KILN WASH AND FIBER PAPER
Because glass tends to stick to many surfaces when it gets hot, it is necessary to protect kiln shelves and molds (not to mention the glass itself) from being damaged. Shelves can be protected in one of two ways: either by applying a primer, or kiln wash, to the top of the shelf or by placing a sheet of specially made paper, called fiber paper, between the shelf and the glass. Although some molds are heat resistant and do not require kiln wash, most molds need to be coated with kiln wash prior to use.
Kiln wash is usually purchased in the form of a powder that is mixed with water to form a thin liquid. This is applied to the shelf with a sprayer or a special brush called a haike brush. Three or four layers (some people use as many as ten) are usually applied, with each layer brushed on in a different direction than the previous one. When the kiln wash dries, it forms a thin coating that prevents the glass from sticking.
If you are concerned about the potential for inhaling the dry particles, it's a good idea to wear a mask when mixing and applying kiln wash. The powder can be harmful if inhaled. Good ventilation is absolutely essential.
Fiber paper, which is made of very fine alumina and silica threads that have been bound together, should also be handled carefully. Its thickness varies from thinner than a single sheet of paper to as thick as 1/4 inch (3mm). Thicker varieties are generally known as "fiber blanket" (if flexible) or "fiber board" (if rigid like plywood).
Before using fiber paper, it should be cut to the desired shape (scissors will work well) and then fired to about 1200 degrees F to burn out the binder. The kiln should be vented during this procedure, as it will smell (like burning sugar) and may give off a black smoke. This smoke is noxious but generally not harmful, but good ventilation is a must.
Once the fiber paper has been fired, it should be used between the kiln shelf and the glass being fired. It will impart a texture to the bottom of the glass; in fact, most fiber papers have a different texture on each side.
Fiber paper works best up to about 1500 or 1600 degrees F. It has a tendency to stick to the glass being fired, but with care you can usually remove the paper in one piece. Loose fibers from the paper can be harmful to the lungs - a good way to prevent this is to wash off residual fiber paper in a container of water, rather than in the open air.
Fiber paper and fiberboard have many other purposes. They can be stuffed in or around a mold to keep the glass flowing in a particular area. They can also be manipulated and used to impart unique textures to the bottom of a piece of glass.
Thicker fiber blanket, which is commonly available in thicknesses up to two inches, can also be useful. It can be used to control the flow of glass inside the kiln or even to make molds for slumping. To make a mold with fiber blanket, the material is saturated with a rigidizing solution and then allowed to dry while conforming to the desired shape. This kind of mold material is commercially marketed under the name of "wet felt."
One particular kind of fiber paper, marketed by Bullseye under the "thinfire" name, is noteworthy because it has been designed to impart a smoother than normal sheen to the underside of the glass. This makes it an ideal candidate for many fusing projects, but the thinness of the paper means that it can generally only be used one time (two or three times at most).