PART II
Art Conservation: Culture Under Analysis BEN B. JOHNSON and THOMAS CAIRNS Conservation Center Los Angeles County Museum of Art Los Angeles, Calif. 90036
the advent of modern methwads of chemical analysis, such as polarography, atomic absorptioa, ITH
neutron activation analysis, energydispersive X-ray emission spectroscopy, laser microprobe analyzer, and spark source mass spectrometry, a great deal of attention has been focused recently on detailed analysis of art objects ( 1 ) with two objectives in mind. Firstly, there are the systematic programs of analyses of a large number of closely related objects in an attempt to fingerprint them by a good statistical profile, Le., trace elements in cast bronzes. Such programs deal with research on the history of technology answering such questions as source of raw materials and techniques in fabrication. Secondly, there are specific analyses for practical purposes. This often involves the analysis of a single sample or even a small group of samples to answer specific questions usually relating to treatment. I n this respect the attention of the chemist is drawn t o relate chemistry t'o authenticity. Until recently anachronisms in the use of materials were a good guideline to follow, but latter-day forgers try hard to mimic the original artist and/or technique. An outstanding example in the use of materials lies in the chronology of white pigment -lead white (PbO) was used since classical times; zinc white (ZnO) made its appearance around 1810; titanium white (TiO) was commercially available around 1920. Evidence of zinc white or titanium white in a 17th century style paint-
ing would certainly raise questions as to its authenticity. At this point it must be emphasized that the sample taken for analysis by the chemist in the museum laboratory is from an original area and not from a recently restored area where later materials might well have been employed. Without a prior physical examination of a painting, for instance, by uv, ir, or X-rays, mistakes might easily be made by a novice to the conservation-chemistry field. Frankel ( 2 ) in a recent article demonstrated the potential use of X-ray fluorescence ( 3 ) in the so-called detection of art forgeries via pigment identification. Such publications can, in this instance, give false impressions that chemistry alone tackles the job whereas interdisciplinary approaches are really necessary. I n matters of authenticity, therefore, three points finally decide the outcome: scientific data, stylistic criteria, and conservator's experience and availability of related comparative material. Instrumentation
Conservation-chemistry covers an extremely wide spectrum of materials for analysis. I n the field of inorganic chemistry one encounters materials from metals, pigments, and stones to ceramics. On the other hand, organic materials encountered are mainly natural products, i.e., gums, glues, resins, oils. This conglomeration prompts the utilization of sophisticated instrumentation capable of handling such diverse materials. Chemists have long been aware
30 A * ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
of the restrictions imposed on their studies by the limits of sensitivity of many of those currently available analytical methods which permit a number of elements to be determined simultaneously in one sample. This has led to the adoption of more sensitive methods for a restricted number of elements. e.g., neutron activation analysis of paper ( 4 ) . Recently, spark source mass spectrometry has provided the chemist with the ability to cover the full range of elements in any sample in a single determination and the ability to detect those elements down to very low concentrations, i.e., parts in lo6. The AEI MS702 spark source mass spectrometer in the Conservation Center of the Los Angeles County Museum of Art is used to tackle a variety of problems encountered in the conservation-chemistry field. Today. spark source mass spectrometry has found its way into even more new avenues of research. I t has been used alongside atomic absorption and neutron activation analysis by Morrison and Kashuba ( 5 ) in the analysis of returned lunar samples (6). Harrison et al. (7) have reviewed the forensic application of spark source mass spectrometry in analyzing such products as hair and glass. Recently, the FBI laboratory in Washington, D.C., has acquired a spark source mass spectrometer. The technique owes its widespread success to its unique capability of inultielement determinations a t low concentrations. With such diverse materials as gums and oils, separation into ma-
REPORT FOR ANALYTICAL CHEMISTS
Conservation-chemistry represents a new and fascinating applied scientific discipline harnessing chemical knowledge to unlock the secrets of the history of technology. Particular attention is paid to ancient metallurgy and pigment and media studies
jor and minor components is of primary importance, followed by identification. T o achieve this ultimate goal on small samples, a combined gas chromatograph-mass spectrometer system must be employed. The AEI 1L1S902 high resolution mass spectrometer a t the Conservation Center is used in conjunction with a Perkin-Elmer F-11 capillary gas chromatograph. Reasons for Analysis
Concentration on chemical analysis of a r t objects is generally prompted by the curator, collector, and a r t historian. H e appreciates the beauty and craftsmanship of an ancient object. However, he is unable t o look into the historical study of technology since there are very few texts on the subject (8). Each object can be considered unique and has locked into its structure and fabrication some knowledge of the properties of the materials used. From the standpoint of his materials, the artist was rarely concerned about any scientific investigation into their nature-his concern was solely directed to physical properties. The aesthetic enjoyment of an object is greatly enhanced when its original creation is fully understood. I n the case of metallic objects, technical analyses can often answer questions as to the source of raw materials (mines, trade), processing (smelting, cupellation), models. casting (molds, alloy composition), finishing (cold working, techniques such as chasing, incising, engraving, joining), and how these various stages are interrelated, if a t all.
