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Old 05-09-2020, 07:25 PM
Steve Lazer Steve Lazer is offline
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Kiln casting

First off I hope all are well safe and remaining sane in this time of lockdown. I have a few questions on kiln casting, something I can do without studio access.

In creating a plaster mold I know Pete mentioned adding alumina to the plaster mold for increased strength. My first question is what type of alumina - tabular or carbonate or ....?

I will be looking at bomma cullet to cast with and am looking for additional guidance on kiln schedule for a very thick piece 8 inches in one dimension.

Thank you
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Steve
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Old 05-10-2020, 08:03 AM
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Pete VanderLaan Pete VanderLaan is offline
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If you can, get a copy of "The Schedules" By Graham Stone. Brad Walker has itt usually at the warm glass board. LIFE'S WORK ANALYZING ANNEALING SCHEDULES FOR CASTINGS.

Roseanna recommended hydrated alumina if one was to add it. There's a lot of ways to go after the mold work and Glassnotes IV covers many of them. We got hijacked in our approaching the molds by COVID 19 and haven't been in the studio at all. There are some great videos on the subject on youtube.

I do tend to think the issue many of us would like to solve is reusable molds. That's why I was leaning to the alumina.
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Old 05-10-2020, 11:26 AM
Shawn Everette Shawn Everette is offline
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The standard one time use is silica and plaster, I've used other silicates like diatomaceous earth with some success.

The best success boost has come from being able to back up a mold with a "container". Either by pouring your mold in a steel box or can, or banding thin sheet around it.

I haven't cast with the bomma yet, but most of the time with cullet you will end up seeing the ghost of every piece. You can sometimes get around it if you do the flower pot method.
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Old 05-10-2020, 12:44 PM
Eben Horton Eben Horton is offline
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I used to have great success with using dental investment plaster to form a 1 to 2 inch skin around my wax and then made a 50/50 plaster silica mix that I reinforced with stainless pins that are sold as a refractory reinforcement. In my opinion the steam release of a wet mold will give you flashing problems more than what material you use so take that into consideration
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Old 05-10-2020, 02:02 PM
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Pete VanderLaan Pete VanderLaan is offline
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Quote:
Originally Posted by Shawn Everette View Post
The standard one time use is silica and plaster, I've used other silicates like diatomaceous earth with some success.

The best success boost has come from being able to back up a mold with a "container". Either by pouring your mold in a steel box or can, or banding thin sheet around it.

I haven't cast with the bomma yet, but most of the time with cullet you will end up seeing the ghost of every piece. You can sometimes get around it if you do the flower pot method.
****
"The ghost of every piece" Indeed. I remember when Gary Beecham was pulling cane at Pilchuck back in '76 and he kept 8 inch diameter PVC tubes that had caps for both ends. As fast as a cane was pulled, it went into a tube until it would be used. Cullet collects dust in the very process of being removed on a rack from the furnace and as Dan Fenton used to say ( RIP Dan) "Glass remembers everything you ever do to it."
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Old 05-10-2020, 04:10 PM
Rick Wilton Rick Wilton is offline
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before you get too far committed, test the bomma cullet out first.

I took a few handfuls of the bomma cullet and threw them in a simple fiber board square mold to see how that would go. Mine came out a devitrified, cordy looking mess.

I suspect that the glass needs furnace temps for a full melt first.

Admittedly, mine was a one off attempt but I can get good clear castings from cristallica that has been through the furnace already.
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Old 05-10-2020, 04:28 PM
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Pete VanderLaan Pete VanderLaan is offline
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Bomma was intended to be very close to SP87 as a glass and SP87 is well known for devitrifying as a casting glass. SP has barium in it which doesn't bode well for being a weaker glass structurally. It does brighten soda glasses.
I'd like to hear more about it going south.
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