Source for MgO in batch

[Marc Carmen]

I could use some help with finding a good source for MgO in batch.

I recently started playing around with magnesium in clear glass but the MgO I have (magnesium oxide from axner) is tinting the glass slightly green at 3%. The oxide itself is a beige color so no surprise here.

Local ceramic supplies carry “Dolocron”, which is a dolomite that advertises .3% Fe2O3. This too seems pretty high in iron. Is there a cleaner source of dolomitic lime around?

Magnesium carbonate doesn’t look like the economical choice but I’d be happy to try it if it’s the right move for a nice bright glass.

[Pete VanderLaan]

I've been here and done it. It won't really help your glass out at all, sadly.

Dolomite is not a reliable name for the material as whether it does or doesn't contain magnesium. Chemical analysis is the only resort.

But the bottom line? It doesn't make a better glass. I wish it did. What makes it better is barium/ and zinc actually. Then , think Potash, not sodium.

Mark Peiser did some great work on showing how changing the stabilizers affected color and it's true. John Croucher had sent me a letter on the subject as well.

These days? I make a high potash, high barium / zinc clear glass. It's really nice.

[Josh Bernbaum]

High barium clears do look nice and bright, especially when you hold them next to the Spruce Pine I'm used to. If I weren't such a color junkie, and made work that relied more on the clear being the star of the show, I'd be melting my own or using one of the Glasma batches.

[Pete VanderLaan]

Indeed the clear from AO glass is a beautiful glass . The expansion at 101 certainly makes for tons of work getting color to fit it at all. Scott Benefield in N. Ireland uses a different GLASMA and indicated recently that it had a massive price increase.

Magnesium ostensibly increases the durability of the glass. I don't usually have testing to check that. The way Mark did it in his class in Arizona was to put a clear on a scale, weigh it and then put it in water for a week, then, weighing it again. I've not tried that.

What Mark did make an effective case for was the importance of the type of stabilizer in a glass and how that affected color. I had always simply looked to the modifier, making potasssium my goto modifier. Mark led it into a whole new direction and with the substitution of Strontium for Calcium as Dave Bross demonstrated, had a fabulous effect on Phosphates.

But magnesium? not so much on a basic clear. Barium has a great effect though.

[Pete VanderLaan]

Originally Posted by Josh Bernbaum

High barium clears do look nice and bright, especially when you hold them next to the Spruce Pine I'm used to. If I weren't such a color junkie, and made work that relied more on the clear being the star of the show, I'd be melting my own or using one of the Glasma batches.

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You might consider trying my VDL clear from Spruce Pine. Tons brighter, higher luster, doesn't devit and is reasonably priced as a spot on 96. Doesn't have quite the same buttery workability. I have worked out a great opal white to fit it.

I expect the glass industry to lose the battle over lithium. It all comes up from Argentina now even though there was a spodumene mine near Burnsville. There's Lithium in Afghanistan as well but good luck with that. It may not simply be a question of price ($1,000 a bag) but one of availability.

[Marc Carmen]

Well what do we mean by help? I know MgO does nothing for density or working properties but Id like to see if small amounts can help reduce devitrification in barium/zinc glass. Or I should say, reduce it even more. I think I see some promise but I’d like to test more. Fig.77 in volf shows it to be even more effective at reducing devit than alumina in a soda lime glass.

Ive worked with a barium zinc glass for a while now for stubborn processes like kiln casting and fusing with blown and solid elements form the hot shop. Lots of heat cycles and opportunity for devit. The glass I’ve been making has worked well but I always wonder if there’s a better formula out there.

I’ll try anything besides resorting to boron. That certainly was Spectrum’s approach to making a 96coe fusing glass. I’ve melted their sheet glass as cullet and you can actually see the boron vapor coming off the glass like faint white smoke.

Pete, what do you and Peiser like as a source for magnesium in color formulas then? I started looking at food grade sources for MgO. Apparently it’s a dietary supplement to help stay “regular”? I just want something that doesn’t turn the glass to shit.

[Pete VanderLaan]

Dolomitic lime is certainly available and so is magnesium carbonate. Both can be had from US pigment. I'm simply saying that I did not see it do any of my glass bodies any good. Rather, it seemed like (subjective) they took longer to fine out.
I think one of the key commentaries Volf makes is over the importance of maintaining a 3 to 1 ratio of Barium to Zinc. Then I think following Labino's old rule of thumb about how to maintain the proper amount of alumina in the glass. If you recall, he said that the percentage of alkaline fluxes in your glass should be divided by eight and that would give you the proper amount of alumina that should be present. Normally that comes in between 3-4 percent.