Were the processes involved indigenous achievements or results of cultural exchange? Ancient Metallurgy
Sampling. Ancient metals and their alloys are sometimes either homogeneous or heterogeneous, and truly representative sampling is difficult to obtain (9). I n particular, the existence of more than 4 wt 7. of lead ( P b ) in a cast bronze (Cu/ Sn) is clearly demonstrated (polished cross section) by the appearance of globules of P b of widely varying size irregularly distributed throughout the Cu/Sn matrix (10), Le., P b is insoluble in the Cu/Sn alloy. It is strongly recommended, therefore, t h a t both X-ray examination and, if possible, cross sections be prepared before sampling a structure. Knowledge t h a t such a structure is under investigation aids interpretation of analyses. One of the greatest problems today is comparing results from two different sources by two different techniques. At the moment a comparative study organized by the International Council of Museums (ICOM) between a number of museum laboratories is underway (11). The same samples are being analyzed by different techniques to document standard deviations and accuracy of results by the various techniques available in the conservation field throughout the world. For instance, Caley (12) has pointed out that the determination of tin by wet chemical methods is known to yield high values by about 10%. On the other hand, spark
source mass spectrometry boasts of accuracy for “trace elements” ( I S ) . With spark source mass spectrometry the question of homogeneity is a critical one since the actual sample size consumed during a typical analysis can be of the order of a few milligrams. The introduction of the ion beam chopper (selective sampling of the positive ion beam) has greatly improved the reproducibility (14) of analysis for both homogeneous and heterogeneous materials by use of the integrating properties of the Q2 photoplate. However, spark source mass spectrometry relies heavily on the use of standard reference materials, i.e., Cu doped with various trace elements a t known concentration levels. The few existing bronze standards are somewhat inadequate and usually too heterogeneous for use in spark source mass spectrometry. I n particular, standards prepared via a dilution technique are extremely heterogeneous (15). Trace element analysis must be treated with some caution when interpretation is attempted. Two main sources of trace elements are the ores from which the metals were smelted and the smelting technique. T o date, detailed trace element analysis by spark source mass spectrometry and neutron activation analysis is fragmentary, and any definitive conclusions are only tent a tive. Historical. I n an attempt to understand the development of metallurgy in ancient times, it is necessary to outline the chronological or-
A N A L Y T I C A L CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
*
31 A
Figure 1. Typical Luristan bronze bar bit depicting a wild mountain goat in profile with its head seen frontally with a bird on its rump (L.2567.67.241, 12.5 x 11.5 cm, bit 21 x 1 cm, w t 613 grams)
der of events that led man to casting and alloy formation. Admittedly, native gold and silver were among the first metals to he used, hut native copper is reported to have been first discovered about 5000 B.C. in the Sinai Peninsula. The importance of copper (and of bronze) lies in the fact that it was extensively used, and its corrosion in soil generally forms a stable patina. Persia, rich in ores of all kinds, was logically a center of early ancient metallurgy (16). By 4000 B.C. the discovery of the reduction of oxide ores by smelting with charcoal took place. Copper ores employed were probably the two oxides, cuprite (CuzO) and tenorite (CuO) and the two carbonates, malachite [CuCOsCu (OH),] and azurite [2CuCOs-Cu (OH),]. Carbon, coal, or coke was heated in the presence of oxygen to form carbon monoxide which in turn reacted with the ore to give molten metal and carbon dioxide. This discovery enabled man t o develop the art of smelting and casting. To cast a pure copper artifact presented some difficulties. Pure copper melts a t 1083OC. It is a very sluggish viscous liquid. Such high temperatures would certainly have taxed the furnace capabilities of the artisans a t this early date. However, a number of pure copper castings do exist, mainly from Northern India and Tibet but usually of a much later date. 32 A
The restrictions imposed by attempting to use pure copper for direct casting resulted in the discovery of adding tin ore to the copper ores during smelting. Tin was utilized from around 1800-1600 B.C. in North West Persia in the form of oassiterite or stannic oxide (Sn0.J. Bronze was therefore born. The addition of tin to copper in the ratio 25:75 reduces the melting point from 1083"to 795'C-a considerable drop. Such an alloy is more fluid and presents fewer problems in casting. Pure copper has the tendency t o shrink greatly on cooling and thereby lose any fine detail of the original mold. Bronze, on the other hand, shrinks less and is more malleable and easier to cold work. Techniques of metal working in the F a r East also developed as the Chinese during the Shang dynasty (ca. 1523-1028 B.C.) and the Chou dynasty (ea. 1027-222 B.C.) achieved the zenith of practical perfection in their bronze castings (10). Another important factor in bronze technology is the use of lead in the Cu/Sn alloy especially where the oasting was to be extensively cold worked. The principal lead ore is galena, lead sulfide (PbS), and might well have been known
