CHEMICAL PROBLEMS in the
FINE ARTS
RUTHERFORD J. GETTENS Fogg Art Museum, Harvard University, Cambridge, Massachusetts
I
T MAY seem strange to some that the field of the described artists' materials from the point of view of fine arts should offer chemical problems. Others, permanence; he laments the passing of the old guild however, may have read Professor Fink's recent system for the training of art'lsts in the proper use of address (6) on "Chemistry in Art" a t the time he re- materials. ceived the Perkin Medal for 1934. We shall try to Turning now to the preservation of works of remote avoid touching upon those particular problems which and near antiquity--and it is with these that this paper he has already so adequately discussed. That there is chiefly concerned-we may point out first that chemshould be a relationship between chemistry and fine istry can be of service to the archzeologist. The arts can easily be realized when one stops and considers archseologist of the past (there are notable exceptions, that the pictorial and plastic artist expresses himself in however) has contented himself in the discovery of substances and materials which are made from chemi- hidden and buried works of art and in their transporcal elements. The artist selects his materials prima- tation to museums for safe-keeping. In the majority rily for their physical properties, among which are color, of cases, he is saving these objects from complete detexture, hardness, toughness, and working qualities. struction. In other cases, he is only hastening the end. The more careful artist takes into account the chemical Many of the objects he finds, such as bronze, pottety, properties of his materials; that is, he considers their glass, frescoes, etc., have been preserved by the parcompatibility with other materials and their chemical ticular environment in which they are found. Many stability to the agencies of time. Unfortunately, many objects have come to an equilibrium with that environartists have given little or no attention to the chemical ment. When they are removed to a new and unfavorproperties of their working materials. In many cases, able environment in a foreign country they are often however, objects of art have suffered vicissitudes of subjected to conditions which bring about their swift time and environment which could not have been fore- and sure destruction. The archseologist of today seen by the artist. should equip himself with the knowledge and the maApplied chemistry should be as useful in attacking terials necessary to recognize and avoid such difticulties. the technical problems of the fine arts as it has been in It is nothing short of vandalism to take an expedition attacking the problems of the paper maker, the metal- into the field without the proper equipment for the lurgist, and the textile manufacturer. The point of preservation and the safe transportation of the objects view of applied chemistry is as important as the or- that are found. Several works (1)(18) (21) have been published recently on the preservation of archmlogical ganized knowledge it makes available. Chemistry has already made notable contributions objects in museums, but little enough has been said to the materials of the pictorial artist. Contrary to about field methods. Of more immediate and general interest are those popular impression, the artist today has a t his command a much better palette than did his predecessor in the problems which face the curator and the restorer of fourteenth and fifteenth centuries. The discovery of works of art in museums and private collections. These several new elements (cobalt, zinc, chromium, cad- problems have chiefly to do with all kinds of two-dimium, etc.) in the eighteenth and early nineteenth cen- mensional works of art. Chemistry can be of service turies led to the manufacture of stable and brilliant in these problems through the following lines of apcolors which have partially revolutionized the artist's proach: (a) in studying the destructive agencies which palette. New and better pigments can still be antici- have brought about the deterioration of such works; pated. Dr. Herbert E. Ives (15) has described the (b) in studying the exact present condition to see what specifications for a simplified palette requiring better progress the destructive agencies have made; (c) in pigments than any yet produced. As yet, chemistry determining the particular materials that have been has offered little in the way of improved media or ve- used in constructing a picture or other work of art with hicles, but in these days of synthetic protective coat- a view to selection of the materials most suitable for ings it can be predicted that something will turn up use in the process of restoration; (d) in deciding upon which will change fundamentally the painter's technic. the particular environment in which a work of art With but an elementary knowledge of chemistry, the should be kept to prevent further deterioration. The literature on the chemistry of art as applied to modern artist would be much more critical in his selection of grounds, priming, vehicles, and pigments paintings is not very extensive, yet it may interest the than he is today. Mr. Nod Heaton (13) has recently layman to learn how much has been written about 587
