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  #26  
Old 02-27-2022, 07:22 PM
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In my afternoon reading after a lot of irritating events, I looked at the ways fluorine gets introduced to a glass. Sodium Silico fluoride was distinctly the worst and cryolite the least corrosive.
Again. Volf makes no mention of fluorine having any capacity to provide a nucleation point. It's in the "Halides" section and I still maintain that the technology applications are pretty marginal.

It is supportive from Volf that both fluorine and Barium really lower viscosity while it neve argues that viscosity is the villain in refractory attack. I t does point to lead fluorides as a big player.
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Old 02-28-2022, 10:17 AM
Marc Carmen Marc Carmen is online now
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Volf pg 572- “with alkali-lime-silica glasses, fluorine increases the liquids temperature and the rate of crystalization. During cooling of the melt, monovalent fluorine facilitates the formation of nuclei as a mineralizer, and the first crystalline phase appears at higher temperatures. However, no changes in crystalization properties occur with the small amounts of halides currently employed in technological practice.”

By “technological practice” I guess he means as a melting accelerator in clear glass? Either way, the raising of liquids temp while lowering viscosity is a recipe for devitrification. While I’m unsure of the liquids temp thing, I know F markedly lowers viscosity at tiny amounts. I just wonder why spb devitrifies more than most glasses and if there’s more to it than just the calcium content.

After a ton of research, I learned the more accurate term “liquids viscosity”, or the viscosity of the glass at its crystallization temp. Low viscosity below the crystallization temp (liquids temperature) means more rapid crystal growth. Seems they started using the term well after volfs book was written.

Yeaaa Volf is pretty vague about a lot of things in a lot of the chapters. The book is somewhat practical, though. Reading any deeper into glass chemistry and you start to see only formulas in mol%’s and more mathematical equations than words.

I think the “DONT DO THAT!” book can pieced together from the archives of this forum. Add fluorine with low ventilation to the book.
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Old 02-28-2022, 10:41 AM
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Pete VanderLaan Pete VanderLaan is offline
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well, "no changes in crystalization properties occur within the small amounts of Halide properties currently employed..." was what I read too. Sp87 has a very small amount of CaF in it indeed but again , it's small. I think a notable addition in SP is barium when coupled to calcium. Clearly it's unhappy as a complete glass. For me, nucleations translates into "A place to begin to grow crystals" That implies multi valent materials and Fluorine is monovalent. My world would include manganese, selenium, iron. Arsenic and Antimony will make opaque glasses too. You just can't share breathable air with them. Fluorine by itself will not grow crystals at all. It does it in the presence of calcium and Alumina.

*****
When I look back on the two years of color classes I gave, a constant comment from the participants was that it made them realize that making your own glass, and color was indeed possible. Everywhere I looked around during the varying stages of the class I saw people so focused on what they were doing and learning. Some students at computers where they were challenged to write their own glass formulas for critique by the larger group- some measuring linear expansion with a dilatometer, some with a polarimeter- some cutting ring tests of laminate glasses at the saw.

This went on all night. I could come in at 2:30 AM and the place was buzzing. The entire approach was the "DON'T DO THAT" approach I think of. Craftweb goes a long ways towards demystifying process and that became a lot of my intent in doing it but it's still not a replacement for melting goop.

My best memory was in Shanghai, teaching Xi Wu about glass. We had to work from a Kanshi character table of the elements but he was so smart, and so anxious to learn. Eveline would sit with us, translating. I told Eveline that he was the most important person in the factory in that if he f***ked up, no one worked. She got it. Today he manages the place. He and his father live there and run it. Late in the night , there they are, mixing, loading and making it all happen. Seeing the formulas I wrote for them in Mandarin Kanshi still cracks me up.
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Old 02-28-2022, 01:28 PM
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So in conversation with Mark regarding magnesium. He liked it. He liked the carb to be specific. He got into it back around '79 when he was interested in lowering the surface tension in glass in the down spout furnace. He sent some of the glass to Corning for analysis and they said it was the most durable glass from studios they had ever seen.

Now keep in mind it had no calcium in it at all. Chock full of strontium and barium though so you can't compare it with SP87 at all. He went on to say that it really benefitted coloring glasses with Chrome, manganese and Nickle. So there's that. Take note, zero calcium so with the strontium in there, it's difficult to say what an isolated effect might be. I still tend to think that SP's issue with devit, which I view as not always producing the same calculable deviation from the normal silicon tetrahedron as being exacerbated by all the calcium in there.
I can get a coke bottle to devitrify too.
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Old 03-02-2022, 12:52 PM
Marc Carmen Marc Carmen is online now
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Pete, thanks for the great info and great stories. It is a good point that fluorine grows crystals with calcium or alumina.

Mark’s approach to a glass including magnesium is very interesting. The durability part mostly, though seeing how these glasses color will be fun. I’m not quite sure yet, but I think I’m actually seeing an anomalous decrease in durability when a portion of calcium is replaced by magnesium even though magnesium is supposed to be a more effective stabilizer. Volf mentions this too with dolomitic glasses and suggests adding alumina. I’m definitely seeing a lower annealing temp though because its taking a 90 or 91 coe glass (E&T exp. factors) to be compatible with spruce pine. Either that or the CaO/MgO mix is higher in expansion than either of their factors alone. My assumption earlier in this thread was that MgO containing glasses would need extra alkali to make up for low expansion. That was completely wrong.

Since the start of this thread I have begrudgingly accepted the fact that going by molar mass is a much more reliable way to work on glass formulas that stray from the standard soda lime type. It really helps to understand glasses that would be, well, chock full of strontium and barium for example.

Looking through these old books, I keep seeing references to Magnesium helping devit resistance. It’s interesting to see two particular types of glass include MgO. One is sheet glass that was made using the Fourcault process, where glass was held at low temp and drawn into sheets. The other is old handmade light bulb glass and fluorescent tube glass, where thin glass was flameworked and had to maintain a pristine surface.
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Old 03-02-2022, 01:57 PM
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well from an industrial standpoint, Dolomite is far more attractive for finding magnesium. The Problem? All the tramp iron. The carbonate is pricey. Interestingly, Mark treats titanium as a flux too. When we were melting those formulas up here in the class, the titanium stuff was something we weren't sure would melt at all. At about 2400F, it just sat there and sat there and then suddenly collapsed. It was hard on pots.

Mark really dismisses calcium. I use as little as I can but making fluorine crystals work, it has to have both the alumina and the calcium. By using cryolite, I can largely attain the density I need with very little whiting. My conversations with him are inevitably fun. He calls out of the blue and immediately picks up a conversation line we had dropped two weeks earlier with no warning. My deafness makes it really hard now since some voice tones are impossible for me to manage and some are easy. He goes back and forth. You are right about the mole calculations. I used to sit here and watch him counting molecules and I'm not exaggerating. He prefers compounds over aggregates since the tramp stuff makes the mole stuff really difficult.

Volf is very interesting when it talks about durability when glass is exposed to steam. I also have Volf "On Technical glasses "and it indeed does look at the stuff very differently.
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