Found moulds are often ceramic bisque or greenware, sometimes glazed and fired for house or other final use. Many other materials, usually metals can also act as moulds. This article will address ceramic materials.
Pick out a mold that is not too complicated, detailed or deep. A shallow bowl or plate with a rim is ideal. When choosing bisque ware to use as a slumping mold, avoid complicated and deep shapes. Do not choose molds with intricate carvings or patterns for slumping. Those shapes would be better for frit casting. Instead, choose shapes with a rim or with gentle curves rather than steep slopes.
Slumping or Draping
Glass has a higher expansion and contraction rate than ceramics. This means that any draping has to be done over gently curved ceramic materials. So the general advice is to avoid draping over ceramics. If you do drape anyway, it is advisable to cover the ceramic with fibre paper in addition to the kiln wash.
You need to drill holes in the proposed mould to allow the air to escape as the glass slumps. You might want to see what holes are drilled on similar plates online. The holes should be small – about 1.5mm. Much smaller and they will get clogged up with kiln wash; much larger and they will mark the glass. The drilling should be from the inside to avoid any break out into the moulding surface.
Greenware is easy to drill, so don’t press hard; let the drill bit do the work.
Ceramic forms that have been glazed require more care to start the hole. The surface is so smooth the drill bit will tend to skitter around. You can place a bit of tape where you want to drill to reduce the movement of the drill. You can also start the hole by using a masonry drill bit and rotate it by hand at the point you want to drill. This will create a “divot” in the glaze to hold your drill. It is also easier to drill, if you sandblast the glazed surface first. This will give a bit of “tooth” for the bit to grip.
You should drill the hole(s) at the last place the glass will fall. In a completely round bottom you drill at the centre. If there is a right angle or steep part near the bottom of the form, that is the last place the glass will touch and so is where the holes should be drilled. Three, spaced equally apart, should be enough.
Preparation for use
Take the greenware and clean it with a mild abrasive pad or nylons to eliminate the mold marks and scratches on the piece. Have any greenware fired to at least bisque temperatures at a ceramics studio. Explain what you are doing and the working temperature. The ceramic does need to be fired high enough to be robust. The ceramics people can give you information on the performance of the ceramic when fired to various temperatures. The bisque mold must be kiln washed before use.
If you are using an already glazed form, you need to remove or roughen up the glaze enough to take the kiln wash. A sandblaster does a good and quick job, but it can be done by hand with wet and dry sandpapers. The process should be done wet to keep any dust from the glaze (a vitreous power) getting into your l
Finally, you should test your mould with glass that has little value, before committing you best efforts to the mould.