Some papers are manufactured already colored right through. We have already spoken of the disadvantages of these tinted papers; the artist should always prepare light-fast tinting for himself.
All papers swell with moisture; they stretch and buckle, which is very inconvenient for painting. Paper should, therefore, always be thoroughly dampened and stretched. To do this, soak the paper in water and lay it flat until it assumes a dull surface, being sure to turn it over frequently.
Then smear the edges with flour paste or strong gum arabic and smooth the paper flat onto a drawing board. A wooden frame made for the purpose can also be used. As it dries, the sheet becomes as smooth as a drum and buckles very little if it is dampened again. After the painting is finished, it is cut away from the pasted edges; so the sheet chosen must be larger than the finished picture to allow for the waste margin.
Although the texture of paper is no indication of its quality, the surface texture greatly affects the finished painting. Paint looks quite different on smooth, cloudy, and rough, cloth-like papers. Smooth papers can be given a texture with fine sandpaper, which if rubbed first
up and down and then across gives a linen or canvas-like effect that shows up especially where it is painted.
Pastel requires a certain roughness of surface. A good ground to roughen the paper is made from skim milk mixed with a little starch flour. The addition of a small amount of pigment will produce a colored ground, but only very small amounts should be added or the pastel will smear. Larger additions of pigment require stronger binding glues, such as gum arabic or capenter's glue.
Carpenter's glue is most frequently used for priming on all surfaces: wood, canvas, pasteboard, or paper. The following recipe is suitable for all of them: soften 70 gr. carpenter's glue (hide glue, which is a transparent yellowbrown) in one quart of water until it is completely absorbed; glue in beads is the best. It swells in a few hours and is more easily weighed than slabs, which take twenty-four hours or more to soften.
When soft, the glue in its container is put into a water bath and heated to 158 degrees F., at which temperature it dissolves completely. It should never be boiled or it will lose much of its binding power. Therefore a glue pot should be used, which also avoids the problem of its sticking to a pan, which it does readily if heated directly. The fillers, or thickening, and pigments are added after it has dissolved and while it remains in the glue pot.
Chalk, baked gypsum (analin), kaolin, or marble dust are used in making a gesso priming. Chalk is for the softest, marble dust for the hardest, roughest surface.
Both the priming and the surface must be warmed when the priming is applied or the paste begins to coagulate and cannot be spread evenly and thinly. A good, even ground must be built up of many thin coats and not one or two thicker ones.
Papers and thin boards, such as cardboard, must be primed on both sides or they curl. This can also happen with hardboard and composition boards which are only primed on one side. The backs require only light priming, or cheap cloth or paper can be stuck on them to prevent the tension being on one surface only.