before cassiterite and bronze since smelting of this particular ore was primarily undertaken to separate out the silver content by a process commonly known as cupellation. T h e dross that forms on the surface of the crucibles containing the melt is continually removed until a shining surface is obtained. This dross contained all the base metal impurities. Separation of gold and silver is achieved by a modification of this process whereby salt is added to the melt to remove the silver as chloride. This so-called cblorination modification has been in practice since the second millennium B.C. I n this way both gold and silver were refined to a fairly high degree of purity. Rarely, however, was cupellation carried to the ultimate extreme. The addition of lead, therefore, to bronze was a practical suggestion and found widespread usage in casting imparting to the alloy desirable extra qualities. The entrance of zinc (Zn) into bronze technology might well have been extremely early especially if either the copper ore or the tin ore contained a natural amount of Zn as an impurity. During the smelting of such ores, however, the metallic Zn vaporizes extremely easily
Figure 2. Krishna Rajamannar Bronzes. South Indian, early 12th century. left to right, Rukmini. Krishna, Satyabhama, and Garuda (M70.69.1;2;3;4.)
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2. FEBRUARY 1972
From
I
and would have been lost to the atmosphere save trace amounts. It was not until around 800 B.C. t h a t brass (Cu/Zn) started to occasionally appear (17). Finely ground calamine [ZnrSizO, (OH)z * 2 H ~ 0 ] , charcoal, and granulated copper are placed in sealed crucibles and heated. The reduced metallic Zn vaporizes and then alloys with the copper since the reaction takes place in a sealed system. Hence the appearance of substantial quantities of Zn in ancient cast bronzes would suggest questionable authenticity. Luristan Bronze Study. A carefully selected group of Luristan bronze horse hits from the Foroughi collection (18) are a t the moment under investigation by spark source mass spectrometry. Such horse trappings are in a very distinctive decoratiive style (Figure 1) and belong to the period during the 7th and 8th centuries B.C. They have no counterparts outside their point of origin-the mountainous province of West Persia. Beginning early in this century, excavations have revealed more and more of these beautiful ancient bronzes belonging to the Luristan civilization. Such prerequisites made them very suitable candidates for a study of the potential of trace element profiles in characterizing artifacts of an ancient culture (19). Drillings were taken from both the cheek plates and bit of one such trapping. I n essence, to date, the results may be summarized briefly since the study has not yet been fully completed. The major constituents (Cu/Sn/Pb) vary from piece t o piece, hut the presence of the same trace elements (Bi, Sb, Ag, Se, As, Ni, Fe, Go) were noted in each analysis within certain concentration levels. A number of the bits, however, were manufactured from native copper and are excluded from the above generalization. Work is continuing to achieve the number of analyses needed t o draw reliable conclusions. T h e noticeable absence of zinc is worthy of mention. A number of forgeries, declared such on stylistic grounds, were also examined. The presence of high percentages of zinc (above 2 wt %) together with other trace elements certainly damned them, scientifically speaking. Such
a combination of science and stylistic criteria is the most effective method in the detection of fakes and forgeries. South Indian Bronzes. Quite recently a n interesting bronze group of the early 12th century A.D. depicting Krisbna and his two wives, Rukmini and Satyabhama, together with his messenger, Garuda (Figure 2 ) , were acquired by the Los Angeles County Museum of Art through the generosity of Mr. and Mrs. Hal Wallis. The occurrence of such a group of high quality and importance is unique. On religious grounds they were probably cast and fabricated (cold worked after casting the rough shape) as a group with Sutras dictating proportions, attributions, etc. This prerequisite, therefore, permitted a detailed study (20) of the chemical composition from piece to piece in an attempt to ascertain if a n exact science was operative or was each composition by chance. It turned out that all four pieces resembled each other fairly closely. Drillings were taken from various locations in each piece and analyzed several times, then averaged. Table I illustrates the typical results obtained for Satyabhama (Figure 3 ) . Many other South Indian bronzes are now under study to determine whether any further correlations can be established.
