Before you use your kiln, you’ll need to take steps to make sure it’s ready for firing glass.
If you’ve never used your kiln before, then you’ll need to prepare it for the first firing. Both the kiln floor and the top surface of the kiln shelf will need to be protected so that your glass project doesn’t stick to them.
Let’s start with the floor of the kiln. The first thing you’ll need to do is to vacuum (or “hoover”) the floor. Using a vacuum cleaner is the best way to remove any dust or other unwanted particles from the bottom of the kiln. When vacuuming, take care not to hit the kiln elements or dig the nozzle of the vacuum into the soft fire brick that usually makes up the walls and floor of the kiln.
Once the kiln is vacuumed, you’ll need to protect the floor. This is so that stray pieces of glass that fall off the kiln shelf will not eat into the kiln floor. The best thing to use to protect the floor is kiln wash, a solution that when applied properly will prevent glass from sticking to the kiln floor.
There’s nothing magic about using kiln wash. Compared to cutting glass, it doesn’t even require much practice. You do, however, need three major ingredients.
• Kiln wash. This usually comes in a powder that needs to be mixed with water. You can buy it or make it on your own if you prefer.
• A jar to hold the kiln wash. Glass is best. A tight fitting lid is essential.
• A brush or sprayer to apply the kiln wash. Applying with a brush requires less preparation, but spray application is generally a bit quicker and leaves a smoother, more even, finished surface.
In addition to these ingredients, it’s a good idea to be cautious when mixing and using kiln wash. The dry powder can be hazardous when inhaled. If you are using a sprayer to apply the kiln wash you should be especially careful and might consider wearing a respirator to avoid inhaling airborne particles.
Mixing kiln wash is simple. Just put the desired amount of the powder into a glass jar, then add the amount of water recommended by the manufacturer. Usually you add four to six parts water for each part kiln wash. Shake the jar to mix the powder and water together. The final mixture should be relatively thin. If you dip your finger in, it will leave a chalky residue.
Most people use a Japanese “haike” brush to apply kiln wash. This kind of brush has very fine bristles that allow the kiln wash to go on more smoothly. You can use a regular paintbrush if you want, but make sure the brush has wide, soft bristles. Foam paintbrushes also work well for kiln washing cool surfaces, but don’t use them with hot shelves or molds.
To apply the kiln wash, it’s often best to pour a bit of it into a bowl to allow easier brush access. (Some artists use a jar that has a mouth that is wide enough to accommodate the brush.) Make sure you shake the kiln wash mixture before pouring, as it settles very quickly. You should also stir the mixture several times while applying to keep all of the particles from settling to the bottom.
To brush on the kiln wash, first dip the brush into the mixture until it is fully saturated. Glide it over the item you want to coat, usually a kiln shelf or the floor of the kiln. Glide in one direction, allowing only the tip of the brush to touch the surface. Don’t drag the brush or draw it back and forth as though you were painting. Apply a thin coat, rather than a thick one.
You should apply at least four coats, each in a different compass direction (right to left, top to bottom, then once more in each of the two diagonal directions). Then allow the kiln wash to dry. If you want this to happen more quickly (air drying can take as long as a day), you can speed up the drying process by heating the kiln to about 500 degrees F (take about half an hour to do this) and then letting it cool naturally. When you fire, leave the kiln door propped open slightly to allow the moisture to escape.
Once the kiln wash is dry, inspect the item to make sure it is covered with a smooth layer. If you want, you can smooth the kiln wash slightly with a soft lint-free cloth – an old pair of pantyhose works well for this purpose. If the item is a mold that will be slumped into, check to make sure that the air holes at the bottom have not been filled by the kiln wash. If they have, just re-poke them using a bent paper clip or similar thin wire.
If you want your shelf to be as smooth as possible, try rinsing your brush in clean, warm water and applying two or three additional coats of warm water to the surface of the shelf. When the water dries, it will leave an exceptionally fine and smooth surface.
The procedure for using a sprayer to apply kiln wash can vary slightly depending on the type of sprayer used. For airbrushes and similar equipment, follow the manufacturer’s recommendations in terms of sprayer preparation and use.
When using a sprayer, the kiln wash should be mixed just as for using a brush — about four to six parts of water for each part kiln wash. Take care not to mix the wash too thick or it may clog the sprayer.
