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Glass Slumping

Glass slumping is the process of heating a sheet of glass until it softens enough to take the shape of a mold. Some of the most dramatic results in warm glass are achieved by slumping.

To slump a fused piece, fuse and slump in two separate firings. Fuse the glass in the first firing; slump it in the second firing.

Here are a couple of questions and answers on slumping:

Q. How do you apply kiln wash or glass separator to stainless steel molds such as a bowl? For me, it always falls off.

A. Heat the stainless steel mold to 250 degrees F in your kiln, in an oven, or with a hair blow dryer. Then spray on the glass separator using an airbrush or even an inexpensive pesticide sprayer available from a building supply store. Shake the glass separator mixture often, because the particles settle to the bottom quickly.

If you heat the mold with a hair dryer, heat from the outside while you spray on the glass separator inside the mold.

The stainless steel mold shrinks more than the glass during cooling. For this reason do not use a stainless steel mold that has vertical sides, such as a steel cup. The sides must be slanted so that the glass can move upward as the stainless steel mold shrinks during cooling. If the glass is stuck inside the mold during cooling, the glass will break.

Q. My slumped bowl cracked. How do you tell if the glass cracked while it was heating up or while it was cooling down?

A. Cracks in slumped glass almost always form as the glass heats up. A hint from Bullseye Resource Center: Do the broken glass pieces fit together?

1) If the pieces do not fit, the glass broke while the kiln was heating up. The pieces do not fit because the glass deformed (slumped) after it broke.

2) If the pieces fit perfectly, the glass broke during cooling. The pieces fit because the glass broke after it had already deformed during slumping.


Last week’s kiln pointer was “Working with Silver Clay.” Don Pearse of Sunbury, Ohio shares additional pointers on firing silver clay:

“You can program a FULL rate to the specified firing temperature of the silver clay. After the prescribed hold time at that temperature, the kiln may be shut off and vented to cool quickly, or the silver may even be removed with long tongs and quenched if desired. (Wear proper protective clothing and glasses.)

“Do not fire the silver on the kiln shelf. It may leave a stain that will be picked up on glass in future firings. Fire the silver on a piece of unglazed tile. I also use a thick layer of dry kiln wash to help support curved silver pieces during firing.”

Thanks, Don, for the useful silver clay pointers.

I always enjoy being with kiln users, whether they fire glass, pottery, china, or silver clay. What they all have in common is the joy of creating. Frances Darby, who founded Paragon, told me, "They are the givers of the world."

When you fire a kiln, you are pursuing ancient forms of art. The Corning Glass Museum owns glass fused pieces dating back to the ancient Romans and Egyptians. Maybe a thousand years from now, long after other signs of our civilization have disappeared, your pieces, too, will appear in a museum.

With best wishes,

Arnold Howard Paragon Industries, L.P. - Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 /

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