I
I
Table 1. Analysis Results for Satyabhama (M70.69.3) Ht, 28 in.; base diam, 8.5 in.; wt, 58 Ib
wt 56
HIP
Neck
Heel
Av
Base
Cu 93.05 92.65 92.45 92.71 93.31 Sn Pb
Bi Sb
Ag As
Zn Ni
co
Fe
2.25 3.50 0.01 0.09 0.08 0.022 0.08 0.50 0.02 0.39
2.21 3.96 0.01 0.09 0.08 0.026 0.11 0.45 0.02 0.39
2.66 3.53 0.04 0.13 0.11 0.04 0.09 0.55
0.02 0.38
2.37
1.62 3.66 4.33 0.02 0.03 0.10 0.09 0.09 0.09 0.02 0.032 0.09 0.05 0.50 0.40 0.02 0.01 0.39 0.03
'4
Mughal Indian Miniature Painting
Although there are several excellent studies of the technique of Indian miniature painting which are based purely on literary sources and tradition passed from generation to generation, until present day very little has been published which deals with the technical examination of actual paintings. Chandra's treatise (21) is a wealth of information but leaves many basic chemical questions unanswered, e.g., whether lead white, which is poisonous and oan turn black in the presence of sulfur, was used by Mughal painters in the 16th and 17th centuries or whether zinc white, which was preferred more recently, was adopted. i The common appearance of ultramarine in Mughal paintings raises a t l natural doubt as to whether lapis lazuli (3Naz0*3A1203'6Si0~'2NaS), Figure 3. Satyabhama, one of Krishna's two wives (M70.69.3.) a costly stone mineral, yielded the ,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
33 A
Report for Analytical Chemists
Figure 4. Page from a Ragamala: Todi Ragini series. Mid.18th century (M71.1.42). Woman with vina stands in grove of trees (9.5” x 6.25”)
blue or whethcr i t was obtained from azurite [2CuCO3~Cu(OH)J,a much cheaper material. Agrawal’s paper (22) is the most comprehensive report to date on materials of Indian painting. It is mainly a survey of historical references hut includes scientific examination of manuscripts as well. To answer some of the questions regarding pigments, a study was undertaken (29) of the miniature paintings (Figure 4) from the collection of Nasli and Alice Hecramanaeck ( 2 4 ) . Pigment Identification. T h e limited number of pigment analyses has precluded any final conclusions at this stage, but some interesting patterns havc evolved. The analysis of the blucs during the early Mughal period (mid-15th to mid16th century) has revealed that genuine lapis lazuli was the most preferred and most frequently used blue, Le., lack of evidence for the suggested use of azurite as a cheap substitute. I n addition Mughal artists preferred to use their pigments in pure form in overlapping 34A
*
layers or in mixtures with a second pigmcnt such as lead white. For white pigments hlughal artists of the first half of the 17th century preferred 1ea.d white whereas in earlier paintings belonging to the 15th and 16th centuries kaolin (A120a * 2Si02 * 2H30) was dominant. Vermilion (HgS) was the most widely used red pigment. Second t o vermilion, minium (PbaOa) was employed, especially in paintings of the 16th and 17th century. Orpiment (As&,) and realgar (AS&), ycllow and orange, respectively, were identified on 16th and 17th century paintings but not in significant patterns to he of diagnostic value. I n a study of the green pigments, both malachite and copper resinate (25) were found. A number of organic pigments were encountered and research is in progress via GCMS t o elucidate their molecular structures. Media Studies
