MATERIALS & METHODS
Thompson, Daniel V., Jr., Research and Technical Adviser, The Courtauld Institute of Art, University of London. The Practice of Tempera Painting. New Haven: Yale University Press. 1936. Fourth Printing, 1946.
Properties of Wood
Wood has certain properties which make it a desirable carrier for painting. It is easily cut and worked to shape for flats, moldings, and carvings: it is strong, not easily bent or broken; and its fibrous structure is readily penetrated by a hot size solution, so as to form an excellent bond between wood and gesso. The disadvantage of wood lies in the unsymmetrical arrangement of its fibers, its grain.
Under the influence of moisture, wood expands, chiefly across the grain; and under the contrary influence of dryness, it contracts in an opposite degree. Humidity fluctuates in most environments, and a wooden panel is therefore constantly subject to expansion and contraction. When these changes are gradual, the wood keeps step with them, and does not suffer; but when they are sudden, a panel will [p. 9] often warp, or split, or change so rapidly as to loosen the bond with the ground upon it. Conditions of moisture at the front and back of a panel may easily be very different, with the result that while one face is being expanded, the other is standing still, or even being contracted. Thus, a panel on a cold wall may be damp at the back from condensed moisture, even though the front be exposed to air dried out by heat or frost. These conditions may to some extent be controlled; and to that extent wood may be used successfully for paintings. [pp. 8,9]
Effects of Humidity
The medieval painter did not have to consider the effects of central heating. In medieval buildings, unheated, atmospheric changes were gradual. The temperature shifted somewhat with the seasons, but the changes were slow, and the humidity of the air was not subject to such violent modifications. In the open atmosphere of an unheated stone building, wood is an admirable base for painting. In a modern house, often hot by day and cold by night, with corresponding sudden changes in relative humidity, parched in winter, and often swollen with damp in summer, wood may crack and warp disastrously, no matter how well it has been seasoned. If air-conditioning in modern buildings becomes a general practice, it may be possible once more to use wood generally for painting; but at present it is risky for any but small works, unless a stable environment can be foreseen. [p. 9]
Selection and Conditioning of Woods
If wood is to be used, a soft wood should be chosen, as Cennino advises. Birch, maple, and oak may be used, but they are heavier, harder to work, more costly, and seem actually to give a somewhat less good bond with the gesso than the soft woods. Bass, whitewood, poplar, and the bay which is sold as mahogany are satisfactory, as [p. 9] long as they have been properly conditioned. Pine, spruce, and fir must not be used; for they contain soft resins which tend to stain the ground and the painting. Whatever wood is chosen should be cut into planks somewhat thicker than the panels to be made from them. It should then be spread out on rafters to dry, and shifted and turned about at intervals for at least a year. This seasoning should take place in a normal atmosphere, the sort of atmosphere in which the panel may be expected to stand after it is done. At the end of a year or two, the seasoned planks may be made up into panels. They will probably have warped a little, and some planing will be necessary to make the panels flat. After this, the wood will probably warp again, so the panels should be placed on the rafters, fully exposed to the air on all sides, for several weeks, and then allowed to stand about the studio for several weeks more, so that their behavior may be watched. If there is any warping, they may be planed down again, and the seasoning continued. It is folly to attempt to paint on a panel which has not demonstrated its stability over a substantial period of time.
