Your hands must be clean when touching glazed ware. Oil and hand lotion will leave fingerprints. If you scrape off or damage the color during handling, you can usually repair it by applying more glaze over the damaged spot before firing.
The difference between loading greenware and glazed ware is that glazed pieces must not touch each other, the floor, or a shelf in your kiln during firing. If this happens, they will be permanently bonded together and ruined by the melted glaze.
The natural expansion and contraction of the kiln’s firing chamber during each firing generates tremendous stresses. As a result, fine grains of dust may form on the firing chamber and should be removed before each firing. Vacuum the walls, bottom and inside surface of the lid with the soft brush nozzle attachment of a vacuum cleaner.
Glaze and clay must expand and contract at the same rate. If the glaze shrinks more than the clay, the glaze will “craze,” forming small cracks. If it expands more than the clay, it will “shiver,” breaking off in sections. Test each clay and glaze combination using clay scraps such as broken bisque pieces.
Use stilts to support low-fire glazed ware during firing. The shelf tops and floor MUST be kiln washed with all purpose, high fire kiln wash for protection from glaze drops. (Apply kiln wash to insulating firebrick floors; do not apply to ceramic fiber.) Kiln wash only the top side of the shelf. Kiln wash on the underside of the shelf will flake off onto glazed ware placed below the shelf.
Glazed pieces must be thoroughly dry before firing and should not be fired with greenware unless both mature at the same cone. Even then, separate the glaze from the greenware by loading glazed pieces in the bottom of the kiln and greenware on shelves above. Low-fire glaze usually fires to a lower cone than greenware, so firing time will be shorter if glazed pieces are loaded separately.
Check to make sure that first, no two pieces of glazed ware are touching each other, the kiln walls, the floor or the shelves; and second, that the underside of the kiln shelf is clean before you place it over glazed pieces. Any dust falling on your ware will cause pinholes.
To eliminate stilt marks, you can prevent glazed pieces from sticking to the shelf by “dry footing.” This is an alternative to stilting. To “dry foot” a piece, remove all glaze from the portion of the piece that will rest on the shelf. Using a wet sponge or a piece of grit cloth, clean off the glaze from the bottom of the ware and slightly above the base so that it will not run down and touch the base. Do not use dry footing for low-fire glazed pieces that will be placed in water while used or cleaned. The unglazed areas will absorb water, which can cause glaze crazing.
Vent the lid with the lid prop for about an hour. Red glazes should be placed in the top of the kiln for extra venting. Separate clear glazes from colors. Load clear glazes in the bottom of the kiln and colors above them. Pieces that go together, such as a cup and saucer or bowl and lid, should be placed next to each other. This helps assure uniformity in color. Keep pieces at least ¼ to ½” apart. The bubbles and gases emitted from glazes can contaminate other nearby pieces. If you are firing ware draped with lace, vent the lid until all smoke disappears.
To keep holes in glazed salt and pepper shakers from closing in with glaze, insert tooth picks in the holes. They will burn away during the firing.
Allow the kiln to cool to room temperature before opening the lid.
Feel free to experiment with firing speed using throw-away samples of bisque. Some glazes look better when fired at a particular speed.
Remove the stilts from the ware after firing by breaking the thin film of glaze holding them. Handle with caution. Remove the sharp stilt edges by rubbing with a stilt stone, electric grinder or hand grinder.