Paint is the mixture of a suitable medium plus finely ground pigment.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Medium is the descriptive terminology applied to the binding agent or vehicle for the pigment particles. Traditionally three major classes exist which can be chemically differentiated: plant gums (gum arabic) which are polysaccharides, glues of animal origin (size) and egg-tempera which are proteins, and drying oils (linseed) which are mixtures of various triglycerides. Historically both gums and glues (including egg products) were used in earliest times. It was not until the 15th century that drying oils such as linseed prevailed and quickly replaced the use of egg-tempera. Gums, however, are still used extensively today as the medium for commercially available water colors. Gums. Starch and cellulose are the most ubiquitous of the plant polysaccharides known. Plant gums also belong t o this general class as demonstrated as early as 1929 by Butler and Cretcher (26) who identified various hexoses (CoH1200) as products from acid hydrolysis of gum arabic. I n essence, plant gums are high-molecular-weight polysaccharides built up by repeated condensation of various monosaccharides (both hexoses and pentoses), The hydrolysis products of such gums from various botanical sources (27) are related to their taxonomic origin (Table 11). Besides the pigment identification of the Mughal Indian miniatures described previously, media samples were also taken (about 0.5 mg) and hydrolyzed with 3% HCI under vacuum a t 105°C for 24 hr ( 2 8 ) . The hydrolysis products were then neutralized with 200 mg Amberlite IRA 68, filtered and evaporated (12 hr a t 45°C) to dryness. T h e residue was then examined by thinlayer chromatography according to Stahl (29) and later by gas chromatography to obtain quantitative results. I n almost every case studied, gum arabic was the preferred medium. One or two cases, however, were of protein origin. Flieder (SO) has successfully used this technique in the identification of both sugars (from polysaccharides) and amino acids (from proteins) in a large number of illustrated manuscripts. Glues. Before the advent of dry-
Table 11.
Composition of Various Plant Gums
Common n a m e
Source
D-glucuronic acid
% D-galactose
Gum arabic
Acacia senegal
16
52
Cherry gum
Prunus Cerasus
12
21
Peach gum
Prunus Persia
7
36
Table 111.
-
D-mannose
L-arabinose
Rhamnose
19
14
10
55 43
Casein,
%
64.9y0
13.8%
Egg-white, % Ovomucoid, 9.2%
Ovalbumin, Gelatin
14
Composition of Various Proteins
Glues, % Amino acid
Xylose
Elastin
Conalbumin,
Lysozyme, 3.4%
Avidine, 0.1%
Glycine
2
27.25
26.7
3.05
5.7
3.8
5.7
Alanine
3.2
11.23
21.3
6.72
4.4
2.3
5.8
Valine
7.2
2.78
17.7
7.05
8.2
6.0
4.8
4.2
Leucine
9.2
3.45
9.0
9.2
8.8
5.1
6.9
4.9
Isoleucine
6.1
1.53
3.8
7.0
5.0
1.43
5.2
5.5
Serine
6.3
3.73
0.85
8.15
6.3
4.2
6.7
4.5
Threonine
4.9
2.36
1.12
4.03
5.9
5.5
5.5
10.5
Phenylalanine
5.0
2.5
6.2
7.66
5.7
2.91
3.12
5.9
Tyrosine
6.3
0.24
1.5
3.68
4.6
3.18
3.58
0.88
1.2
3.0
0.3
10.6
5.4
3.6
4.9
2.72
1.4
1.64
0.51
3.8
6.7
6.8
0.47
2.06
1.41
Tryptophane Proline
1.2 11.3
Hydroxyproline Cystine
15.47
13.5
13.24
1.6
0.34
0.35
4.6
1.35
Cysteine
5.2
Methionine
2.8
0.63
Aspartic acid
7.1
6.7
1.1
9.3
13.3
2.03
13.0
0.95
Glutamic acid
22.4
11.56
2.4
16.5
11.9
6.5
18.2
9.7
4.32
6.6
1.04
0.96
Histidine
3.1
0.7
2.35
2.57
2.15
Arginine
4.1
9.04
1.3
5.72
7.6
3.7
12.7
6.5
Lysine
8.2
4.37
0.5
6.3
10.0
6.0
5.7
6.2
Hydroxylysine