One might suppose that by securing old wood, using old table tops, paneling, floor boards, which had seasoned for years, this process might be shortened. So it may; and this is excellent practice. But the use of old wood does not eliminate the need of further seasoning if the surface is interfered with by sawing or planing. No matter what the wood may be, once the panel is made up it should be watched for some months before anything is painted on it, and even before it is gessoed. I have seen a twenty-four-inch panel cut out of an eighty-tear-old walnut table-top warp in an arc two inches high, though the wood had stayed perfectly flat in the table-top for eighty years before. [pp. 9-10]
Casein Glue for Joining
If it is necessary to glue up a panel, the best thing to use is the cheese and lime glue which Cennino describes in Chapter CXIL. [The Craftsman's Handbook, p. 68.] I have seen a medieval panel, made up with this cement, which was  so completely eaten away by worms that the cement which originally joined two parts together stood out in a thin ridge half an inch high, with the marks of the grain of the wood visible on both sides of it. It is durable, adhesive, and, once dry, insoluble in water. This insolubility is a great advantage; for the surface of a panel is kept moist for a long time in the process of gessoing, and ordinary glues tend to soften dangerously if they are kept damp. If the panel is so wide that it cannot be cut from a single plank, two or more planks may be put together with this glue, with broad clamps to hold them while it dries, of course. Moldings may be glued to the panel with it, and nailed through in addition with copper nails, or brads. There are several proprietary casein glues on the market now which are good and useful, but for gluing wooden panels Cennino's lime-casein recipe is better. The alkaline principles of the trade casein glues are more soluble than lime, and may do damage; and the dried film which these products leave behind resembles a glue, while the lime mixture resembles a cement, and is better suited to use in a joint between two pieces of wood. Cennino's recipe is easily followed, using any lean cheese, "mouse cheese," or even "cottage" cheese, consisting of fresh curds strained from the whey. Be sure to get pure lime, and not the compound material called "Hydraulic lime" which many builders use instead. [pp. 10-11]
Precautions against Warping
Several methods are used to control the movement of wooden panels. The most satisfactory, when well performed, is cradling. To form a cradle, high, narrow strips of wood are glued to the back of the panel in the direction of the grain. In these strips, notches are cut, to fit next to the panel, and into these notches transverse strips of wood are fitted but not glued. These transverse strips act as a firm but gentle spring, resisting the tendency of the panel to warp. Good cradling is rare. Most workmen err in making the grain-wise strips too wide. The design and arrangement of the fixed strips, the location of the transverse strips and their adjustment in the notches, call for expert workmanship and judgment which not every carpenter possesses. [p. 11]
People who do not know the character of wood often suppose that it can be kept in shape be strong measures. A student of mine once fastened bars of iron securely to the back of a narrow panel, across the grain, believing that they would make it impossible for the wood to warp. The wood did not warp; it couldnÍt--so it cracked instead! Warping may be discouraged by the gentle action of a good cradle; but this expedient is applicable only in the case of well-seasoned wood. No cradle can be expected to oppose the violence of wood in which the sap has not been properly dried and set. The seasoning of wood brings it into equilibrium with the atmosphere, and well-seasoned wood will accommodate itself to gradual atmospheric changes of limited extent without difficulty and without special precautions. If the changes are rapid or extreme, as they often are under modern conditions, a good cradle is needed to keep the panel in shape while they act upon it. [pp. 11-12]
Another method of treating wood for use in panels is by lamination. If a panel is built up in layers, with the grain of the wood in one layer opposing that in the next, it is possible to make a beautifully stable thing of it. Unfortunately, far more bad panels of this sort are made than good. The arrangement of the grain is often ill considered; inferior and badly seasoned woods are often employed; and the adhesion among the component parts is often far from perfect. I once purchased a plywood panel from a firm of colormakers of international reputation and had the disconcerting experience of seeing it come apart, layer by layer, in the course of little over a year. I have been well pleased with a plywood made for airplane manufacture out of very thin sheets of rotary-cut veneer, birch and [p. 12] mahogany, cemented together with a blood-albumen adhesive chemically not unlike Cennino's cheese and lime glue. This kind of plywood is prepared for oil painting with a special gesso by an American artists' colorman. None is offered for sale, as far as I know, ready prepared with a proper gesso for tempera. Cheap, nondescript plywoods must be avoided at all costs; for they are sure to blister or peel. A good cabinetmaker can put together splendid laminated panels to order, if he can be persuaded to interest himself in the painter's problem. He will usually prefer to use hot hide glue to fasten the layers together, and this is good practice. If he is open to suggestion, he may be furnished with a supply of cheese-lime glue, and this is even better. If he shows signs of wanting to glue the layers with cold liquid glue, he is not a good cabinetmaker, and should be shunned; for he cannot be cured. [pp. 12-13]
Wood Pulp Boards
The natural structure of wood can be broken down mechanically or chemically, and the fiber converted into artificial boards. There are many boards of this sort on the market, for the building purposes; but few of them are good for painting. If wood is ground up, it has to be put together again with an adhesive such as glue, and the resulting product is both too weak and too hygroscopic for a large panel of reasonable thickness. Poor wood and low-grade glue are usually employed in these ground wood pulp boards. Still worse are the boards made from wood reduced to pulp by the action of chemicals. Sulphites and chlorides remaining in the pulp are injurious to the painting and to the fibers of the panel itself, and boards of this sort often disintegrate within a few years, turning yellow and brittle like old newspaper--to which they are, after all, equivalent. Good pulp boards are made, without the addition of any foreign adhesive, out of wood alone, by subjecting the pulp to the action of superheated steam and hydraulic pressure. The substance of the wood itself is made sufficiently cohesive by the moisture and heat to be welded together by the action of the press.
