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Testing Glass with Polarizing Filters

Rotate the top polarizing filter until it darkens. Any stress points will appear in the clear base layer of glass.


Testing Glass with Polarizing Filters

Recent Q&As: Temperature drifts during hold; Kiln shuts off before slow cooling segment

A Kiln Story: Pottery Thrives at a Michigan School

Memorable Quote

News: Glass Pioneer Boyce Lundstrom Passes Away



You can test glass for compatibility with inexpensive polarizing filters. In this article I will show you how to fire glass samples and set up the polarizing filters. This test would make a good school science project, too.

Pieces of glass that are heated and fused together in a kiln must be compatible. This means the pieces must flow, expand, and contract at a similar rate. Otherwise they will break after they have been fused together. Sometimes the breakage occurs weeks after a piece has been removed from the kiln.

Making Glass Test Strips

Fuse small pieces of glass onto a clear base layer until the edges are rounded. The test will determine whether the small pieces are compatible with the base layer.

The photo shows four test strips:

Top strip: Bullseye 6” x 1 3/4” white base fused with Spectrum green, white, and blue 1” squares

Middle strip: Bullseye 6” x 1 3/4” black base fused with Spectrum white, blue, and red 1” squares

Bottom strip: Bullseye 6” x 1 3/4” clear base fused with Spectrum green, white, and blue 1” squares

Right strip: Bullseye 4” x 1 3/4” clear base fused with 1” square float (window) glass

Setting Up the Polarizing Filters

Order a sheet of 8 1/4” x 11 3/4” polarizing film from Bullseye Glass Company. Cut the filter into 2 – 5 7/8” x 8 1/4” pieces. The filters scratch easily; handle them carefully. Store them in protective plastic page protectors.

Place a filter on a clean light table, coated side down. (To determine which side is coated, make a tiny scratch in one corner. The coated side scratches off easily.) The coated side should go toward the glass to avoid scratches from contact with glass samples.

You can make your own light table. I used a graphic arts light table that we’ve had at Paragon for many years. The top is 1/4” clear sand-blasted glass illuminated with fluorescent tubes. The type of light source is not important. Dim the overhead lights.

Place the glass samples on top of the polarizing filter. Hold the other filter over the samples and rotate the filter until it turns dark. Stress in the glass will appear almost magically as glowing halos around the edges of the test squares.

Analyzing the Test Pieces

The brighter the glowing halos, the greater the incompatibility between the glass. The base layer of glass should be clear. As you can see in the photo, the top and center test strips do not show halos, because the base layers of glass are white and black.

I tested Bullseye, rated COE (coefficient of expansion) 90 against Spectrum, rated COE 96. So it is not surprising that halos formed around the test squares.

If you buy one brand of glass that has been tested and labeled “fusing compatible,” you probably won’t need to test with polarizing filters. Carefully label glass storage containers with the COE number to avoid mixing different types of glass.



Q. During a 15 minute soak in my digital kiln, the temperature continued to climb the whole 15 minutes. It didn't do any real harm because it was a load of planters, and the glaze just came out a bit shinier (nice, actually), but I don't want to risk it for a load of my painted tableware. With that glaze, 5 degrees makes a huge difference.

A. The soak will hold the temperature for the length of time that you programmed. The temperature may keep climbing a little, however, at the beginning of a soak if the soak was preceded by a fast temperature climb. This is called temperature overshoot. After a few minutes, the temperature will stabilize.

You might also be seeing temperature fluctuation during the soak period. This may not indicate an actual change of kiln temperature but rather a change in the temperature readout. Temperature fluctuation could be due to a loose thermocouple wire. Current-carrying wires that are too close to the controller or the thermocouple wires could also cause temperature fluctuation.

A relay that is stuck in the "on" position can cause the temperature to rise during a soak.

Q. We have fired our kiln to bisque and are now trying to switch to a Ramp-Hold slow cool down, but the controller keeps clicking the kiln off reading CPLt (complete). We have programmed the temperature below what the kiln is reading, but it keeps clicking off.

A. To fire a cool down in a hot kiln, program a temperature for segment 1 that is higher than the current kiln temperature. If the target temperature of segment 1 is lower than the current temperature, the kiln will shut off shortly after you start the firing. For segment 2, choose a lower temperature with a slow rate.



Bonnie Staffel of Charlevoix, Michigan wrote, “Eleven years ago on my 80th birthday, my family set up a foundation in my name to give grants to pottery students, workshops, and other worthy projects. Many of my friends donated money to start the fund.

“This morning I was invited to see the result of one of these grants. It was issued to the Boyne City School in Michigan to buy elements for their kilns, parts for their electric wheels, and other improvements to the school studio. Their teacher was clever enough to do the labor himself. In the days when so many schools are dropping the arts and ceramics programs, this school is thriving. The teacher is well informed, and the students are learning to throw and hand build. Each student has a new iPod where they could Google my name and see my work before I arrived. The Boyne City School is fortunate to have such a dedicated teacher.”

Bonnie Staffel is a generous, giving person. She has dedicated her life to making beautiful pottery. Thank you, Bonnie, for sharing your story.



“Obstacles become inspiration as soon as you stop fighting the challenges and embrace them.” Jeff Cronenweth, film maker



Boyce Lundstrom passed away December 2, 2012. His son Patrick wrote, “Over a year ago, Boyce Lundstrom was diagnosed with a rare form of brain cancer and was given only three to six months to live. Boyce made no plans to die. He approached death like he lived life. He refused what everyone told him was reality and created a new reality for himself.”

Boyce Lundstrom first became interested in glass while studying pottery at San Jose State University in 1965. He helped start Bullseye Glass Company in 1974. He loved glass fusing because it blended colors together without the lines of traditional stained glass. Since he could make his own glass at Bullseye, he wanted to produce fusing-compatible glass. He used polarizing filters for his testing. This was the beginning of the modern glass fusing movement.

Glass fusing dates back to the ancient Romans and Egyptians. But during the late 1970s when Boyce started experimenting, few people, including stained glass artists, had ever heard of glass fusing.

In 1983 Bullseye began making fusing-compatible stained glass. This revolutionized the art form, making it easy to fuse a wide range of colors. In 1985 Boyce sold his share of the Bullseye Glass Company and bought Camp Colton, an 80-acre children’s summer camp. Camp Colton, nestled amongst streams and moss-covered trees 22 miles east of Woodburn, Oregon, became a teaching center for glass fusing. Artists came from all over the world to attend Boyce’s 13-day classes.

In 1985 Boyce taught a two-day glass fusing workshop at the Paragon factory here in Mesquite, Texas. We fused fish from glass packets that he brought, and Boyce fired them in 8-sided, 22 1/4” deep Paragon ceramic kilns. I imagine the fish project was inspired by the trout lake near Camp Colton. At that time his book “Kiln Firing Glass: Glass Fusing Book One” was one of the few sources of information on fusing.

In 1993 Boyce began recycling glass bottles into architectural tile. Later he added clay tiles to his selection.


Yesterday a carton of stoneware mugs arrived from Mel Jacobson in Minnetonka, Minnesota. The women in the office each selected a mug this morning. “This one would be perfect for coffee.” “That one is adorable.” “They’re each different.” “I don’t see a signature. Oh, here’s a stamp near the bottom.”

I imagine that while your friends are out shopping, you are busy making gifts in your kiln. Nothing warms the holiday season quite like hand-made gifts.

Thank you,

With best wishes,

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

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

Copyright 2012, by Paragon Industries, L.P.

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