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Old 11-08-2018, 11:02 AM
Gabriel Greenlaw Gabriel Greenlaw is offline
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Why do some colors turn transparent when heated?

Hey All,

Haven't actually ever posted on here before because that darn search function is so handy.

Couldn't find an answer to this random question however: Why do some colors go transparent when heated and then return to their "normal" opacity when cooler/cold.

I'm thinking of those colors you use that when you're say heating the lip, the lip goes clear, while the rest of the body of the piece remains normal, and then as it cools down, it returns back to the same opacity as the body. Is it simply a form of striking even though this process doesn't alter the color? After all, the final piece looks like the color bar in the end. Random Thursday thoughts.

Many thanks!
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Old 11-08-2018, 12:07 PM
Josh Bernbaum Josh Bernbaum is offline
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Could just be a property of the fluorine, if that's the type of opaque you're working with. If you ever gather a fluorine from a color pot, you'll notice it's transparent and then strikes opaque fairly quickly as it cools off. Which is really nice to see any bubs or imperfections in the gather beforehand..
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Old 11-08-2018, 12:14 PM
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Pete VanderLaan Pete VanderLaan is offline
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different chemical make ups determine things that look the same.

Most opaques ( not all) have a sufficient amount of calcium and alumina in them to form crystals in combination with Fluorine. You really need the three elements present but it can be done with two. Fluorine by itself won't go opaque. At percentages above 4%, usually the fluorine combines with alumina or calcium that is present in a base glass., The combination causes crystals to form in the glass. Usually they strike at around 1900F or in that range. In the pot, the glass is clear. Take it through that range back and forth are the opacity is diminished, then reaffirmed. Eventually the crystals lose their capacity to form up and what you see is a weaker and weaker opaque.

In colloidal strikes, again, strands form up that are long and stringy. They too are responsive to different heat ranges but it's not calcium or alumina driving it. It is the metallic oxide, usually in the presence of a source where crystals are happiest forming up, sometimes referred to as "Nucleation Points" Those points are often atoms that are multivalent. Iron, selenium and manganese are found in this type of strike frequently. Copper ruby, gold ruby, silver opals are all of this nature. Some , like Gold really want the presence of Lead in the formulation . Many are found to also have the lower form of Tin. Colloidal color is much touchier to get repeatable results with.

The you also have another form but it's not a strike, being the glass with 6% arsenic in it along with heavy doses of lead to keep things in solution. Those usually come to you under the name "enamel" . Most aren't even 50 % silica.

Finally you have a group using Phosphates as glass formers instead of relying on Silica, or Boron. Phosphates melt seemingly homogeneously and are clear being drawn from the pot but if allowed to get quite cold, and then reheated, they undergo a phase separation where one type of glass is embedded in another and appear as a milky emulsion which indeed has opalescence in the form of a bazillion bubbles but allows transmittable light to pass through it as a fiery amber. Really beautiful glasses. Most effective in substantial quantities on the pipe .

making most of these glasses at home require operator experience. They're likely to have fluctuating linear expansions, some staying compatible for very short periods of time. The arsenates are very toxic and are better avoided without a good guide at your side.
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Old 11-08-2018, 10:56 PM
Gabriel Greenlaw Gabriel Greenlaw is offline
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Incredible. Thanks for the excellent information there. This all really makes me wish I had paid more attention to the chemical foundation that our art is built upon.

I'd imagine that these types of color would tend to off gas when being worked as rod, since they would if you were making it yourself?
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Old 11-09-2018, 08:32 AM
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Pete VanderLaan Pete VanderLaan is offline
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Fluorines are almost always outgassing. As rod, I don't tend to think it's a health issue. As powder, if you smell something pungent, that's fluorine. A well ventilated powder booth is important. As the temperature of a glass goes up, the micrograms released also go up but for the most part that's a batch issue.

I would say that it's rare that I see a studio with decent ventilation.

The colloidal bunch are quite stable. Lead could be an outgas issue but really only if it's a pot glass.

Phosphates? I don't really know. It's not really any sort of toxin to begin with.

Enamels? That sweet smell is Lead Arsenate. That feeling like you just got the flu is from arsenic.

You can always learn your chemistry. I had no formal training. I don't think Croucher did either, but that could be wrong. Buy a copy of Volf, get coloured Glasses by Weyl. Get Helmer. Build a little pot furnace. Make goop. Stir, Heat, for forty years and you're way better off.

Writing a practical book on making glass is just not economical. There aren't enough interested people who would buy it. I considered it but I wager selling a print run of 200 would be hard.
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Old 11-09-2018, 09:01 AM
Eben Horton Eben Horton is offline
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Stick your nose over the opening of a bowl that you are opening that has an interior color of gaffer lapis blue and you will know for sure that fluorine outgasses. It is ripe and pungent. It only took two pieces to make me overlay the color after getting a whiff of that.
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Old 11-09-2018, 01:51 PM
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Without using it, Why do I think that Lapis Blue is an enamel? Is it pungent or sweet?
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Old 11-09-2018, 03:40 PM
Eben Horton Eben Horton is offline
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Quote:
Originally Posted by Pete VanderLaan View Post
Without using it, Why do I think that Lapis Blue is an enamel? Is it pungent or sweet?
Pungent. I know itís lead free if that makes a difference
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Old 11-09-2018, 07:38 PM
Josh Bernbaum Josh Bernbaum is offline
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I haven't gotten many strong blasts of any acrid odor off the fluorines I've melted and worked with here. Yes, I have a couple times when my nose was closer than needed to a fairly fresh gather. I haven't done those melts in a while but just the other week, I stuffed a cup that I'd made previously with a homemade white. We made that into a large-ish bowl, and my face was kind of close to it at times but nothing detectible this time, odor-wise.
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Old 11-10-2018, 08:01 AM
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Pete VanderLaan Pete VanderLaan is offline
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It's not much of a whiff from a color rod. You need nice full pots to really clean out the sinus system.
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Old 11-10-2018, 08:35 AM
Eben Horton Eben Horton is offline
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It is with gaffer lapis. Trust me.
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Old 11-10-2018, 01:14 PM
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The analogy I sometimes use for people is candle wax. I'm not a chemist, and I slept through most of my high school physics class, so my observations may or may not be on point at times. Correlation does not equal causation.

I do really like the Sam Skoals fudge recipe Henry put in Glass Notes 4.0. Says a lot more about this process than I think most people realize.

Mr. Skoals knew what he was doing.
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