particular phases of this study. We have considerable contemporary technical literature from classical times down to modern times, but a discussion of these early sources of information is beyond the scope of the present paper. S i r Arthur Church (2) in the late nineteenth century gave us a now classic work on the chemistry of paints and painting. Professor A. P. Laurie of London has been studying the technical aspects of painting for over forty years and he has written several books (16) (17) and many papers dealing with painters' methods and materials and with the microchemical examination of pictures. Considerable interest along these lines has centered in Munich where Dr. A. Eibner and Dr. H. Hetterich have been investigating the pigments and media of old pictures. Dr. A. M. de Wid has published his work (3) on the application of microchemistry to the scientific examination of paintings. Periodicals devoted to the technical phases of art are not numerous. Technical Studies in the Field of the Fine Arts, a quarterly journal published for the Fogg Art Museum, attempts to cover recent developments in the field by means of articles, notes, book reviews, and abstracts. Mouseion, published by the International Museums Office in Paris, has scientific articles on museology in general. Our primary interest at present is in the study of paintings and in the problems that face the picture restorer. One of the phases of this study is the development of a technic for examining pictures to ascertain their present condition. If the preliminary examination shows that the condition is bad, further observations and study are made so that the proper remedy may be found. We believe each work of art that comes into the hands of the restorer should he treated as a clinical case-as a patient in a hospital. Detailed observations should be made, accurate and permanent records kept, and subsequent restoration should be based on the peculiar conditions a t hand rather than upon a general routine. MICROCHEMICAL ANALYSIS
Microchemical analysis has its place, along with radiography, ultra-violet examination, and detailed optical study, in the scientific study of works of art. It is often a necessary supplement to those other types of examination in getting information concerning the state of preservation, technical construction, origin, and authenticity of such works. Microchemical analysis can frequently answer specific questions as to pigments, mediums, primmgs, grounds, and nnderlayers of paint film. Visual and ultra-violet examination stop short a t the surface. The Rontgen rays are so penetrating that they reveal gross structure rather than minute detail. With the microscope and microchemical analyses one is able to peer beneath the surface and examine the interior. Since the microchemical examination of works of art is a comparatively new science, we may stop to consider some of the details of this interesting approach as applied to the study of pictures.
Getting a fair sample for analysis is one of the most diicult parts of the work. Of course, the analyst is limited to the takiig of a very small-that is, a microscopicsample. He must be sure that he does not mar the picture in any way. If one is only interested in the media or varnishes used, one can usually get a fair sample from the edge of a picture, which part is generally covered by the frame. However, if the analyst is interested in the pigments, he has very often to take his sample from the middle of the picture. If this is the case, he tries to take the sample from along some crack or blemish or other inconspicuous place where it will not be noticed. Sampling can be done with a small needle or harpoon. This operation can be carried out under a binocular microscope so that the operator can see at all times just what he is doing. Prof. Laurie (16) mentions the use of a small hypodermic needle which is cut off and sharpened and used like a miniature cork borer. We have been working on the development of a special instrument for this purpose (7). This instrument carries a cut-off hypodermic needle rigidly mounted in a manipulator in front of a hinocular microscope in such a way that the point of the needle can he turned and thrust into the paint film directly in the field of view. The sample is ejected by a plunger and is caught by a glass slide, which can be brought by means of a small gear device directly under the point of the needle. The sampling is usually done in a special examining room in the museum and the sample is then carried to the chemical laboratory in a small cupped slide. In the laboratory, the sample is examined with the aid of the chemical microscope. An equipment for the microchemical examination of pictures has already been described (11). If it is felt that there is no particular interest in the structure of the paint jihn the specimen may be examined on a glass slide directly, without mounting it. This is done with low-power magnification. First, behavior of the specimen to organic solvents is noted. To free the pigment from the medium, the sample is next dissolved in a 5 per cent. caustic soda solution. This reagent is particularly effective in disintegrating old linseed oil. When freed from the medium, the pigment particles may be examined for their optical properties, such as refractive index, color, hirefringence, etc. Particle size and shape are carefully noted. Very often pigments may he identified by their optical properties alone. This is done whenever possible since the sample is not destroyed and hence only very small samples are required. If the optical properties alone do not permit identification, recourse to chemical tests is necessary. The pigment particles may be dissolved and the constituents tested by various chemical reagents. All these tests can be carried out under a microscope. The identification of some pigments is comparatively simple. When there are complex mixtures it is sometimes very difficult and requires considerable time and patience. When the medium is soluble in organic solvents, it oossible to effectthe se~arationof ~ i m e n t is- sometimes A