ing oils in the 15th century, glues and egg-tempera were widely used, although isolated examples exist of the use of oils as early as the 13th century. This takes the conservator-chemist into the realms of protein and amino acid chemistry. Here again the hydrolysis products (amino acids) have a direct bearing on the origin of the protein used (Table 111). One can easily distinguish ( S I ) casein (from milk), from animal glues (gelatin and elastin), and from egg-white. The two observed cases of protein as media in Mughal Indian miniature painting were both of animal glue origin. Drying Oils. Vegetable drying oils consist largely of triglycerides (glycerol ester of fatty acids) of five fatty acids: palmitic, stearic,
0.76
oleic, linoleic, and linolenic (Table IV) . The analysis of triglycerides by gas chromatography is still somewhat in the development stage ( 3 2 ) . To date, analysis of such molecules (mol wt approx. 950) has been achieved by transesterification of the triglyceride entity into the methyl esters of the three parent fatty acids which are then volatile enough to undergo facile gas chromatography on nonpolar columns. Table V lists the composition of a number of the more important vegetable oils in terms of fatty acid content. The iodine value is the percentage of iodine chloride, calculated as iodine, which is capable of being absorbed by the oil and is a direct measure of the total amount of unsaturation in the oil. Gunstone and Padley (33) have
recently proved that the distribution of the fatty acids as esters on the glycerol backbone confornis to a modified random distribution (Table V I ) . A novel new way of rapid and sensitive analysis of triglycerides in oils has recently been developed by
Table IV. Principal Fatty Acids in Drying Oils Name
Double Molecular bonds, formula no. Location
0
Palmitic
C16H3202
Stearic
C18Ha602 0
Oleic
CiaH~02
Linoleic
C18H3202 2
Linolenic
C,,H3,02
1 3
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
9 9, 12 9, 12, 15
35A
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Report for Analytical Chemists
Table V. Average Component Characteristics of Some Drying Oils
Oil
Unsaturated entities Iodine palmitic value stearic, yo Oleic
+
Source
Perilla
Perilla ocimoides
198
7
Linseed
Linurn usitatissrnum
180
10
Candlenut
Aleurites rnoluccana
164 132 136
13 14 11
Soyabean
Glycine hispidu
Sunflower
Helianthus annus
Hites (34) on the basis that a molecular-weight distribution can simply be measured by the resultant mass spectrum of the oil, Le., measurements on the various M + and (11 - 18) + ions. Positional isomers, ho\vever, cannot be distinguished. In spite of this, the rapidity of such measurements overcomes the other conventional and tedious methods of esterification followed by chromatography. A method to locate ultimate double bonds in triglycerides has been developed by Serck-Hanssen (35) using a rapid isothermal gas chromatographic determination of the main and end carbon chains split off as monocarboxylic acids by permanganate oxidation of the oils in acetone during a few minutes a t room temperature. Autoxidation of Drying Oils. The mechanism by which conjugated species polymerize in an autoxidation process is not yet fully understood, but it is certain that a system involving the reaction of free radicals is responsible for the construction of a three-dimensional macromolecular structure (36). ~~
Table VI. Triglyceride Composition of Linseed Oil Triglyceridea
333 332 331 330 322 321 320 310 300 Others
Wt
%
22 15 18 10 4 8 5 6 1 11
3 , 2, 1, an,d 0 refer to the acids lin,olenic, linoleic, oleic, and saturated specles, respectively.
