Molds for glass slumping are surprisingly easy to make. The key is to make the mold out of the right material which will withstand the 1,200 degree temperature in the kiln. This example explains how molds are made from clay. There are two basic types of molds in glass slumping. Hump molds are used to form the interior of the slumped glass. The glass sheet or piece is placed on top and slumps over it, it at the right temperature assuming the same shape. A slump mold, on the other hand, is bowl-shaped into which the glass melts.
Find a small bowl of any material which has the shape you wish to create in glass. This will be the mold for the slump-mold. Knead some clay as you would with bread dough, to give it an even consistency. Use as much as you need to cover the exterior of the the small bowl. A one-pound piece should be sufficient for a mold of a small bowl.
Roll out the clay with a rolling pin so that it is large enough to drape the rolled slab over the small, overturned bowl. Cover the bowl with cloth so that it will release from the clay, Press the clay it against the bowl so that the clay assumes the exact shape of the bowl.
Let the mold try a little and lift it off the bowl. It should be dry enough that it retains its shape when separated from the bowl. Use a needle tool to poke small holes in the bottom of the clay mold. Create four holes near the bottom. This is important as it will let the trapped air out of the bowl as the glass slumps into it during the firing.
Create a flat, steady foot on the mold so that it will stand straight in the kiln, even with glass loaded on top. Use a damp sponge to create a smooth surface on the inside. Use it to remove sharp edges and fingermarks as well. Focus on the inside of the mold, as this is where the glass will drop and from a bowl.
Let the clay dry and bisque-fire it in a kiln. This will burn out any impurities and harden the clay for handling. It would make sense to make several molds at once to fill the kiln. Make some additional molds in different sizes to do so. Fire your molds to bisque temperature and let it cool down before unloading.
Apply several coats of kiln wash using a brush. Kiln wash will prevent the melting glass from sticking to the clay mold. Now the slump mold is ready to load into the kiln with glass placed on top.