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Old 04-12-2019, 11:55 AM
Louis Copper Louis Copper is offline
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Trouble kiln casting with billets made from Spruce

Hi everyone... I haven't posted in years, but I am in need of some wisdom, and decided to see what you thought.

I recently had the opportunity to make some of my own colors using SprucePine batch and various metal oxides in a high temperature kiln. At first I only made small test samples just to tweak the color. I made two colors that I really liked, a golden yellow and a blue, and invested in enough batch and chemicals to make perhaps 400 lbs of the two..

I had some time with a furnace, melted the batch, and then formed billets by ladling the glass into a frame on a marver table, and then putting them into an annealer. The billets were beeeuuuutiful.

Then I tried to kiln cast with this glass, using a flower pot as a reservoir, and flowing the glass into a plaster/silica mold. and it was a disaster... extreme veiling and devitrification, with a lot of bubble formation at the mold wall. The shapes were 10" diameter spheres, solid castings, so the top temp hold time and annealing time were necessarily quite long. Smaller castings with other glass in the same kiln run were clear. (glass specifically made for casting.) ((all molds were plaster/silica of similar mix))
It seems that I overlooked one TINY detail about this glass. I have now been informed that batch for blowing is formulated with a goal to make glass of even viscosity over a wide range of temperatures. Casting glass batch is formulated (I am told) to resist de-vit and flow at a lower temperature. And I clearly did not use it as intended....

So my questions are these...
What is a typical formula for a blowing glass batch?
And a casting glass batch?
Are the fundamental ratios of glass former, flux and stabilizer different in the two?
Or is one a kind of subset of the other, but with additional additives to provide the altered characteristics needed for casting?
Is there any feasible additive (or set of additives) that can be added to batch made for blowing (if possible, specifically Spruce Pine, since that is what I now have on hand) such that it will produce glass that is robust enough for casting?

The glass may not be a total loss, if I can find a threshold of heat work that it can tolerate that can still permit thin castings. So far all heat work tests, even as short as possible, show de-vit..

And one final request for input... Is the following analysis correct? Please correct/clarify it for me if you can.
blowing glass stays for days in a furnace, and resists cords and property changes over time, and yet does not perform well at the much lower temperature when it comes to components within the glass beginning to crystallize and cause devitrification. In hindsight I guess this is because at high temperature the heat is sufficient to keep everything molten and crystallization is not possible.. degradation happens over time by a slow loss of flux from the melt. When this same material is much lower in temperature it is in a temperature region where it's component elements can begin to crystalize out of the melt. Casting glass has added components that reduce the mobility of the materials that tend to crystallize so that the crystals do not form as readily and increases the complexity of the melt in a way that interferes with and slows the crystallization characteristic of de-vit.

Any thoughts would be helpful
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Old 04-12-2019, 12:27 PM
Shawn Everette Shawn Everette is offline
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I've never had great success with Spruce as a casting glass. You can do it, but devit is common, and it tends to look cordier.

To avoid devit in any situation you're going to want to crash the kiln after your high temp soak down to your anneal. I'm not sure what the devit temp is for Spruce, but most kiln glasses you're looking in the 1250-1350 range. You also might have more success with a pre filled mold and either spruing or topping off. If the devit temp is around your casting temp, the more it has to move, and the longer you leave it there, the more it will crystallize.

I also know that in some situations certain colors are going to be more prone to devit.
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Old 04-12-2019, 01:46 PM
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Pete VanderLaan Pete VanderLaan is offline
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The questions surrounding what makes a good casting glass as opposed to a good blowing glass sound simple, but they're not.

Shawn is certainly on the money when he says SP87 is a lousy casting glass. It's quite true and making billets of it and then melting a second time is tweaking the nose of Buddha.

Modern Glass practices by Sam Scholes lays out the parameters for what different materials do to a glass and they are worth noting. In my color classes I try to stress that you can't make a good colored glass until you can make a good clear. I also think it's covered to a certain extent in Glassnotes.

If I wanted to make a casting glass that worked in the Anne Robinson process with the flower pots, I'd be looking at having materials that would lower the viscosity a good deal. There are the following to consider: Boron coming from borax, Fluorine from Fluorspar ( calcium fluoride) Lithium carb, and barium if you aren't doing lead.

It will need alumina in a ratio taking the total alkaline fluxes and dividing by eight, so, a 16% flux glass will need 2% alumina. You will need more than 16% though. Boron lowers viscosity but it also eats at your furnace. It is not a favored nation material in my toolbox. Fluorspar is powerful stuff and also attacks refractories as well as all electrical parts. Barium can be substituted in a glass for calcium or lead. It brightens the glass notably and helps reduce viscosity. At over 3% it will start to give you trouble. It really makes blue colors jump. I really like potassium in my glasses but they increase viscosity while increasing luster. It also really enriches color. I view Soda Ash and calcium as sort of trash materials. I use them but I don't lean on them beyond the basics. . Keep your silica content at 70% or so. Mine has a good deal of potassium in a nitrate form which helps with the melt. You need Antimony as well, not much but don't start without it.
My formula, available from Spruce Pine won't devitrify, has high luster but is a bit more viscous than SP87. It's the potassium that does that, so, it might take an increase in temps to flow. Beyond that, Gaffer makes great casting glasses as does Bullseye. Both are pricey. The colors from Gaffer are infinitely better.

That's a real nutshell. The subject takes much more time if you are picking at it. There's many things I've left out. I think a thorough search of the archives may surprise you. There's a lot in there from the last 18 years.
Where are we going and why am I in this basket?
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Old 04-13-2019, 08:49 AM
Eben Horton Eben Horton is offline
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Have you considered taking the billits and re melt them in a furnace and use the glass to ladle cast? Spruce pine does indeed struggle with kiln casting but it is great if you hot pour it into sand or graphite molds. My experience hot pouring it into plaster is limited, so I can’t comment on that.
<eben epoiese>
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Old 04-13-2019, 09:18 AM
Shawn Everette Shawn Everette is offline
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You'd want to reinforce the plaster or use R&R, that much heat kills what strength the plaster has left. I've seen it done poorly, and a kiln blow out is not something you want to deal with. Then there is the trick of juggling a ladle into a kiln and getting a clean poor.
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Old 04-13-2019, 11:55 AM
Louis Copper Louis Copper is offline
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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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