%Linoleic
5 16 49
20 20 10
55
23 16
74
Linolenic
68 53 28 8 0
Farmer (37) was the first to demonstrate that oxidation may occur a t reactive methylene groups in the fatty acid entities with the formation of a hydroperoxide. I n the past, classical theory had demanded that oxidation involve the production of a cyclic peroxide via the double bonds themselves. Oxidation is now regarded as a free radical chain reaction as follows involving only the reactive allylic methylene groups in both linoleate and linoleneate chains:
Initiation:
RH
Propagation: R
complete easy to use chromatograph from less than $2,000
+R +H
+
0 2
+ R02
Ro2+ RH +
ROOH
+R
Termination: 2R02 + R-0-0-R R+R+R-R Ro,+R+ R-0-0-R Dimerization can therefore occur between unsaturated fatty acid chains on separate triglycerides (interpolymerization) or between fatty acid chains on the same glycerol backbone (intrapolymerization) . The initial number of unsaturated sites available within these triglycerides is the key to the drying process. Polymerization proceeds a t room temperature via an autoxidation process involving uptake of atmospheric oxygen. Via such a mechanism a network of bonded triglycerides is formed retaining the pigment particles within the polymeric framework. I n addition t o this polymeric structure, some triglycerides may remain unchanged owing to lack of sufficient unsaturation to participate in the autoxidation process, i.e., palmitic and stearic. Decomposition may also occur
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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Report for Analytical Chemists
(possibly with the hydroperoxide formed) with the production of a number of smaller molecules. Stolow (38) has already undertaken a detailed study of the mechanism of this polymerization process and a number of the controlling factors affecting the rate of polymerization in an attempt to relate the chemistry of such a polymerized system to real time. It has been demonstrated successfully that such tentative correlations do in fact exist, but much more experimental data must be collected to evaluate the program statistically. Stolow (38) encountered a number of compounds by gas chromatography that he was unable to identify by normal procedures. It is hoped that the use of the combined gas chromatograph-mass spectrometer system at the Conservation Center will help solve these problems. At the moment, however, research is underway a t the Conservation Center to further the basic work on the gas chromatography of triglycerides. New Materials
It is often necessary for the conservator-chemist to study the properties of new materials not only from the point of view of their application to conservation practices but also as a potential material for use by contemporary creative artists. Such a case is polyurethane elastomers which have already been used by prominent artists such as Claes Oldenberg in his composite molded relief of the Chrysler airflow car (39). After the first limited edition was published, the polymeric material discolored badly, and studies were initiated a t the Conservation Center to determine causes. Research into the basic components of the resin used revealed that an aromatic diisocyanate had been employed together with a suitable polyol. Polyurethane elastomers based on conventional aromatic diisocyanates are prone to yellow (i.e., oxidation) and lose gloss on exposure to sunlight. Replacement of the aromatic component by an aliphatic saturated hydrocarbon molecule such as hexamethylene diisocyanate removed the ability of the elastomer t o undergo further re38 A
action after casting (@). Tests on this new suggested formulation (i.e,, prolonged exposure to ultraviolet) indicated that no such discolorn t’ion was likely to occur in the future. These discoveries necessitated a republication of the art work with new castings made of the more stable formulation.
(13) R. M. Elliott and P. Swift, A p p l . Spectrosc, 21, 312 (1967). (14) P. G. T. Vossen, AKAL.CHEM.,40, 632 (1968). (15) F. Brown, MS702 Users Meeting, St. Louis, Mo., 1971. (16) H. E. Wulft,: “The Traditional
Role of Analytical Chemistry
p 572. (18) “7000 Years of Iranian Art,” Smith-
Today’s public consciousness of cultural heritage has elevated Conservation to a new significance in the museum world. However, lack of suitably qualified analytical chemists has somewhat hindered rapid advancement in the field. The marriage of the two disciplines, chemistry and art, has not yet been formally conceived academically although it has existed to some extent in certain talented individuals throughout the world. Rapid development of more scientific laboratories in the U.S.A. devoted to this topic is under discussion by Congress and will necessitate qualified staff which do not formally exist. Fresh manpower oriented t o this new discipline and committed to the principles of Consemation will be in demand in the very near future. References (1) Symposium on “Application of Spec-
trographic Techniques in the Museum Laboratory,” 10th National Meeting of Society of Applied Spectroscopy, St. Louis, Mo., 1971. (2) R. Frankel, Isotop. Radiat. Technol.,
8, 1 (1970). (3) R. S. Frankel and D. W. iiitken, A p p l . Spectrosc, 24, 557 (1970). (4) R. 1,. Brunelle, W. D . Washington, C. M. Hoffman, and M. J. Pro, J . AOAC. 54. 920 (1971). (5) G. H . Morrison and A. T. Kashuba, ANAL.CHEM.,41, 1842 (1969). (6) K. M . Reese, ibid., 42, 26A (1970).
