In an attempt to save money on molds, I went to the local ceramic store, bought a shallow bowl and have used it for many firings. It works.
Pick out a mold that is not too complicated, detailed or deep. I started out with a shallow bowl and have added a plate. I’ve found that something which requires a gentle slump or has a rim and gentle slope works fine.
Take the greenware and clean it with a Scotchbrite pad or nylons to eliminate the mold marks and any scratches on the piece. You then have to drill holes in the item to allow the air to escape as the glass slumps. You might want to see what holes are drilled on similar plates online. I drilled holes in the middle of the bowl– its deepest part– and along the sides with a drill bit– 1/16″ or so. I did it by twisting the drill bit by hand. The greenware is soft and easily drilled at this point. Don’t press too hard; let the drill bit do the work. Two to three holes work. Clean the areas around the holes. Smooth out the greenware with a sponge or the Scotchbrite pad. Then usings a wet sponge, evenly moisten the piece. Greenware has to be bone dry before firing. The moistening is meant, I guess, to allow the clay to dry evenly. (Ceramics has its own issues.)
I had the greenware fired to bisque at the ceramic store. Yes, I could have fired it in my kiln, but the moisture of the clay can be hazardous to the kiln and firing clay requires a different approach. I left it to the professional. I explained what I was doing and how hot the glass gets and the greenware was fired to accomodate the glass.
The bisque mold must be kilnwashed before use.
A caveat: while using bisque molds, the general consensus I’ve gotten is that the mold won’t last. Working with glass, we sometimes do a fast cool-down to prevent devitrification. That’s great for glass, bad for ceramics. I use a natural cool-down for glass I slump in my “cheap” molds and haven’t had problems with devit. One mold has been used 12 times while the other has been used 4 times.
Your ceramic or pottery store might be able to create a different clay mixture to create a stronger mold, especially if you want to use the mold for glass casting. I’d definitely let them know how hot you intend to go because they can fire the clay at different temps.
I’ve also cut a terra cotta flower pot in half lengthwise to create a sconce slump mold. That mold has worked for 3 firings and cost me a dollar or two. I try to find the pots made in Italy or the US because I remember reading somewhere that they seemed to have a better clay.
When choosing bisque ware to use as a slumping mold, it’s best to select a piece that has the appropriate shape for slumping. Avoid complicated three-dimensional shapes and deep bowls; also stay away from molds with intricate carvings or patterns in the clay. Instead, select a shape with a rim around the edge or with gentle curves rather than steep slopes.
Before using bisque ware as a slumping mold, you’ll need to prepare it for slumping by drilling holes in the bottom of the mold. You’ll also need to kiln wash the mold to prepare it for firing. These topics will be covered in part two of this tip.
Once you’ve selected your bisque ware, the next step is to prepare it for use as a slumping mold. This usually involves drilling several small holes in the bottom of the mold. These holes allow air to escape during slumping; all but the most shallow molds will require that holes be drilled.
In addition to the mold, you’ll need an electric drill and the appropriate bit. Drill bits should be small — 1/16″ or 3/32″ is ideal, but 1/8″ can also work. A masonry bit will work well, but good holes can be drilled with bits made for drilling in wood or other materials.
Though not essential, a drill press will make the drilling easier and will virtually guarantee good holes. But even if you don’t have a drill press you can do a good job with a handheld drill, so long as you drill slowly and carefully and with the drill held straight up and down.
For best results, drill holes in the deepest part of the mold. You should drill at least two (and three is better) holes, spaced equally apart in the mold. You can drill from either the bottom or the top of the mold; use whichever approach is easier. Drill slowly, and be especially careful when starting and finishing the hole — these are the times when the drill is most likely to slip and cause a chipped spot in the bisque ware.
Once the holes are drilled, clean the edges around the hole by sanding the mold lightly with fine sandpaper. Then kiln wash the mold, allow the wash to dry, and you’re ready for slumping. Handled carefully, a bisque ware mold should last for dozens of firings.