I am reluctant to recommend any commercial article by name, partly because of a strong prejudice against seeming to advertise a [p. 13] trade product, and partly because conditions of manufacture are not necessarily permanent, and what one has found good in the past may not be found good in the future. In view of the difficulty of finding satisfactory materials for panels at short notice, however, I will say that my students and I have for seven or eight years used a commercial building board made under the name "Masonite," both a thin sort called "Prestwood" and a thicker, called "Quarterboard." I cannot take the responsibility of recommending these products; but I know of no objection to them as they are now produced, and my experience of several hundred panels made from them has been entirely satisfactory. For large works, two or three thicknesses can be glued together. This material can be steamed and bent or pressed into any required shape. [pp. 13-14]
Closely related to the wood pulp board is linen paper, made from linen rags beaten and shredded into pulp, and a small a mount of size to act as a binder. Paper is, of course, flexible; and rigidity is essential in a carrier for the nonflexible gesso. Good linen paper would be an admirable carrier for paintings of moderate size if it were solidly mounted and protected from the mechanical danger of puncture. This can be done by stretching the paper on a wooden frame, with a sheet of aluminum between the paper and the stretcher. The edges of the metal should be rounded off; and it should fit the stretcher exactly, but should not be fastened to it. The paper may be damped, stretched, and pasted or glued in the usual way, and tacked to the frame for good measure. Panels of this sort may be made easily up to 20" x 24". Without the metal backing, the stretched paper would not be firm enough to stand the pressure of scraping or burnishing, and would be easily pierced by a blow from front or back. An alternative method is to stretch the paper over a sheet of aluminum which in turn lies on [p. 14] a sheet of plywood or building board, gluing the turnover of the paper to the back surface of the board. A steel-faced plywood is made for airplane manufacture, and may be used for this purpose, though it is rather heavy. Paper is best backed with metal, and should not be fastened to it, but only stretched over it. In all cases, a thick, pure linen, handmade paper should be employed. [pp. 14-15]
There have been many efforts to use metals themselves as carriers for gesso grounds; but they have not generally been rewarded with success. The chief difficulty is, of course, to make the gesso adhere properly to the metal surface. But even when this can be accomplished, the result is not altogether satisfactory. Except for aluminium and magnesium, and alloys of those, metals in panels of sufficient thickness to be rigid are usually objectionably heavy. And if it should ever be necessary, for any reason, to remove the painting and ground from the original carrier, as must sometimes be done with old paintings, a metal panel makes heavy labor for the restorer. A successful method was patented some years ago for making panels with oil-gesso on an artificially roughened aluminium surface. In general, however, the lack of porosity in metals makes the bond with the ground uncertain; and I know of no reliable method of overcoming this difficulty which would commend itself to a practicing craftsman. [p. 15]
Gesso grounds for painting need not necessarily be rigid. It is possible to paint in tempera on canvas, as Cennino directs. [The Craftsman's Handbook, p. 103.] The best [p. 15] , material for this purpose is a fairly closely woven linen crash or twill, preferably the later, as it has a firmer structure. This should be washed thoroughly, to remove the manufacturer's sizing, and to make it change as much as it will. Two washings with hot soapy water, and thorough rinsing and drying after each, should be a minimum of preparation. Tack the linen to a canvas stretcher, taking great care to get it evenly taut all over, and to have the tacks properly lined up with the weave. Size it with one to sixteen gelatine solutions, made as described later in this chapter [see documents on sizing]; and when the size is dry, put on one coat of gesso sottile [see document] made up with some of the same one-to-sixteen size, adding a teaspoonful of white granulated sugar to each eight ounces of the size. The sugar makes the ground a little hygroscopic and hence flexible, but should not be used in excess. The gesso is applied to the stretched, sized canvas with the blade of a spatula, and worked into the grain. Then as much as possible is scraped off again with the blade. If an ordinary oil canvas stretcher is used, the canvas will be badly marked unless the upper faces of the stretcher sticks are cut away more steeply than usual. Actually, special steeply beveled stretcher sticks should be made to order for this purpose.
