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THE MEDIUMS USED IN PAINTING
THE mediums used by artists to mix with their pigments are very much the same to-day as they have been through long periods of time.
For wall-painting the pigments, merely mixed with water, are laid on the wet lime surface of the plaster, the binding medium being the crystallized carbonate of lime, which is slowly formed by the combination of the carbonic acid gas in the air with the lime. Mr. Noel Heaton has shown that this method was used in painting the frescoes in the Palace of Knossos, and the weight of evidence and research is in favour of the same method having been used at Pompeii. The frescoes of the Italian Renaissance were painted by this method. Technical details have varied, but the principle of using the carbonate of lime to form the binding material is very ancient.
Gum-arabic was used by the Egyptians, and is used by artists to-day for water-colour painting, the pigments being ground in this medium.
Size was used by the Egyptians, and by the Greeks and Romans, and throughout the Middle Ages for wall decorations. To-day it is used principally by the house-painter and the scene-painter.
The history of the egg medium is more obscure. Its use for certain special purposes is mentioned by Pliny (A.D. 28—79) and in manuscripts of the Middle Ages; but it is not till we come to the treatise on Painting by Cennino Cennini, at the beginning of the fifteenth century, that we find a full and detailed description of the use of the yolk of the egg as a painting medium. The pictures of the painters of Italy during the fourteenth and fifteenth century, and probably earlier, were principally executed in this medium.
The first mention of the use of the drying vegetable oils as media for painting occurs in the manuscripts of the eleventh or twelfth century. There is evidence of a long northern tradition in the use of this medium before the fifteenth century, after which it gradually replaced egg and became the universal medium.
Only one medium used in classical times has fallen out of use, namely, beeswax. Pliny tells us how this medium was used. The pigments were stirred in with the melted wax, and the work executed partly with the brush and partly with bronze modelling tools. In order to paint with this medium in a cold climate the panel or canvas must be artificially warmed. Each stroke of the brush must be rapidly put in place and cannot be altered.
Mr. Burns has painted quite successfully in this medium, the canvas being placed with its back to a hot fire. The finished picture, when polished with a cloth, closely resembles an oil painting. Wax is a fairly permanent medium, but readily collects dust and dirt. Mr. Burns tells me that his picture painted twenty years ago is in excellent condition; but owing to the wax accumulating dirt, he varnished it some time ago, quite successfully, with copal oil varnish. This confirms the statement made in the fifth century by the physician Ætius, that wax pictures should be varnished with a drying oil.
Examples of wax pictures, on the whole in excellent condition, which were found by Professor Flinders Petrie in Egypt, are of about the second or third century, and are to be seen in the National Gallery. With the exception of such pictures preserved by the Egyptian sand, I am not aware of any others. Possibly the examination now going forward of the Russian ikons may reveal some other early examples of wax paintings. Encaustic painting with wax dissolved or emulsified in a volatile medium was at one time fashionable ; it is quite different from the classical technique. An interesting account of the attempts made to revive wax painting will be found in the History and Methods of Ancient and Modern Painting, by James Ward.
The Lucca manuscript of the eighth century mentions only two mediums for painting—wax for painting on wood and glue for painting on parchment. I have already stated that the first description of a vegetable drying oil as a painting medium occurs in a manuscript of the eleventh or twelfth century. This suggests the possibility that the use of drying oils for painting was discovered some time between the eighth and eleventh century.
It is sufficient for our purpose that the mediums used by artists to-day have the tradition of centuries of use behind them, and that modern chemistry has not up to the present discovered any other medium, with the exception of casein, that has been found suitable by painters.
Of these mediums the most important is oil, and it might be supposed that after so many centuries of use there would be nothing new to say of the properties of this medium or of its correct use. Unfortunately owing to loss of studio tradition, less is known to-day of some of the properties of this medium and of its correct use than was known to the Van Eycks and their followers, and the main purpose of this book is to discuss the properties of the oil medium and how it should be used. The drying oils first known to painters were linseed oil, expressed from the seed of the flax, and walnut oil, extracted from the kernels of the walnut. Hempseed oil is also mentioned. Later on poppy oil was added to the list. To-day walnut oil is little used, linseed oil and poppy oil being used in grinding the pigments by the modern artists' colourmen.
