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Kiln posts, thermocouple and bubbles

For discussion of processes related to using Bullseye glass, including kilnforming and kilncasting, torchwork, blowing and stained glass.

Kiln posts, thermocouple and bubbles

Postby yeruchem » Sun May 06, 2018 8:03 pm

My new kiln is 13.5" deep and the thermocouple is 9 inches from the bottom.
I was originally advised to use 4 inch kiln posts but recently was told that using 6 inch kiln posts would be better. The reason I was given was that the glass being closer to the thermocouple, would lessen chance of bubbles forming. Can someone share their own experience with me? Thanks.
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Re: Kiln posts, thermocouple and bubbles

Postby jestersbaubles » Fri May 11, 2018 7:29 pm

This sounds to me as if someone is giving incorrect information :mrgreen: .

Bubbles form because of trapped air. Air is often trapped because the edges of the glass seal before the middle of the glass "relaxes" and drops to the glass below it. A process called a bubble squeeze will help the glass to drop in the middle first, before the edges seal, pushing the air outward as the glass continues to drop from the center out to the edges.

If you have a side-firing kiln, your glass may be more prone to sealing on the edges first and causing bubbles. The further away from the elements your glass is, the less chance you have of this happening. However, the best way to avoid bubbles is to incorporate a bubble squeeze into your firing schedule. Bubble squeezes are generally accomplished by a slow (50 deg F / hr) ramp from 1100-ish to 1250-ish, with a long hold at 1250 (60 minutes).

There is a lot of info on the internet about bubbles. I have a blog post with several resources about bubbles -- most of the info is how to create them (rather than avoid!), but there is also mention of a Bullseye educational video, "Beating Bubbles".

here is my blog post: http://jestersbaubles.blogspot.com/2016/10/fused-glass-bubbles.html

here is the Bullseye video: https://videos.bullseyeglass.com/videos/beating-bubbles/
(a small annual subscription fee is required for Bullseye's educational videos, but they are well worth the cost)

I hope this helps.

Best, Dana
Dana Worley Fused Glass Designs
dba Jester's Baubles
http://www.jestersbaubles.com
http://www.jestersbaubles.blogspot.com
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Re: Kiln posts, thermocouple and bubbles

Postby yeruchem » Tue May 22, 2018 9:29 am

Thank you, Dana.
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