The stretching of a canvas which is to be prepared on the stretcher requires a little extra care. The raw, unsized linen is hard to stretch evenly. The work must, however, be done accurately, beginning with the center thread on each side, and tacking at equal intervals toward the corners. The strain must be distributed evenly, and it is usually necessary to draw the tacks and replace them several times before a perfect result is secured. No slack should be left to be taken up later by driving in the corner pegs. It is wise to start with the pegs driven half-way in, so that the strain can be relaxed and restored slightly as the canvas shrinks and stretches during the sizing and gessoing. When finished, a canvas of this sort is better left permanently [p. 16] on the stretcher and not rolled; but it can be rolled fairly safely, if necessary for transport, if it is turned around a tube of large diameter and the face of the painting kept on the outside. A flexible canvas prepared by a patented method of controlled tension is available commercially in England. [pp. 16-17]
Finally, we may consider the possibilities of painting in tempera directly on plaster walls, or applying gesso grounds to existing plaster surfaces. Either of these procedures is possible; but neither is advisable for serious work under ordinary modern conditions. For large or informal decorative purposes, it is often desirable to paint directly on an existing wall; but for these purposes a broader working tempera will be wanted than that described in this book. [See documents in the folder on Artificial Emulsion Painting]. Extensions of tempera technique to large-scale decorative painting cannot be treated in detail in this introductory discussion; but they are easily mastered by anyone who has learned the basic principles of tempera handling. It is possible to paint on a wall in pure egg tempera; but modern builders' plaster usually contains injurious salts, and the result cannot be guaranteed. The wall may sometimes be sized and papered. Gesso may be applied to a papered wall; but it is a laborious process, if much space has to be covered. It is not safe to apply gesso to fresh plaster directly, or indeed to any plaster of the kinds now almost universally used. An old wall, plastered with pure lime and sand, may be painted in tempera, and gessoes wherever perfect smoothness is required; but gypsum plasters, plasters containing alum and magnesia, and plasters laid over rough coats which contain Portland cement, are unsuitable grounds for any kind of painting, whether in tempera or oil. Tempera painting of the type under discussion here is particularly applicable, however, in connection with paneled walls, or architectural woodwork in general. [p. 17]
Preparation of Panels
There are several good ways of preparing a panel to be gilded or [p. 17] painted, and a thorough craftsman may want to understand them all. A beginner, however, simply wants to know one good working system which will not require experience that he has not had. The rules for gessoing which follow here will produce good results in the hands of any careful workman if they are followed exactly. They have been tested by many people over many years, and found reliable. The type of gesso that they produce is entirely satisfactory for all ordinary purposes; and if you learn to handle it easily and familiarly, you will acquire the judgment and experience necessary for success with the more perfect but more complicated process described by Cennino, [discussed in the document Cennino's Gesso]. Even after you have learned to work in Cennino's way, you will still find plenty of use for this simple method. [pp. 17-18]
Materials [preparation of panels]
You will need some gelatine [either leaf or granulated], some whiting, a double boiler, balances, a measuring glass, a stove of some sort, or a supply of very hot water, a bowl or two, some cheesecloth, and a proper brush. The best brushes for the purpose are what house painters call "sash tools." [p. 18]
Standard Size Solution
Weigh out an ounce of gelatine, and put it to soak in sixteen ounces of cold water in the top part of the double boiler; and when it has softened up and swelled thoroughly [which should not take more than ten or fifteen minutes], put some boiling water into the bottom part of the double boiler, replace the top part, and set it on the stove. It is not necessary for the water to reach the bottom of the top part of the boiler, and if it does, it is apt to boil over. All you want is a cloud of steam rising from the water and heating the upper container and its contents. Never heat this size solution over hot water. It is very easy to burn it or discolor it if you use direct heat, and it will be perfectly safe over the water bath. Stir the gelatine until it is dissolved, and make sure that no undissolved bits stick to the bottom or sides of the container. If you are doing a small job, you will not need all the solution at once, and you may [p. 18] pour half of it off into a bowl, to be used later on. Cover it, and keep it in a cool place. [pp. 18-19]
Sizing the Panel
The panel may be sized with the hot solution alone; but it is a good plan to add a little whiting to the liquid at this point, in order to provide a better tooth for the coats of gesso which are to follow. A tablespoonful of whiting, more or less, in the eight ounces of hot size is enough. Put it in and mix it up thoroughly with a sash tool. Keeping the mixture as hot as the boiling water underneath it will make it, apply it quickly and evenly to the surface of your panel. A small panel may be brushed over with the hot size from end to end, and then "straked" off from side to side with the brush almost dry. On a large panel, the size will cool and set before you can get it covered; and it is a good plan to make a practice of painting out sizes and gessos in squares about five or six inches each way, brushing [p. 19] the mixture on vigorously in one direction, and immediately straking it off in the other direction, running the brush lightly and crisply across the marks left by the first application. This will go far to obliterate brush strokes and uneven covering, and you will find that you can get the size mixtures on smoothly before the gelatine which they contain has a chance to set. Once gelatine has set on the panel, it cannot be stirred up again with the brush without making the work untidy. Your brush should hold plenty of size, and should be dipped often into the hot mixture. If the size gets cool as it is put on, it stays on the surface of the panel, and does not penetrate as it will if it is hot and liquid.
