragments

depends on their volatility and polarity.

To obtain better chromatographic

behaviour, derivatisation techniques have

been introduced. The derivatisation

reaction is achieved together with pyro-

lysis by adding a reagent to the solid

sample.

The procedure introduced by Chall-

inor [69], the so-called thermally assisted

hydrolysis and methylation (THM),

consisting of pyrolysis in combination

with the methylating reagent TMAH

(tetramethylammonium hydroxide), has

shown to be advantageous for the anal-

ysis of organic binders and protectives.

However, in the strong alkaline environ-

ment of TMAH, side reactions, such as

isomerisation of double bonds of unsat-

urated fatty acids [70] and methylation at

the carbon atom in the 2-position [71],

may occur at high temperature and the

use of milder methylating reagents has

been proposed.

Trimethylsilyl derivatisation is em-

ployed as an alternative to methylation

for converting compounds containing

active hydrogens, such as carboxylic acids

and alcohols, into less polar derivatives in

order to improve GC analysis. For this

reason we have proposed hexamethyldi-

silazane as a silylating reagent in pyroly-

sis analyses. The method is fast as it

requires no sample preparation apart

from the application of the silylating

solution and operates under milder con-

ditions. This condition is an important

feature which distinguishes pyrolysis –

silylation from THM with methylating

reagents. Figure 1 reports the derivatisa-

tion reactions.

In this paper a review of the experi-

ence in the application of analytical

pyrolysis to the diagnostic of works of art

achieved by our research group in the last

decade is presented.

Experimental

Pyrolysis were carried out at 600–700 �Cfor 10s using a Chemical Data System

(CDS) 1000 pyroprobe heated filament

pyrolyser (Oxford, USA) directly con-

nected to the injection port of a Varian

3400 gas chromatograph coupled to a

Saturn II ion trap spectrometer (Varian

Analytical Instruments, Walnut Creek,

USA). A Supelco SPB5 capillary column

(30 m, 0,25 mm I.D., 0,25 lm film

thickness) was used.

Samples (0,1-1 mg depending on the

type of the sample and in particular of

the organic material percentage) were

544 Chromatographia 2003, 58, November (No. 9/10) Review

inserted into a pre-pyrolysed quartz

capillary tube. To achieve derivatisation

reaction 3–5 lL of the reactant are added

to the sample in the quartz capillary

before pyrolysis. Tetramethylammonium

hydroxide 25% in water, hexamethyldi-

silazane 99% were used for methylation

and silylation respectively.

Egg

Egg is composed of yolk, an emulsion

between a colloidal solution of phos-

phorilated proteins together with lipids,

and glair, an aqueous colloidal solution

of proteins, mainly albumins, with low

quantities of fats. Albumins are globular

proteins readily soluble in water [72–73].

The lipidic fraction of egg is composed of

triglycerides, cholesterol and phospho-

lipids. Phospholipids are triglycerides

in which a phosphoric group esterifies

one hydroxy group of glycerol. When

the phosphoric acid is esterified with

choline the resulting group of com-

pounds are called lecithins. The phos-

phorous content represents 0,9% of the

dried whole egg.

Egg can be detected through the

presence of lipidic and proteic markers.

Under normal pyrolysis conditions a

pattern of fatty acids similar to that ob-

tained from drying oils is detected.

Moreover, the presence of hexadecanit-

rile can often be noted as a possible

marker of the simultaneous presence of

lipidic and proteic fractions and indole

and methyl indole as degradation frag-

ments of triptophane [74–76]. These

markers are of low intensity with respect

to fatty acids and for this reason it is hard

to distinguish egg from a drying oil. A

pattern of cholesterol derivatives has

been detected in some samples. In other

samples they are completely absent,

indicating their low stability with ageing

and for this reason they cannot be con-

sidered as markers.

Under methylating conditions the

main peaks are the methyl esters palmitic

and stearic acids, while azelaic acid

dimethyl ester is in low concentration

[75–78].

Recent papers report results obtained

by pyrolysis-silylation [79–80]. Besides

the fatty acid trimethylsilyl esters [81] the

tris trimethylsilyl ester of phosphoric acid

has been detected [80]. This compound

derives from lecithin, but it has not been

detected for instance from casein, which

Figure 1. Supposed mechanism reaction of a) methylation with TMAH, b) silylation with HMDS.