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from the medium by means of a micro-extraction apparatus designed for the purpose (10). In this apparatus the tiny sample is held in a small platinum filter cone and subjected to the percolating action of hot organic vapors arising from a reservoir below and condensed in a cooling chamber above. Even more difficult than the identification of pigments is the identification of media and varnishes. This is just as important as, if not moreimportant than, the identification of pigments, for these reasons. In the first place, the various technics employed down through the centuries differed in respect to the media used and not in respect to the pigments. It is really the medium that determines the technic of painting. A limited number of media have been used in the past. Beeswax was used in classical times; it can still he identified on certain ancient paintings found in Egypt. Egg yolk and egg white were the media used by the early Italian masters. Linseed oil did not come into general use for panel painting until the fifteenth century. Mixed technic employing underpainting in egg tempera and overpainting in oil were known. Some emulsions were used. Casein, glue, gums, and resins have all had their use in the construcbon of pictures. Secondly, the restorer is a great deal more concerned with the medium used than he is with the pigments, because it is much more liable to be the failure of the medium and the varnish than the failure of the pigment that has led to the deterioration of a picture. I t is more difficult, as we have said, to identify the medium used. The media are not so diverse as the pigments, but they are much more complex in themselves. They are all organic substances. There are no outstanding color reactions for any of them. There are, to be sure, some color tests for the proteins as a class, and for the resins, but these are of limited service on a miaoscopic scale since they lack intensity. Nitrogen, common to all proteins, can he identified in very small amounts. The microchemical identification of phosphorus, in a simiiar way, helps to establish the presence of egg yolk and casein. Tannic acid is fairly specific for animal glue, and precipitation with alcohol for gum arabic in aqueous solution. Preferential staining with organic dyes promises to be of assistance in differentiating proteins and drying oils. It is only recently that attention has been directed t o the importance of the identification of media in old paintings. Dr. Eibner (4) and Dr. Hetterich (14) have recently outlined the present status of this particular phase of picture study. Occasionally, it is desirable to study the structure of the paint film itself. The paint film, from the ground up to and including the varnish, usually consists of different layers. We often desire to know just what these layers are, and the relationships between them. A technic for findmg out has been developed. A very small chip of the paint film, barely visible to the naked eye, is embedded between two small blocks of para&. The block of para& is mounted in a microtome and a section is cut with a razor through the paint films at
right angles to their flat sides. This gives a plane surface which may he viewed by reflected light. By this means very interesting facts concerning the method and materials used in painting the picture are disclosed. One has also an excellent opportunity to get permanent records in the form of photomicrographs. The question naturally follows here: Of what value is the information that is obtained in microchemical examination? In the 6rst place, as has been stressed above, the facts obtained are of immediate concern to the restorer. A knowledge of the media enables h i to clean the picture more intelligently and a knowledge of the pigments enables him to use, in retouching, pigments that were used by the artist and thus avoid changes in values which frequently develop with fnrther aging of the restored areas. I n time, the miaochemical examination of many pictures should contribute much to the history of the technic of painting. Little enough is known at present as to just how the artists of different periods painted certain types of pictures. Miaochemical examination promises to be one of the most searching tools for prying into methods of technic. Furthermore, microchemical examination very often throws light upon the authenticity of pictures. We believe, however, that attempts to determine the authenticity of objects of art by microchemical examination alone is a very hazardous undertaking. Such examination, in certain instances, has contributed important facts and furnished certain evidence which has been of aid to the art critic. Occasionally, fakes can be-detected through the presence of a pigment because it is definitely known that such pigment was not used in the period that the fake is supposed to represent. Modern copiesunless the copyist is very clever -can sometimes be spotted by this means. PRESERVATION AND RESTORATION OF PAINTINGS
The preservation and restoration of paintings constitute a field which offers many problems for attack