( 7 ) W.W.Harrison, G. G. Clemena, and C. W. Magee, J . AOAC, 54, 929 (1970). (8) “Art and Technology-A Symposium on Classical Bronzes,” S. Doeringer, D. G. Mitten, and A. Steinberg, Eds., M I T Press, Cambridge, Mass., 1970. (9) E. R . Caley, “Analysis of Ancient Metals,” Pergamon Press, Yew York, X.Y., 1964. (10) R. J. Getten!, “The Freer Chinese Bronzes-Technical Studies. Vol 11,” Smithsonian Institution, Washington, D.C., 1969, p 124. (11) R . M. Organ, Amhaeometry, 13, 27 (1971). (12) E . R. Caley, “Critical Evaluation of
Published Analytical Data on the Comparison of ilncient Metals” in “Application of Science in the Examination of Works of Art,” Boston Museum of Fine Arts, 1967.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Crafts of Persia, MIT Press, Cambridge, Mass., 1%6. (17) R. J. Forbes, “Extracting, Smelting and Alloying” in “History of Technology,” C. Singer, A. R. Hall, and E . J. Holmyard, Eds., Oxford, England, 1954,
sonian Institution, Washington, D.C.; 1964. (19) P. R . S. Moorey, Archaeometry, 7, 72 (1964). (20) B. B. Johnson, “Krishna Rajaman-
nar Bronzes: An Examination and Treatment Report” in “Krishna: The Conherd King,” P. Pal, Los Angeles County Museum of Art, to be published, 1972. (21) M. Chandra, “The Technique of Mughal Painting,” The, U. P. Historical Society, Lucknow, India, 1949. (22) 0. P. Agrawal, “A Study in the Technique and Materials of Indian 11lustrated Manuscripts,” paper presented at ICOM Symposium, Amsterdam, Holland, 1969. (23) B. B. Johnson, “The Technique of Indian Miniature Painting,” paper presented at Svmoosium in Indian Art. LOS Angeles Cointy Museum of Art, to be published, 1972. (24) “The A4rts of India and Nepal: The Nasli and Alice Heeramaneck Collection,” Boston Museum of Fine Arts, 1966. (25) R. D. Harley, “Artists’ Pigments c. 1600-1835, Buttenvorths, London, England, 1970. (26) C. L. Butler and L. H . Cretcher, J . Amer. Chem. Sac., 51, 1519 (1929). (27) F. Smith and R . Montgomery,
“Chemistry of Plant Gums and Mucilages,” New York, Y.Y., 1959, p 106. (28) L. Masschelein-Kleiner and F. Tricot-Marckx, Bulletin Institzct Royal dzc Patrimoine Artistique, Brussels, 8, 180 (1965). (29) E. Stahl, “Thin-Layer Chromatography,” Academic Press, New York,
S.Y.,
1965. (30) F. Flieder, Stud. Conserv., 13, 49 (1968). (31) “Traite de Biochemie Generale,” Masson, Paris, France, 1952. (32) R. Watts and R. Dils, J . Lipid Res., 9 , 4 0 (1968). (33) F. D. Gunstone and F. B. Padley, J . Amer. Oi2 Chem. Sac., 42, 957 (1965). (34) R.A. Hites, *4NaL. CHEM.,42, 1736 (1970). (35) K . Serck-Hanssen, Acta Chem Scand., 21, 305 (1967). (36) G. H. Hutchinson, J . Oil Color Chem. Ass., 41,474 (1958). (37) E. H . Farmer, Trans. Faraday. Sac., 38, 340 (1942). (38) N. Stolow in “Application of Science in Examination of Works of Art,” Boston Museum of Fine .4rts, 1967. (39) “Profile Airflow,” Gemini G. E . L., Los Angeles, Calif, 1969. (40) Modern Plastlcs Encyclopedia, 1970-71, p 224.