Figure 2. Reconstructed ion chromatogram obtained from a) pyrolysis of a egg painting layer b)pyrolysis-methylation of a egg painting layer c) pyrolysis-silylation of a egg painting layer. DKPS isthe abbrevation of diketopiperazines.


is a phospho-protein. Under methylating

conditions the corresponding methylated

compound has not been detected owing

to its low molecular weight. A study is

now being carried out on the effect of the

influence of inorganic pigments, which

seem to strongly influence the detection

of this compound. For this reason at the

moment it cannot be considered as a

marker.

In this study we have given attention

to the proteic component and in par-

ticular the distribution of diketopiper-

azine, characteristic of glue (see below).

It has been stated that even in egg there

are some diketopiperazine compounds,

but with a different distribution, than

in glue pyrograms, and of very low

intensity.

In Figure 2 the pyrograms of an egg

painting layer under the three pyrolytical

conditions are shown, while Figure 3

shows the mass spectra of palmitic acid

and of the corresponding methyl and

trimethylsilyl esters.

Drying Oils

Drying or siccative oils are mainly

mixture of triglycerides. Linseed, walnut

and poppy oils are the mostly used in

painting.

Normal pyrolysis conditions lead to

the formation of fatty acids, mainly

palmitic acid, and stearic acid, but also

a sequence of shorter chain acids. The

thermal treatment also produces a se-

quence of alkenes coming from decar-

boxylation of the fatty acids. The

pyrograms are characterised by fatty

acid tailing peaks with a distribution

similar to that obtained from the lipidic

fraction of egg. It is known [73] that

during the siccative process dicarboxilic

acids, in particular the C9 azelaic acid,

are formed by the reaction of oxygen

with the polyunsaturated chains of tri-

glycerides (in particular linoleic and li-

nolenic acid). Azelaic acid allows to

distinguish a drying oil from the egg

lipidic fraction but; under normal

pyrolysis conditions, owing to its strong

polarity, it is difficult to detect.

Under methylating conditions the

methyl esters of palmitic, stearic, oleic

(C18: 1: 6) and the dimethyl esters of

azelaic and suberic (C8) acids are the

main products. The sequence of the low

molecular weights fatty acids methyl

esters is also present [75, 78].

In silylating conditions the same

pattern of peaks is obtained [79]. At the

moment we are studying the effect of

the inorganic pigments on the detection

of carboxylic acids. Figure 4 gives the

pattern obtained by a linseed oil painting

layer under three pyrolytical conditions.

Glue

Animal glue is obtained from connective

tissues, skin, muscle tissues, bones and

hides of animals and fish as boiling water

extracts. Collagen, present in these tis-

sues, is a fibrous protein formed by three

strands strongly held together by hydro-

gen bonds between the hydroxy group of

hydroxyproline and the hydrogen of

adjacent glycine groups. When boiled for

a long period it is partially degraded due

to the separation of the three strands to

form a sort of gelatine. On cooling, the

material has powerful adhesive property

[73].

In our first papers [74–78, 81] we

identified as pyrolysis markers of animal

glue fragments arising from hydroxypro-

line such as pyrrole (with its methyl

derivatives) and diketodipyrrole. Diketo-

dipyrrole is derived from the cyclisation

and dehydratation of the dipeptide

hydroxyproline-hydroxyproline. The

resulting diketopiperazine (see the reac-

tion scheme in Figure 5) can rearrange

losing two other molecules of waters to

form this very stable compound [82].

In a recent paper we have applied the

silylation reaction to study proteinaceus

binders. As regards glue we have not

obtained derivatised molecules, but we

have focused our attention on the char-

acterisation of the other diketopiperazine

derivatives [80]. We have found that be-

sides diketodipyrrole there is a charac-

teristic distribution of compounds which

contain proline and other amino acids.

The most important are the PRO-GLY

and the PRO-PRO diketopiperazines.

Figure 6 shows the profile obtained by

pyrolytic silylation.

Figure 3. Mass spectrum a) palmitic acid b) palmitic acid methyl ester c) palmitic acid silyl ester.


Milk

Milk typically contains 5.5% of fat, 4.9%

of lactose and between 3 and 5% of

proteins. Casein, the principal protein, is

a phosphoprotein, which contains about

1% of phosphorous in the form of

phosphoric acid combined with serine

and glutamic acid [73].

Milk and casein are the most difficult

binders to be detected by pyrolysis. The

proteic fragmentation gives rise to very

poor pyrograms and the markers are of

such low intensity that in real samples they

are difficult to observe.

In particular in our first investigations

under normal pyrolytical conditions we

focused our attention on milk and found

two markers of the saccharidic fractions,

furanmethanol and maltol. These two

compounds contain hydroxy groups and

so they give rise to broad peaks [75–76].