Light, the oxygen of the air, and water vapour convert these oils when exposed in thin layers into a tough, elastic, transparent solid which consists principally of a substance which the chemists have named linoxin. We shall have to discuss this drying process in more detail later on, as the conditions of drying must be closely studied to avoid the danger of cracking. The film of dried oil has two properties which are disturbing to the painter. It becomes of a brownish yellow with age, and it apparently has the property of making the pigments gradually more translucent and deeper in tone.
It is to these causes that the lowering of tone of pictures painted in oil is due. It is therefore essential that the artist should have a thorough understanding of how these changes are going to affect different pigments and different methods of painting, in order to avoid serious lowering of tone and serious changes in the whole colour scheme.
The story told by Vasari of the discovery of how to paint in oil by the Brothers Van Eyck has misled artists for generations. So supreme a technical result so quickly arrived at, suggested a secret and lost medium, and Vasari himself hints at such a mystery. But when we realize that Van Eyck was the final expression of craftsmanship to produce a certain æsthetic result, with some three hundred years of experience and tradition behind him, we look rather to a study of his methods than of his medium for an explanation of his results.
The main facts are, that, with the exception of an occasional obscure reference or isolated recipe, the account of how to grind pigments in oil and to paint in oil is essentially the same in the writings of Theophilus and Eraclius, in the twelfth century, in Cennino Cennini and other fifteenth-century manuscripts, and in Vasari in the sixteenth century. Chemists may dispute as to refinements, and the results to be obtained by different methods of preparing the oil described in these old recipes ; the extent to which resins were dissolved in the oil; and what diluents were known. These details we may leave aside for our present purpose.
The accumulation of evidence is in favour of the conclusion that these painters were painters in oil, but probably on a solid under-painting in egg ; the extent to which this solid under-painting was carried being a matter for discussion. A study of these and later pictures, more especially those that are half-finished, reveals a supreme knowledge of the behaviour and properties of the dried oil film. No sharp line can be drawn between the perfection of preservation of the pictures of Van Eyck and his followers, and of some, at any rate, of the pictures of the later painters. More especially is this true of the Dutch school, at a time when the oil medium was undoubtedly firmly established.
There can be no doubt that oil pictures painted on wood are much more brilliant and better preserved than pictures on canvas.
The first step necessary for the student to-day is to learn something of elementary optics before he can understand how to handle the most pliable and, at the same time, the most treacherous of mediums.CHAPTER 2
THE WRITTEN EVIDENCE ON EARLY PAINTING METHODS IN OIL
IN the following chapters I have tried to bring together as shortly as possible such documentary information of which I am aware on early methods of oil painting Before beginning the account of such evidence as I have been able to collect, it is, I think, necessary to state how, in my opinion, such evidence should be used.
In the first place it is probable that the written evidence is both inaccurate and incomplete. The examination of the innumerable recipes published in early manuscripts compels one to the conclusion that in most instances the writers of the manuscripts were merely compilers, and had little practical acquaintance with the methods in use in the studio. Even when the writers are themselves painters, it is notorious that the account of technical processes written by those engaged in using them is often unreliable, and that essential details are omitted owing to the writers' familiarity with them, resulting in irritating obscurities. We must therefore take such information for what it is worth, and not regard it as conclusive.
In the next place it is essential that this information be examined in a purely scientific spirit, without the making of unjustified assumptions. Unfortunately, too many writers on this subject have begun with a preconceived theory, and have had to twist historical evidence to prove a particular thesis. Let me give an example of how I think such evidence should be used. Dioscorides describes the preparation of nut and poppy oil. Oleum Cicinum was also known. These oils were therefore known in his time, but neither he nor Pliny mention their property of hardening into an elastic film, or their use as a medium for painting or the making of varnishes. This being so, until either written evidence is obtained, or the examination of contemporary works of art has proved the use of drying oils for such purposes, we must assume that in the time of Pliny the technical use of such oils for paint mediums and varnish was not known. Any assumption beyond this is mere speculation, and leads nowhere.
So far as I know up to the present, no such use of drying oils has been found in the examination of objects of this or an earlier time. It is true that Greek physicians mention the astringent properties of linseed, but are using the phrase in a medical sense, and it is also evident that a linseed poultice was known in his time, but there is no mention of the extraction of linseed oil.