from the chemical point of view. Only a few of these problems are purely chemical, hut the equipment and methods of a chemical laboratory are very useful in prosecuting their study. We have, first, the varnish and protective coating problem. For centuries it has been the custom of painters and restorers to cover paintings with some clear varnish. This is done to give them a h i s h and to protect the painted surfaces from moisture, corrosive gases of the atmosphere, and dust. There is little doubt that in many cases certain varnishes that have been used on pictures have done more harm than good. The dark-brown, muddy tone of many old pictures is usually nothing more than dirty and discolored varnish on the surface. Many varnishes bemme yellow with age. Many have a d i e r e n t coefficient of thermal expansion and a different shrinkage rate from that of the paint film below. This is the most probable cause of much of the disfiguring crackle on pictures. There is no question, however, that there is a need for some protective mating which can be applied to the surfaces of pictures to protect the paint film from dust, the actinic rays of the sun, water vapor, and such corrosive gases of the atmosphere as hydrogen sulfide and sulfur dioxide. At the International Conference for the Study of Scientific Methods for the Examination and Preservation of Works of Art, held in Rome in October, 1930, it was mmrnonly agreed that the greatest prohlem in restoration today is that dealing with protective coatings. A Committee on the Restoration of Paintings and the Use of Varnish was appointed. This committee drew up a set of specifications for the ideal varnish. These specifications are: (1) it should protect the picture from atmospheric impurities; (2) its cohesion and elasticity should be such as to allow for all ordinary changes in atmospheric conditions and temperature; (3) the elasticity of the paint film and tissues under the varnish should be preserved; (4) it should be transparent and colorless; (5) i t should be capable of being applied thinly; (6) it should not bloom; (7) it should be easily removable; and (8) it should not be glossy. It is further added: "Concerning the question of the best protective coating for a picture the Committee cannot recommend, a t present, one that fulfils all the qualities specified." To find a varnish that will come up to all these specifications is a pretty large order. To our knowledge, the industrial laboratories have not been called upon to make a am-forming material that will meet such specifications. There are possibly many film-forming materials, both natural and artificial, which have never been seriously studied by the industrial laboratories interested in protective coatings for the reasons that they are available only in small quantities and have a restricted use or because they cannot be developed economically for commercial purposes. Such materials will have to be developed or investigated in academic laboratories. The cost of restorative materials is of small importance as compared with the value of some works of art-or even the labor spent in preserving
them. We are attempting to make some coutributions to this problem. Quite a large number of materials have been used for protective coatings. Spirit varnishes, made by dissolving natural resins, such as mastic, dammar, sandarac, and some copals, in turpentine or other organic solvents have been used from the time of the old masters to the present. Linseed oil, balsams, and waxes have enjoyed a minor use. More recently, the colorless synthetic resins have been suggested and they show some possibilities. A question is frequently asked when a. particular protective mating is considered: How permeable is such a film to the moisture of the atmosphere? We have conducted a series of experiments which have thrown some light on this p d c u l a r question. To study this problem, it was necessary to prepare thin a m s of equal thickness from a large variety of spirit varnishes, oil varnishes, and lacquers. This was done by dipping a thin, transparent paper made from hydrated spruce pulp into a solution of the varnish or lacquer. After draining and drying, a small disc of the 6lm was cut, having a thickness of about 0.002 inch, measured by a dial gage. This film was clamped over the mouth of a bottle between two flat metal discs having an orifice 30 mm. in diameter. The bottle, holding about 10 cc. of water, was placed in a dry chamber held a t about 5 per cent. relative humidity and a t a constant temperature of 3 2 T . The bottles were weighed each day and the loss in weight of water for each 24 hours was taken as indicating the permeability of the film. These measurements show that the natural resins as a class are about 10 times as permeable to moisture as the waxes, like beeswax, and that the drying oils and synthetic resins are about 100 times as permeable as beeswax (9). At the present time, a test chamber with which we hope to get some information on the durability of protective coatings and the relationship between protective coatings and paint films is in use. In this insulated test chamber is a metal drum which holds a number of test panels; these panels can be subjected to a cycle of periods of high and low humidity and high and low temperature. The procedure is much like that used in the accelerated testing of commercial paints, except that the factors are not quite so intense, but they cover a longer period of time. One serious difficulty encountered in connection with old pictures painted on canvas is that the canvas becomes brittle and unsafe for supporting the paint 6lm. A picture in such condition must be lined. This operation consists in simply joining a new canvas to the back of the old one by means of a lining adhesive. Here the question of the choice of adhesive arises. Different restorers use different adhesives. Some are of doubtful value. Some restorers use glue, others waxes, and still others mixtures of various kinds. In order to settle in our own minds the proper choice of lining adhesive we set under way some time ago a series of experiments (25). the purpose of which was to test these adhesives. We took pieces of old canvas and
FIGURE2.-THE ILLUSTRATION (a) IS A CHINESE PAINTING DONEON A MUD WALL Before transfer it was very fragile and would not support its own weight; ( b ) shows the painting after the paint film and priming were transferred to an asbestos-cement board and cleaned.
