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Testing Glass for Fusing Compatibility

Examining the compatibility test piece on a light table. Photo courtesy of Bullseye Glass Co.

Here is a simple, interesting glass test you can perform with your kiln. In addition to glass samples, you will need two polarizing filters.


When glass changes temperature, it expands and contracts. The rate at which glass changes size is called the coefficient of thermal expansion. If you fuse two glass pieces together and one changes size faster or slower than the other, the fused piece may crack--even several months after fusing.

When different glasses have a close enough coefficient of expansion to fuse successfully, they’re called fusing compatible. Buy glass labeled fusing compatible. Or fuse glass that has been cut from the same sheet, which guarantees compatibility.

Fusing glass is rated with a coefficient of expansion number (i.e. COE 90 or COE 96). The pieces that you fuse together should have the same COE number. Carefully label glass storage containers with the COE number to avoid mixing different types of glass.


1) To test glass for compatibility, fuse half-inch square sample pieces of different glasses onto a larger base piece of clear transparent. It should extend beyond the small sample pieces by half an inch on each side. One of the sample pieces should be cut from the base piece.

2) Heat the glass to a temperature that completely rounds the edges of the small sample pieces.

3) After the glass cools, place a polarizing filter under the glass and another filter over the glass. Look at the glass with light shining through it (hold it over a lamp). Turn one of the filters until the filters are at their darkest. (Polarizing sheets are available from Edmund Scientific and photo supply stores.)


If you see a halo around the edges of the small glass samples, this usually means the glass is not compatible. If you see no halo, the glass is fusing compatible.

Why did we include a sample square cut from the base transparent glass? It tests for annealing. A halo around that piece means the glass was not annealed properly. Perform the test again, this time cooling more slowly through the annealing range.


Five weeks ago I mentioned that you can sometimes salvage broken kiln shelves. Karen Sullivan in Claremont, California wrote, “I use broken cordierite/mullite kiln shelf pieces to prop up work that is glazed all over. I form a tripod support to balance the work. If the contact area is small between the object and the shelf piece, the shelf will knock off easily, and you can grind off any small mark from the piece.

“I fire to cone 11. This method works with any clay including stoneware and porcelain. The broken shelf pieces that I use are small, 1 - 3 inches, uneven, ragged. I prop them on their side, so often only a small shelf edge touches the glazed piece. I hammer the shelf pieces to break off bits of glaze that stick, so my shelf pieces become a dwindling pile.”

Thanks, Karen, for sharing this pointer.


Last week I invited readers to visit Paragon’s recommended reading list. Today I added more books and videos. Go to . Select Products, then Books & Videos from the drop menu. Or click here:

Paul Ringo of Lake Charles, Louisiana wrote, “Arnold, I'd like to suggest 'I'm all Clay, You're all Clay' by Jan Parzybok. It's a very good instructional video for basics, it's inexpensive, and Jan Parzybok is a good (almost playful) teacher. He's also got a video about throwing one-piece goblets that is thorough and useful.”

Thanks for the recommendation, Paul. You can order the DVD at this link:


We have just revised the SC-series instruction manual. It is crammed with new information, such as changing the door, adjusting the door latch, firing decals, etc. You can download the manual at . Select Support, and then Instruction Manuals from the drop menu. Or click here:

Pointer for Internet searches: To find a publication in the list of manuals, hold down the Ctrl key and press F at the same time. Type a key word in the "Find" window of your browser. Example: For lid replacement, type "lid." I use this when I am visiting a website and want to locate a key word quickly.


A question for those who own a Paragon Janus kiln: What are your favorite features of the kiln? What results are you having with a multi-purpose kiln?


Texas weather is unpredictable. Yesterday the sky was clear and bright as employees left Paragon for the day. A moment later the sky darkened, and rain pelted down. Just after I left, the sudden storm blew in one of our large plate glass windows. This was after a summer with so little rain that in Mesquite, it is illegal to water your yard more than once a week.

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 /

Feel free to email this Kiln Pointer to friends. To read back issues Click here or go to, select “Support,” and then “Kiln Pointers” from the drop menu.

To respond to this Kiln Pointer, press Reply. Your email will go directly to Arnold Howard.

PRIVACY NOTICE: Under no circumstance do we share or sell your email address.

Copyright 2006, by Paragon Industries, L.P.

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