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Old 04-13-2019, 10:55 AM
Louis Copper Louis Copper is offline
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Join Date: Apr 2019
Location: Evanston (Chicago) IL
Posts: 4
Louis Copper is on a distinguished road
Thanks and follow-up

Thanks Shawn, Pete and Eben…

It was great of you all to take time to comment, and I learned a lot overnight.
In a way I feel comforted (though the problem has no easy fix) just knowing I am not crazy and other people know what I’m talking about.

Shawn, as a little test, I cast pieces of these billets in a little test kiln I have… to form a little block about 1” x 2” x 2” in a plaster/silica mold.. I did it as fast as I could in the kiln, up and down, and crashed the kiln after 15 minutes for the chunks to form into the block.. All together, up and down, I’d say the glass was at 1250 – 1550F for about one hour. I still got devit, so kiln casting with the glass I formed already is probably not feasible.

The spheres were prefilled as much as possible, and the remaining glass was in the flower pot. There was a clear difference between the preloaded and the glass that flowed out of the pot, we could see the level on the casting, and the preloaded glass was better, but still massively crystallized.

I have had that experience with different Bullseye colors de-vitrifying to different degrees. (all surface de-vit though.) for now though, that difference is laughably small compared to the big de-vit happening due to the basic glass.

Pete.. I gather Buddha does not like his nose tweaked! He responded emphatically. Thanks for the reference book leads.
I read my Glassnotes during this project, but I found it to be at a ‘general information’ level. I will get my hands on the Modern Glass Practices book.

Thanks for the tip about lowering the viscosity… I am wondering if you could say more about why that was your immediate thought and a possible way to address my issue..
Your boron suggestion had occurred to me, but I was thinking it would be better to add a boron source that did not also add sodium… so I was going to buy boric acid instead of borax. Am I shooting myself in the foot somehow?
Similarly, if I add fluorspar, I simultaneously add both Ca and F… would the Ca portion need to be backed down to allow the addition, and would the effect of the F be greater on the flow properties than that caused by the Ca?
As far as Li goes, I thought one of the key differences between Spruce Pine and other blowing batch is the presence of lithia, and I was associating the very high level of de-vit with SprucePine batch with it. ( I was recently told that the Gaffer batch recipe that Spruce Pine also makes does not contain it, and works better for my type of casting process..) So I clearly have a misguided understanding of what the Li is actually doing or why more of it might be good.
The idea of changing batch with additives that eat up equipment pains my conscience, as the equipment we have access to is not ours, and we best not be changing that aspect of the batch very much. Does boron do immediate damage at the lowest levels? I certainly wouldn’t want to shorten my friends’ equipment life. F too? Ba is new to me as an ingredient, and I will look into that one.

Would you mind explaining the source of the rule of thumb you are using for the alumina being 1/8 the total flux? What is your understanding of the chemical reason that is helpful and the right amount? Perhaps it is an empirical finding based on deep experience, but I would be interested in your thoughts about why it works.

I found your insights about the materials and the addition amounts to be very helpful and interesting. As a newbie to glass formulation, it is the levels that are difficult to get a clear picture on… and seem to be the most closely guarded part of the ‘trade secret’ of commercial art glass. So thanks for sharing that.. It is good food for thought, and immediately puts more realistic bounds on sample tests, and saves a ton of effort.

Do you mean that you also have a formula batch that Spruce Pine makes and sells? (is the Gaffer formula yours?) Sorry if I am talking to the inventor without knowing you were involved! Would your formula work for kiln casting do you think?

Lastly, your point about searching the archives is well taken… I will try to do some homework searching the archives before I jump in with questions. It is so tempting to just ask, but if the questions have been tackled before I should give that a try first. I struggle sometimes to find effective key words, but the truth is I need to give the archive a whirl and see how it goes.

Eben, thanks for your thought about ladle casting. Ladle casting is awkward in the studio I have access to… I am told they have done some, but the kilns are not designed for it, and the furnace and kilns are quite far apart. I like the graphite mold idea, and have a couple shapes I want to pursue, so will look into it. People I have talked to about ladle casting into plaster/silica give very mixed reviews, which is not surprising considering the decomposition temp of the plaster.

Bottom line… thank you everyone for your input. I got a ton of value from the exchange.. Lou
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