In our recent paper on the pyrolytic

silylation of milk [80] we have found a

better marker of the saccharidic fraction

(trimethyl-silyl oxymethyl)-2-furaldehyde

[83]. Also in milk a characteristic distri-

bution of diketopiperazines containing

proline has been detected. In Figure 7 the

pyrolysis-silylating profile is illustrated.

Beeswax

Natural waxes are composed of esters of

saturated fatty acids, saturated long

chains (C14–C33) monoalcohols and, in

lower amounts, free fatty acids and long

chain hydrocarbons [73].

As an example of the application of

pyrolysis of wax, a beeswax pyrogram is

given in Figure 8 [84]. The sample was

taken from an Egyptian sarcophagus;

Egyptians often used the encaustic tech-

nique which consists of using a saponifi-

cated wax obtained after treatment with

lime as a painting binder.

Under normal pyrolysis conditions a

sequence of n)1 alkenes is obtained. Near

each peak corresponding to the alkene,

Figure 4. Reconstructed ion chromatogram obtained from a) pyrolysis of alinseed oil painting layer b) pyrolysis-methylation of the layer c) pyrolysis-silylation of the layer.

Figure 5. Formation scheme of diketopiperazines during pyrolysis.


smaller quantities of the homologous al-

kane and diene are present. As a natural

product, the odd carbon hydrocarbons

dominate, while in an industrial paraffin

wax the odd hydrocarbon content is

equal to the even content.

Challinor analysed waxes under

methylating conditions obtaining a char-

acteristic distribution of fatty acid methyl

esters and fatty alcohols [85].

Polysaccharide Compounds

Up to now we have focussed our atten-

tion on cellulose and have tested the dif-

ferent pyrolysis techniques on its

characterisation [83, 86–87].

Pyrolysis of cellulose leads to the for-

mation of levoglucosan (1,6-anhydro-b-D-glucose), as one of the main peaks. This

is a typical anhydrosugar formed by other

carbohydrates containing glucose. How-

ever, its detection can be difficult because

of its polarity which may cause adsorp-

tion in the Py-GC interface. Reactive

pyrolysis could be used to improve GC/

MS performance of these polar markers,

through the derivatisation of OH groups.

Pyrolysis of cellulose under methylat-

ing conditions using tetramethylammo-

nium hydroxide produces mainly

methylated deoxyaldonic acids [83] with

the permethylated anhydrosugars only

being detected as trace amount [88].

Methylated deoxyaldonic acids are

distinctive markers for carbohydrates

capable of providing structural informa-

tion [83]. However, they are not specific

markers for monosaccharides like

anhydrosugars as the formation of deox-

yaldonic acids occurs with racemisation

of C-2 and reduction of C-3 with loss of

structural information. In addition the

strong alkaline environment created by

TMAH favours the production of deox-

yaldonic acids with shorter and larger

chain lengths so that different monosac-

charides produce the same deoxyaldonic

acid. For instance glycolaldehyde and

glycerinaldehyde give rise to the same

products as C6 sugars [89].

Recently, pyrolysis/silylation with

hexamethyldisilazane (HMDS) has been

proposed as an alternative to pyrolysis/

methylation for the characterisation of

carbohydrates [87]. Silylated derivatives

of levoglucosan have been obtained as

important products of cellulose by on-

line pyrolysis-GC/MS with HMDS along

with silylated hydroxylated furans and

pyranones [87]. However, levoglucosan is

found as a minor derivative, while the 4-

monoTMS, 2-monoTMS and 2,4-diTMS

derivatives, are found as major products

(Figure 9).

Diterpenic Resins: Sandaracand Copal

Sandarac is mainly composed of com-

munic acid (a dienic compound which

can easily polymerise), sandaracopimaric

acid (an abietane compound) and some

phenolic compounds.

Under normal pyrolysis conditions

totarol, a phenolic molecule, was identi-

fied as marker of sandarac, while for

copal a series of sesquiterpenoid com-

pounds characterised by 204 as molecular

weight and 161 as base peak in the

chromatogram [90].

The principal identified products un-

der silylating conditions were trimethyl-

silyl esters of various substituted

hydrogenated naphthalene carboxylic

acids (m/z 240, 308, 306), and the TMS

ester of sandaracopimaric acid [93] as can

be seen in Figure 10.

Manila copal exhibits a pattern of

abietane compounds similar to that of

sandarac, with the TMS ester of sandar-

acopimaric acid present in a small

amount. However it can be differentiated

from sandarac by the presence of addi-

tional unidentified compounds with

characteristic mass spectra [92].