The first mention of a use of a drying oil is made by Ætius in the fifth century. He describes the preparation of linseed oil, and after describing the preparation of walnut oil, states that it is used by gilders and encaustic painters to preserve their work owing to its property of drying. The first description of the preparation of an oil varnish, by dissolving resins in a drying oil, is found in the Lucca Manuscript, supposed to be of the eighth century, a recipe which is similar to that given in the Mappæ Clavicula.
It is not till we come to the manuscript of Theophilus, supposed to be of the eleventh or twelfth century, and the manuscript of Eraclius, supposed to be of about the same date, that we find an account of the use of a drying oil as a paint medium.
Certain vegetable oils on exposure to the air in a thin film have the property of absorbing oxygen from the air and being turned into an insoluble elastic film. It is impossible to draw a sharp line between a drying and a non-drying oil ; but it is sufficient for our present purpose to note that poppy, walnut, and linseed oils were known as drying oils in mediæval times, and these are the oils used by artists to-day.
The oil as extracted from the seed or nut by heat, pressure, or by boiling with water, is far from pure, and contains mucilage and other impurities. Such an oil dries very slowly. It can be rendered more siccative by the following processes—boiling, passing air through it, and exposure to sun and air. All such processes also purify the oil, many of its impurities separating during the treatment. If the oil is repeatedly shaken up with water and exposed to sun and air, the mucilage and other impurities are separated, the oil is bleached and at the same time becomes more siccative. Such methods of purification are, we shall find, very old, and are practised by some artists' colourmen to-day.
The oil so prepared dries quite sufficiently fast to be used for modern oil painting. There are also modern chemical methods of purification which are used for commercial oils.
If it is required to hasten still further the rate of drying, the oil is boiled, or mixed in the cold and then exposed to light with certain metallic compounds such as litharge and white lead and compounds of manganese and cobalt. These compounds dissolve to a slight extent in the oil, and render it more siccative. An oil varnish is a solution of a resin in a drying oil.
These general facts about drying oils will enable us to follow the account which I now proceed to give of the old recipes.
The bleaching of oils by the sun was known in the time of Dioscorides, and the boiling of oils with litharge was known in the time of Galen (A.D. 103—193).
I shall quote in the first instance from the manuscript of Theophilus :—
"Take linseed and dry it in a pan, without water, on the fire. Put it in a mortar and pound it to a fine powder; then replacing it in the pan and pouring a little water on it, make it quite hot. Afterwards wrap it in a piece of new linen; place it in a press used for extracting the oil of olives, of walnuts, or of the poppy, and express this in the same manner. With this oil grind minium or vermilion, or any other colour you wish, on a stone slab, without water ; and with a brush paint over the doors or panels which you wish to redden, and dry them in the sun. Then give another coat and dry again. At last give a coat of the gluten called vernition, which is thus prepared."
"Take any colours which you wish to apply, grinding them carefully in linseed oil, without water; and prepare tints for faces and draperies, as you did before in water colours; distinguishing, according to your fancy, animals, birds, or foliage with their proper colours."
"All kinds of colours may be ground in the same oil and applied on wood, but only on such objects as can be dried in the sun. For having applied one (coat of) colour you cannot add another until the first be dry, which in images (figures) and other paintings is too long and tiresome."
"All colours employed on wood, whether ground in oil or in gum (water), should be applied in three successive coats. The painting being thus completed, place it in the sun, and carefully spread over it the gluten vernition. When this begins to flow with the (sun's) warmth, rub it gently with your hands. Do this thrice and then let it remain until it is thoroughly dry.
"There is also a kind of painting on wood which is called translucid, or by some, golden ; it is produced as follows : Take a sheet of tinfoil—not varnished nor tinged with yellow, but in its natural state, and carefully polished—and line with it the surface which you wish to paint. Then having varnished the foil, grind colours very finely with linseed oil, and spread them extremely thin with the brush; so let the work dry."
Excerpted from The Painter's Methods and Materials by A. P. Laurie. Copyright © 1988 A. P. Laurie. Excerpted by permission of Dover Publications, Inc..
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