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The drawing materials enumerated so far, lead or graphite pencils, chalks, charcoal, and pastel, constitute a group, as they are all erasable. The softer they are the more easily they rub out. Hard pencil is the least erasable; it has to be pressed quite hard onto the paper before it makes any mark, and even so the mark is never more than dark gray. Only the very softest graphite gives anything approaching a black line or mark, and there are some solid graphite pencils available nearly four-tenths of an inch thick which can be used like pastels without a holder. They are useful for largescale drawing and for the application of large areas of shading and for almost brush-like effects.
Some white, red, and black chalk leads are made which are nearly as hard as soft graphite pencils and give almost as firm a line. If a stroke is made in pencil or chalk with sufficient pressure it cannot be completely erased. Charcoal is different. It is so soft that the stroke can be wiped away completely into a gray smudge. Most pastels are the same, although their hardness varies. The factory aims at producing pastels with as near uniform hardness as possible, but some colors harden with time so that ultimately they will hardly mark at all unless the surface is very rough and hard, like wall plaster, or is primed with marble or pumice dust and size.
A drawn line is made by the surface mechanically rubbing something off the drawing instrument. The surface of even quite "smooth" paper is rough enough for drawing pencils and chalks, although it is much weaker than rough paper or the sandpaper used for sharpening points. The rubbed-off particles of the pencil are held weakly and mechanically to the drawing surface.
Only a few particles penetrate into the pores from pressure or rubbing. The smaller the particles the deeper they penetrate and the less easily they smudge or rub out. A rubber eraser is adequate to remove pencil marks from a firm paper which is not fluffy or fibrous. It removes the graphite particles which have penetrated the paper mechanically, but it always rubs away some of the paper, although a soft rubber does it less than a hard one. Plastic rubber, on the other hand, lifts up the particles of color by sticking to them.
It is not made of rubber, but of just the right mixture of clay and vaseline to be malleable and not to leave either clay or vaseline when pressed onto the paper, and yet be sticky enough to pick up particles of graphite dust. fresh bread crumb is even stickier and does not affect the paper; however, it must, of course, be completely free of fat or grease. It is the ideal cleaning material for restoring work, since it does not damage even the most delicate paper.
All pencil and chalk drawings are made without the third, binding element essential to a durable picture. Binder is therefore sprayed onto the paper after the drawing is finished, fixing the more or less loosely attached particles of color to the ground. All binders used in this way are called fixatives. In general, any adhesive material can be used as a fixative; the choice depends on the colors and ground.
The most usual, universal fixative is made from a two per cent solution of shellac in alcohol, as sold commercially. The basis of shellac is the resin of East Indian fig trees, which is extracted by punctures from plant lice, the hemiptera or semiaphides. The resin contains some of the wax secretion from the insect, which reduces the brittleness of the resin. The wax is dissolved only in the alcohol under heat, although the resin itself remains solid. If you wish to prepare shellac fixative yourself, you must warm the bottle of shellac and alcohol carefully in a water bath from between 140 and 158 degrees F. The bottle must not be more than half full, for some of the alcohol evaporates and could cause an explosion as it expands.
A simple blow pipe works quite well for spraying the fixative. If the drawing is large it is best to make a spray with a cork and rubber bulb. The spout and ascension pipe get encrusted in time with shellac; they can be cleaned with a thin steel wire and rinsed out by squirting a few times with pure spirits. It would be quite possible, theoretically, to brush on the fixative, but even the finest hair brushes rub off some of the particles of color and smudge the drawing.
Loose pigments used to be fixed in the following way: before drawing, the support was first primed with reversible glue, such as gum arabic, gum tragacanth, or carpenters glue. After the priming had dried, the drawing was done as though on ordinary paper and then steamed, so that the glue softened and fixed the particles of pigment onto the page from underneath.
The advantage of this method is that the pigments are not embedded in the binder as they are when covered with a spray, and their surface is unaffected. The disadvantage is that the glue is sensitive to damp. This, though, is a disadvantage shared by all painting which contains glue, including all watercolors. Some experience is necessary to ensure the correct thickness of the glue; if it is too thin it will not hold the pigments sufficiently, and if too thick it tends to crack and scale off.
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