Diterpenic Resins: Pinaceae

Under normal pyrolysis these resins are

characterised by the presence of frag-

ments of abietane compounds [90].

Figure 6. Reconstructed ion chromatogram obtained from pyrolysis-silylation of a glue paintinglayer. The dipeptide abbreviations on the tops of the peaks are related to the corresponding

Figure 7. Reconstructed ion chromatogram obtained from pyrolysis-silylation of a milk paintinglayer. The dipeptide abbreviations on the tops of the peaks are related to the correspondingdiketopiperazine compounds.


Under silylating conditions an intense

sequence of TMS esters of abietane acids

can be detected. With the exception of

sandaracopimaric acid, these esters are

not detected in Manila copal and in

sandarac and could be used as distinctive

markers of pinaceae resins [91]. Figure 11

is the pyrogram of colophony, as an

example of pinaceae resins.

Triterpenic Resins: Dammarand Mastic

Dammar is a mixture of tetracyclic com-

pounds of the dammarane series and of

pentacyclic compounds derived from ur-

sonic acid. Mastic has a composition

similar to dammar and this is reflected in

the GC-MS traces obtained from normal

pyrolysis and with derivatisation condi-

tions.

In conventional Py-GC-MS [90],

dammar can be distinguished from mastic

by the presence of the sesquiterpenes

C15H22 and C15H26 with characteristic

pairs of ions at m/z 202 + 159 and m/z

206 + 163, respectively [81]. A dehydro-

genated sesquiterpene can be used to

characterise mastic.

With silylating conditions [91] a series

of silylated compounds with molecular

ions at m/z 238, 240, 242, 256, 316, 330

and 342 are revealed in the chromato-

grams of both dammar and mastic. In

Figure 12 these compounds are indicated

in the profile obtained from dammar.

Although their structure was not identi-

fied, these silylated compounds could be

used for the positive identification of

these resins. The sesquiterpenes obtained

under normal pyrolysis are still present

and can be used to distinguish dammar

from mastic.

Synthetic Resins

Analytical pyrolysis can be easily applied

to the study of synthetic polymers [92–

101]. However only a few papers in the

literature deal with the analysis of such

products used in the art field [40, 42, 44,

50, 60–61].

The synthetic polymers used in the

artistic field are of different types. In par-

ticular the most important products are

acrylic and methacrylic derivatives, poly-

fluorourate and polyvinylic compounds.

Recently we were able to analyse

samples from the Scrovegni Chapel

(Padova, Italy) with the aim of detecting

the products used in the past restorations

[102].

Four main products were detected by

normal pyrolysis: polyvinylacetate, an

acrylic resin, polystyrene and a polyflu-

orourate polymer as shown in Figure 13.

Organic Pigments

Most pigments in paintings are of inor-

ganic origin except for a few natural

colorants mainly used as dyes. In this

category a class of pigments used in the

past were the antraquinone dyes, the so

called mordant dyes for the procedure of

production of the colour. The pigments

were obtained by adding to the solution

of the colour, extracted from plants or

insects, an aluminium salt to obtain a

precipitate. To fix the colour, the mate-

rials were previously immersed in the salt

solution and then the colour was applied.

Antraquinone dyes contain com-

pounds derived from the 9,10 anthra-

Figure 8. Reconstructed ion chromatogram obtained from pyrolysis of an Egptyan sarcophaguspainting layer: evidence of beeswax.

Figure 9. Reconstructed ion chromatogram obtained from pyrolysis-silylation of cellulose: 1 ¼ 6-anhydro-b-D-glucose 2 TMS ether, 2 ¼ 6-anhydro-b-D-glucose 4 TMS ether, 3 ¼ 6-anhydro-b-D-glucose 2,4 TMS ether 4 ¼ 6-anhydro-b-D-glucose 2,3,4 TMS ether.


cenedione. Alizarin and purpurin are the

most important of the plant extracts,

while in the insect extracts carminic acid

and chermesic acid are the most abun-

dant.

Pyrolysis of alizarin under methylat-

ing conditions produces the partially or

totally derivatised compound, while from

purpurin only benzoic acid methyl ester

and dimethylphtalate are obtained. The

same results for purpurin have been ob-

tained by analysing the natural extracts

treated with alum [103].

Another important historical dye is

indigo which belongs to the vat dyes. The

material was immersed in the colourless

solution obtained by extracting the plant

(Indigofira tinctoria). Standing the mate-

rial in the sun gives rise to a blue insol-

uble coloration thanks to the oxidation of

the indoxile present in the solution to

indicano [103].

