MATERIALS & METHODS - Painting - Oil Painting - Supports and Grounds
Rigid Supports: Wood Panels - Cardboard - Standard Masonite Presdwood - Plywood - Metals - Grounds for Rigid Supports
Flexible Supports: Paper - Textiles - Commercial Products - Grounds for Flexible Supports
It is most important that, without exception, textiles that are to be primed with an oil ground receive a coat of size. The size protects the textile fibers against the action of the linseed oil, which would cause the textile to become brittle and to disintegrate.
The textile should be sized just enough to make the next layers of paint or priming adhere well to it. If the support is sized with too much glue, it will become brittle and nonabsorbent, and the paint will adhere to it badly and may even crack or peel off, as it [p. 101] would from a glassy surface. If the support is sized with too little glue, the support may absorb too much binder from the next layers of paint or ground. The paint may then powder off when it dries, as though it were applied to a blotter. [pp. 101-102]
2. Gently heat the glue-and-water mixture in a double boiler until the glue dissolves completely, making a brown brothlike liquid. If an ordinary metal container is used instead of a double boiler, be sure to stir the glue as it heats, so that the heavy particles do not settle to the bottom and scorch. Never boil the glue. When boiled, it loses its strength and must be discarded.
3. Be sure that the raw linen support is free of oily films or dirt. It should be stretched on a frame as outlined in the previous section.
4. Apply warm glue solution to the support with a 2-inch bristle brush. Start at the center of the stretched fabric and make the coating as thin and even as possible. A heavy coat of size is undesirable. To prevent the fabric from raveling or fraying, apply glue size all the way to the edges which have been folded over to the back of the stretcher strips.
Notes [Sizing Procedure]
A. A coat of size in which there is too high a proportion of glue or which is too thickly applied may leave a continuous glossy layer of glue on the surface of the support. This may crack easily, especially on canvas. The size should sink into the support completely, filling its pores to some extent but not producing a separate film on its surface. [p. 102]
B. Rabbitskin glue seems best at a sizing material, but weak solutions of casein or gelatin may be substituted.
C. It should be noted that hide or rabbitskin glues vary somewhat in strength according to the age and source of the material. Therefore the recipes are given with room for adjustment within limits, such as from 1 1/2 to 2 ounces [av.] of glue for size.
D. Size, like all glue solutions, spoils rather quickly and loses its strength. Discard it as soon as it begins to give off a bad odor [two weeks maximum]. It will keep better if stored in a clean container at a cool temperature. On cooling, it will gel and must be gently heated to become liquid again.
E. Rabbitskin glue begins to set as it cools. In thin films it begins to "dry" by cooling and gelling and then hardens more thoroughly as the rest of the water evaporates. Its dry film remains hygroscopic--that is, it takes in water easily and swells as it does so. In fact, if hot water is applied, the glue can be redissolved.
A sized support may be made more moisture resistant by brushing it with a 4 percent formaldehyde solution easily obtainable in a drug store.
F. The shape and degree of fineness of the granules of the glue, as it is sold to day, vary so much that a teaspoonful of finely granulated glue may hold much more glue [by weight] than does the same teaspoonful of the same glue in the form of rougher, bigger granules. Consequently when an accurate measure is wanted in a glue recipe, one should determine the amount of glue to be used by weighing it on a scale. All glue recipes in this book indicate the amount of glue in ounces avoirdupois [av.]--that is, by weight. An inexpensive small postal balance scale [p. 103] can be bought at most stationary shops. It is accurate for weights from 1/4 ounces [av.] to 8 ounces [av.], graduated in 1/4 ounces, and is a most useful item of equipment for the artist who makes grounds or colors. However, if no scale is available, the proportions of the recipe given or glue size can usually be approximated by the following method:
b. Gently add dry rabbitskin glue until the level of the water rises an additional 1 1/4 to 1 1/2 fluid ounces--that is, just a little higher than the mark on the measuring glass that indicates 17 fluid ounces.
c. Put the glue-and-water mixture into the double boiler and add to it 16 fluid ounces of water to bring the total volume to 32 ounces of water plus the immersed glue.
d. Allow the glue to soak 3 hours, or overnight if possible. Then heat and dissolve the glue as in step 2.
When it is first put into water, one ounce by weight of glue usually displaces 3/4 of an ounce [by volume] of water.
g. The sized canvas should be allowed to dry under natural conditions. The canvas should not be exposed to intense heat from radiators, stoves, heat lamps, or direct sunshine in order to accelerate the drying.
[Kay, Reed. The Painter's Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.]
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