joined them with new canvas a t the back, using a va- of a softener rests chiefly upon its physical properties riety of the commonly employed lining adhesives. These and its mechanical practicability, considerations as to lined canvases were placed in a closed chamber where chemical stability and compatibility with the subthe atmosphere was saturated with water vapor a t stances of the old paint film cannot be ignored. Still another problem facing the restorer is the choice room temperature. After some weeks they were taken out. Some had mildewed badly. Some showed cleav- of solvents for the removal of old and discolored varage planes in the lining adhesive. The tests showed nish films from the surfaces of pictures without harmquite convincingly that adhesives of the wax and wax- ing the details of the painted surface and with no harm resin type are better than others. Of course these to delicate glazings that may be present. In other specimens were subjected to exaggerated conditions. words, the "skinning" of the picture must be avoided. However, the test does furnish us with some definite Some old varnish films are very refractory, while others information on the question. More work must be can be removed without diiculty by the selective acdone to settle this problem. These tests may not tell tion of organic solvents. A study of the use of solvents us what lining adhesive is best for all conditions. They as related to problems in cleaning and restoration is planned. may tell us only where the best compromise lies. Frequently, it becomes necessary to transfer an old Directly connected with the question of lining adhesives is the brittle and hardened nature of old paint paint film from the ground upon which it has been films. Before lining a painting having a cupped, painted to a new ground because of the complete discracked, and brittle surface, it is frequently necessary integration and decay of the old ground. The probto soften the paint film so that it will not crack further lems in such work are largely mechanical. There is, under the pressure required in joining the old and the however, one type of transfer which has drawn our new canvases. At the present time, linseed oil and particular attention and that is the transfer of Oriental various mixtures containing such softeners as glycerin, wall paintings (24). These wall paintings, many of sugar sirups, and essential oils are used. All of these which are several centuries old, are usually painted on have their shortcomings. It is going to be a real prob- a dried clay ground. As this ground offers such a weak lem, one requiring a study into the fundamental na- support, i t is often necessary to transfer the paint film, ture of old paint a m , to find a softener which is satis- and that alone, to canvas or other ground. Here it factory from all points of view. Although the choice was necessary to find a satisfactory h a t i v e which
would bind the particles of the friable surface together Europe start to blister after they reach this country and in reality make a new paint 6lm of it. After some because of the greater extremes of humidity prevalent experimental work, we chose for this purpose one of here. In many museums a lively interest is being taken the new synthetic resins, a polymerized vinyl ester. in this important question. The artificial regulation This resin is a satisfactory fixative, it does not discolor, of the humidity in museums appears to be the only and, best of all, if used in moderate amounts, it does answer. Eventually museums will adapt the humidity not greatly change the character of the surface. After of various rooms or even exhibition cases to the specific the surface is sprayed with the vinyl ester dissolved in objects they house. Mr. G. A. Rosenberg (23) and a suitable solvent mixture, a cloth and paper facing is Mr. J. Macintyre (19) have both written recently confixed to the surface with glue or other water-soluble cerning the importance of control of humidity in the preservation of adhesive. With antiquities. the surface now Directly reprotected a t the lated tothissame front, the piece problem is that is turned over of mildew and and the clay is fungus action. removed from Several instances the back, great of the destrnccare being taken t i v e action of when the paint mildew on paint6lm is reached. i n g s h a v e reWhen the back cently come to has been made our attention. properly smooth, Mildew will atit is joined with t a c k pastels, the aid of a press paintings on silk, to the canvas or water colors, and other ground even oil paintwith more of the ings, if they are vinyl ester. Afstored in damp ter drying, the places during the facing is removed summer. Aninwith water, the teresting probsurface cleaned, lem of this kind and holes a n d arose in connecfissures f i l l e d tion with the with a clay lutPanama Canal ing. The canmurals a t Balboa vas-supported Heights, Canal pieces are now Zone (26). Acjoined and fas(a) By transmitted illumination, and (6) by oblique illumination showing different cording to the tened to a large zones and layers: (1) the metallic copper core, (2) cuprous chloride, (2a) silver l a t e s t informareenforcedwoodchloride. 126) imoure cu~rouschloride. 12c) silver chloride. (2dl limonite.. fZe) . , Dure . cuproui