Spray the wash evenly from side to side, covering the entire surface with a thin, smooth layer of kiln wash. It may be necessary to go over the surface several times to ensure sufficient coverage.
Because of the risk of in advertently spraying kiln wash on kiln elements, it’s a good idea to use a brush for applying kiln wash to the floor of a kiln. Also, a respirator or mask is recommended when using a sprayer.
Some people say that you should reapply kiln wash to a kiln shelf after each firing, others claim that if you apply ten or more coats (rather than the four mentioned above) you can get by for longer without reapplying. Try both methods and see which works best for you. In either case, the kiln wash will need to be reapplied when it starts to flake off or shows other signs of not covering sufficiently.
To reapply kiln wash, you must first remove the old kiln wash. The temptation is to just brush new kiln wash over the old, but that won’t give you a smooth finish you need. Remove the old kiln wash by scraping the shelf with a paint scraper or putty knife. You may also use a wire brush or sandpaper. If you have the equipment, an electric sander can quickly remove the old kiln wash.
It’s not a good idea to breathe in the dust from the dried kiln wash. If your removal process is particularly vigorous or if you’re sensitive to the dust, you should wear a respirator. The kiln wash particles may be hazardous if inhaled, so make certain you have good ventilation
When the old kiln wash has been removed, just brush or vacuum any loose dust and other particles off the shelf and reapply fresh kiln wash.
Items other than kiln shelves will last much longer between coatings. Although it should be avoided if the smoothest possible surface is desired, on many items new kiln wash can be applied directly over old without difficulty. Scraping or otherwise removing the old kiln wash from items other than kiln shelves is always necessary if the wash has built up significantly or if it is beginning to flake away.
Protecting the kiln shelf can be done with either kiln wash or with fiber paper. (Using both is not necessary.) If you decide to use fiber paper, first cut it to fit the shape of the shelf (scissors will work well). Then fire it to about 1400 degrees F (760C) to burn out the binder that holds the fibers together. The kiln should be vented during this procedure, as it may smell (like burning sugar) and may give off a mild dark smoke. Although the smoke from the burning binder is not harmful, the odor can be unpleasant. For this reason, good ventilation is essential when pre-firing fiber paper.
Once the fiber paper has been fired, it is ready to use. Most fiber papers have one side that is relatively smooth and a second side that has a light texture. You can fire with either side facing the bottom of your glass.
Fiber paper can also be re-used, but to do this you must peel it away very carefully to avoid ripping. This process works best if the fired item is allowed to totally cool before removal. Despite your best efforts, some fibers of the paper will probably stick to the underside of the glass. These loose fibers are potentially hazardous if inhaled, so the best way to remove them is to place the glass item under water and lightly rub them off. Wear a respirator if your warm glass activity requires you to work with loose fibers in the open air.
Unless the fiber paper has become soiled, it should be saved and re-used. In addition to its use as a shelf protector, fiber paper can be used in several other ways, the most common of which is to control the flow of glass during a firing. Some artists like to kiln wash over the fiber paper in order to create a more desirable texture on the underside of the glass being fired. There’s no harm in doing this, but either kiln wash or fiber paper alone are sufficient to keep the glass from sticking to the shelf.
In addition to the kind of fiber paper discussed above, which is generally about 1/4″ (3mm) thick, it’s also possible to purchase a “thinfire” shelf paper that is designed to protect the shelf for a single firing. As the name implies, thinfire paper is paper thin. It is relatively expensive, expecially compared to the cost of kiln wash, but it does have the advantage of expediency and also leaves an slight sheen to the underside of the fired item.
While it doesn’t matter whether you place your item to be fired on the kiln shelf prior to putting the shelf into the kiln or afterwards, you should make certain that you do not place the shelf directly on the kiln floor. It should be raised off the floor at least half an inch, so that air can circulate under the shelf.
The best way to do this is to use kiln posts, short “legs” that are usually made of the same kind of ceramic material as the shelf. Three posts are sufficient, although four may be used. Select posts that that are about half an inch high and place them under the shelf in the kiln. These support posts do not need to be kiln washed, but there’s no harm in doing so if you prefer.
Once the kiln floor and shelf have been protected and the shelf is supported in place, you are ready to begin your warm glass projects.