Pyrolysis with methylation produces

characteristic fragments such as 2-

(amino)-benzoic acid methyl ester and

2-(methylamino)-benzoic acid methyl

ester.

Inorganic Pigments

Some inorganic compounds can be anal-

ysed by means of analytical pyrolysis as

they produce volatile derivatives under

thermal treatment. Cinnabar, for instance

(HgS, red pigment) gives metallic Hg

atoms which can be detected. A broad

peak appears in the chromatogram with a

characteristic mass spectrum showing the

isotopic pattern of mercury [104] (Fig-

ure 14), deriving from the thermal

decomposed salt.

Arsenic compounds such the yellow

pigment orpiment (As2S3) give As4 on

heating and this appears as a regular

peak. Its mass spectrum is characterised

by m/z 300 (As4 molecular weight) as the

base peak and following peaks with losses

of 75 (As) [84, 105] (Figure 15).

Patinas on Stones or Bricks

Degradation patinas on stone or brick

buildings show the presence of aromatic

hydrocarbons deriving from the incom-

plete combustion and alkanes and alk-

enes whose distribution can give

information about the biogenic or

anthropogenic origin of the deposition

[106–109].

Figure 10. Reconstructed ion chromatogram obtained from pyrolysis-silylation of a sandaraclayer.

Figure 11. Reconstructed ion chromatogram obtained from pyrolysis-silylation of colophonylayer: 1 ¼ Pimaric acid TMS ester, 2 ¼ Sandaracopimaric acid TMS ester, 3 ¼ Isopimaric acidTMS ester, 4 ¼ 6 Dehydrodehydroabietic acid TMS ester, 5 ¼ Dehydroabietic acid TMS ester,6 ¼ Abietic acid TMS ester.

Figure 12. Reconstructed ion chromatogram obtained from pyrolysis-silylation of dammar.Numbers on top of the peaks indicate characteristic ions (M+) in the mass spectrum.


Patinas on Bronze

The study of patinas on bronze art ob-

jects is important in understanding the

mechanism of formation, the binding role

and the use of organic products in con-

servation [110–112]. A study on some

samples from the statue of Neptune in

Bologna [112] showed that the pyrograms

were characterised by a homologous ser-

ies of aliphatic hydrocarbons, carboxylic

acids, phthalic anydride and phthalimide.

In Figure 16 the profile obtained by

pyrolysis methylation of a sample from

the Capitolium She-wolf revealed the

presence of a drying oil as a protective

coating.

Bituminous Materials

Analytical pyrolysis can be easily applied

to the characterisation of bituminous

materials found in vessels or old bottles

[113]. The characterisation of the mate-

rials can be useful for dating or historical

purposes. In our paper the content of a

red bottle has been examined and the

fatty acid profile suggested the presence

of a vegetable oil.

Chinese lacquer

Chinese lacquer is produced by the tree

Rhus verniciflua. It consists mainly (60%)

of acetone soluble/water insoluble mate-

rial, 30% of water and 7% of water sol-

uble polysaccharides. The components of

the major hydrophobic fraction are mix-

tures of similar substances of general

formula given in Figure 17. The side

Figure 13. Reconstructed ion chromatograms obtained from samples from Scrovegni Chapel evidence of a) polyvinylacetate b) acrylic resin c)polystyrene d) polyfluorourate polymer.

Figure 14. a) Single ion monitoring for m/z (198 + 200 + 202) obtained from pyrolysis of a layercontaining cinnabar b) mass spectrum with evidence of Hg atoms.


chains are mainly of 15 and 17 carbon

atoms and may be saturated or have 1,2,3

double bonds [73].

Pyrolysis with methylathion of a ter-

racotta jar sample has showed evidence

of Chinese Lacquer [106] by the presence

of substituted dimethoxy phenols.

Conclusions

Pyrolysis both in normal and in derivat-

isation conditions is a useful procedure

for the GC-MS analysis of the organic

fraction in works of art. The main

advantages are the limited amount of

required sample and the possibility to

simultaneously analyse organic compo-

nents belonging to different classes, while

with traditional techniques specific pre-

treatments for different matrices are nec-

essary. Analysis of polar compounds can

be performed thanks to the use of deri-

vatisation reactants.

This technique allows the characteri-

sation of different organic matrices and

of some inorganic pigments.

Our research group is engaged in the

experimentation of this technique and it

is our intention to verify its applicability

field and its limit testing it with new

derivatisation reagent or to the new

materials used in modern art.

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Received: May 13, 2003Revised manuscriptreceived: Jul 14, 2003Accepted: Jul 23, 2003


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