Glass is fired on a kiln shelf and not directly on the kiln bottom. You can also slump glass over a mold. The kiln shelf and sagging mold must be coated with glass separator to keep the glass from sticking to them. Glass separator is similar to kiln wash.
Pour the separator carefully from the bag to avoid creating airborne dust. Do not breathe glass separator dust.
1. Mix the separator with water following the directions on the bag. Stir.
2. Use a haik brush (which is a Chinese soft-bristle brush) or a soft paintbrush to apply the separator to the shelf. The haik brush lays down a more even coating than a paintbrush. Each time you dip the brush into the glass separator mixture, swirl the brush around the bottom of the container. This is necessary because the separator settles to the bottom.
3. Give the shelf two or three thin coats, changing the direction of the brush stroke each time.
4. Moisture in the kiln shelf can crack glass or cause bubbles during fusing. So dry the shelf thoroughly before use. Let the separator dry overnight. You can speed drying by placing the shelf in the kiln on 1/2" posts, venting the lid, and heating to 300 degrees F for 30 minutes. Let the shelf cool before placing glass on it.
5. After the separator has dried and your shelf is cool, you can smoothen the separator by rubbing your palm lightly over the shelf.