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Preventing Glass Bubbles, Part 2


Preventing Glass Bubbles, Part 2

Recent Q&As: Wadding under flat clay slabs; applying rigidizer to a ceramic fiber kiln

Announcement: A glass fusing display at the Haitian embassy in Washington


“Cityscape” is one of my favorite glass pieces. A group of beginning students made it during a 1985 seminar taught by Roal Enix at the Paragon factory.

I am using “Cityscape” to illustrate this article on glass bubbles, because even one bubble would have ruined this large piece. (To see Part 1 of this article, go to the online 02/02/2005 Kiln Pointer.)

To prevent glass bubbles in large projects:

1) Lay a ruler on edge along the shelf surface. Do you see a depression in the shelf where the bubble forms? If so, turn the shelf upside down and try using the other side. (Do not throw away a warped shelf. Save it for small glass projects such as pendants that won’t be affected by the shelf warpage.)

2) Moisture in the shelf and fusing molds can cause bubbles, because the moisture trapped under the glass forms steam. To be sure that a shelf is completely dry, place the empty shelf in the kiln and heat to 300 degrees F. Allow the shelf to cool. Load the shelf with glass while the shelf is still warm. Some people pre-heat large shelves before every firing.

3) Debris on the shelf can burn and form gases under the glass, causing bubbles. Make sure the shelf is clean.

4) Ramp up slowly after you reach 1000 degrees F. Fast firing can contribute to bubbles in large pieces, especially between layers of glass.

5) A sheet of ceramic fiber paper between the glass and kiln shelf helps eliminate bubbles. The paper is porous and allows air to escape from under the glass.

6) Place several tiny slivers of glass under the outer edges of the base piece. As the glass softens, gaps under the edges will help air to escape. To blend in, the glass slivers should be the same type and color of glass as the base piece.


Last week I included a pointer from Vince Pitelka on using what he calls “wadding” under large clay pieces.

Q. What is wadding?

A. Wadding is a strand of clay rolled out to the diameter of a pencil. You can make wadding by rolling the clay in your hands.

Q. I understand that wadding is used for high-fire pieces. Wouldn't stilts around the underside of a low-fire bowl or platter do just as well?

A. Yes. For earthenware pieces, stilts would serve the same purpose as the wadding that Vince described.


Rigidizer is a liquid that hardens the surface of a ceramic fiber firing chamber. (Ceramic fiber is a white material used in the SC-2, QuikFire, J-14 kilns.)

Q. How do you apply rigidizer to ceramic fiber?

A. Using a paintbrush, apply only one layer of rigidizer to the firing chamber. The fiber absorbs the rigidizer somewhat like a sponge, so you will dab it on rather than brush it on. That's why it takes only one application.

The ceramic fiber surface should be dry to the touch before firing the kiln. The first time you fire after applying rigidizer, hold the kiln at 200 degrees F for 20 minutes.

Q. Should you apply rigidizer to a ceramic fiber blanket?

A. Please consult your kiln manufacturer.


A Kiln Pointer reader and friend, Dr. Judy Fisher, of Mercy Outreach Ministry International, Inc. invites you to a benefit for the Clean Water and Sanitation Project In Haiti: "An Evening With the Ambassador" at The Embassy of the Republic of Haiti, 2311 Mass. Ave NW, Wash DC 20008.

Saturday, December 2, 2006, 6:00 – 9:00 PM

Come meet the Honorable Raymond A. Joseph, Ambassador to the U.S. and learn more about the Clean Water Project. View a collection of wearable glass art from the hot glass studio of Dr. Judy Fisher, a teddy bear showcase, handcrafted gifts for the holidays, and participate in a fundraising auction.

Dress - Business / Refreshments

301-390-0024 /

Dr. Fisher, I wish you much success with your laudable benefit.

With best wishes for the weekend,

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 /

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

Copyright 2006, by Paragon Industries, L.P.

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