When the whole surface of the panel has been thoroughly covered in this way, let it dry at least overnight. Do not attempt to hasten the drying of this or any of the other materials used in gessoing or tempera painting. Gradual, even drying is essential to successful work. The workroom should not be overheated, as in a hot, dry atmosphere the surface of a size coat may dry and harden while the under part is still quite wet. But the room should not be too cold either; for size mixtures are awkward to handle if the air is chilly enough to make them set quickly. They are slow to dry in a cold, damp room. Take particular care not to place panels which are being gessoed in any spot where direct sunlight may fall on them while they are drying; for the heat of the sun on the damp gesso is sometimes enough to cause bad cracks. They should be set out of a draught, also. Overnight in a room which has been comfortably warm during the day is a safe and convenient rule; for size is always best dried in a falling temperature. If you can give this coat two or three days to dry, it will be still better. [pp. 19-20]
Applying Moldings and Ornaments
If there are any moldings to be applied to the panel, they should be sized in the same way with the hot gelatine and whiting mixture. This sizing should be applied [p. 20] to the whole surface of the moldings, tops, sides, and bottom. To do this, it is convenient to size them in one position, and when the size is dry to the touch, turn them over, and size the other side. They may then be supported on pins or other light supports until they and the panel are entirely dry. To fasten them in place, they should be set in position on the panel, fitted and joined where necessary, and their outlines then marked on the panel with pencil. Soak a piece of carpenter's or cabinetmakers' hide glue overnight, and take it out of the water and melt it by itself. If necessary, add a little hot water to it; but it should be very strong. Put a thin coat of this on the bottom of the molding, and a thin coat also on the panel where the molding is to come. Immediately set the first piece of molding in place, sliding it up to the pencil line, so as to avoid air bubbles; and promptly apply enough clamps to hold it in close contact, using blocks between the jaws of the clamps in order to distribute the pressure. With a slip of hardwood, remove any excess of glue which may be forced out at the edges, so as to leave the joints clean and crisp. Wooden moldings [p. 21] should be further secured by the use of copper brads. Countersink the heads of the brads, but not too deeply.
Carved wooden ornaments may be fastened to the panel in the same way as the moldings around the sides. Any degree of relief may be secured in this way. Small castings are sometimes useful too. They may be made with plaster of Paris mixed with a solution of glue instead of with water; an ounce of glue in a pint of water gives an extremely hard, tough casting. These castings are glued in place with strong glue, and the gesso applied over them, so that they look as if they were integral with the panel, carved out of the same material. Composition materials of many kinds can be used for combinations of sculptured elements with the painting: papier mâché, mixtures of sawdust and glue; but we are concerned with the simplest usual operations here, and must not go too far afield. Moldings applied to the panel and gessoed along with it, gilded or colored in place, form for many purposes the most satisfactory sort of frame. Very simple elements will sometimes do all that is required.
The longer you leave the clamps on the moldings, the better. Twenty-four hours is a minimum for good practice. When the clamps are taken off, you will often find that there is a little work to be done with the gouge or chisel, to improve the joints and angles. All loose fibers should be cut away sharp, and any projections caused by careless cutting, or fragments of glue or foreign matter accidentally caught on the surface, should be removed at this stage, or they may show up in the gesso surface later on. [pp. 20-22 ]
[Thompson, Daniel V., Jr., Research and Technical Adviser, The Courtauld Institute of Art, University of London. The Practice of Tempera Painting. New Haven: Yale University Press. 1936. Fourth Printing, 1946.]