Mark is a phosphate enthusiast, and his work has shown how dramatically the color changes occur when the quality of the stabilizer is improved. We both think that calcium is a real junk additive for glass, and it's used because it's so cheap. Same with soda.

I'm not really at liberty to say where I think SP87 should be different. I have the formula, but I don't have the inclination or permission to be specific about it. Always remember what Nick said about the stuff. I'm disinclined to say it here since it makes Tom mad.
But build your glass around more potassium, less sodium and more Zinc and Barium and I think you will be a happier camper. Minspar feldspar is your friend. Or you could try my formula. John Croucher gave it his highest praise when he said "I can find nothing wrong with this glass." As I've said, Spruce Pine will mix it for you. So would East Bay It's not godawful expensive at all. Tell them they have to clean the mixer first. Little stuff.

[Marc Carmen]

I’m aware that dolomite and mag carb are available, I just wanted to see if anyone here had experience finding a low iron magnesium source. I just ordered some food grade MgO that the seller claims is pure white. I’ll report back here about it if anyones interested.

I don’t think I’ve seen the 3:1 ratio of barium to zinc thing. Is that in “chemical approach to glass”? I’ll dig around for it when I have time to read.

Alumina at 3-4%? is that a typo?

I appreciate subjective observation from people who know their melts. Well fined glass is a big deal to me and if MgO makes for a troublesome melt then I’ll abandon the experiment. Again, I’m mainly looking for more resistance to devitrification in kiln casting from the formula I’m otherwise happy with.

[Pete VanderLaan]

My opal white is almost 7% alumina. Marks Marco Blanko is almost 20% alumina. Sometimes glass gets so different that it seems to defy the rules, The rule for soda glasses that Nick applied said take the fluxes and divide them by eight. That would be lower but then again, you want a glass that doesn't devitrify. Alumina becomes your stiff upper lipped friend.

As to magnesium, I'm just saying it never did me much good at all. I think the mag carb from US Pigment is white. I tend to think the Lithium in SP87 causes a lot of those kind of problems. I also could be wrong.

[Jordan Kube]

If you want to help devit get rid of the calcium.

[Pete VanderLaan]

I have a message in to Mark about his experience with Magnesium. I know he's far preferential to titanium and phosphates . Jordan is right about the calcium. It's junk. But it's cheap junk.

[Marc Carmen]

At first I thought it would be as easy as replacing the relatively small amount of calcium left in the formula. I could do it with even more barium and dissolve the pot in the process. Maybe that would be ok for a special glass here and there, but not for an everyday clear. Magnesium or zinc work, but then making up for the low expansion would leave it with an alkali content well over 18%, something even high alumina wouldn’t fix as far as chemical durability goes.

That leaves us with strontium. I tried a strontium/barium/zinc crystal and it made for a very nice glass but it did horrible in the kiln. Much more surface devit than even spruce pine. Some research afterward revealed that strontium is well known to increase the liquids temp. It has its place in opalines, but not here.

It seems like at least a little bit of the cheap junk is staying in the mix. It sure does its job as far as chemical durability.

I’d love to hear anything Peiser has to say about it.

[Pete VanderLaan]

The protocol from the 1979 Corning GAS conference had their engineers saying "Melt your glass as cold as you can get away with and then fine it as cold as you can get away with. "

The point is that no glass is more corrosive than any other glass, it's simply a question of the viscosity of it attacking the wall of the pot or the furnace wall. Barium indeed will cause viscosity to drop. Turn it down some.

In my fluorines, I melt at 2050F and turn it up to 2090F to fine for three hours. I think I could do it colder. The glass does not particularly corrode the pot but I have destroyed absolutely brand new pots in one day by trying to melt at 2300F. I work my fluorines at about 2000F, sometimes colder.

In actuality, calcium does not do it's job on durability as Jordan noted regarding devit . Calcium is cheap and that is its claim to fame. When Mark was calculating the costs of his studio glass back in 1978 ( worth reading that paper) that he thought it would run $10.00 lb. I don't think it was ever that bad, but good glass costs money. Mark's interest was in what the stabilizer did for the color and he has always been a stickler on durability. He doesn't like Calcium really and truly.

[Marc Carmen]

I wonder about the whole corrosive glass thing and I’m not sure if its all just about viscosity, though that’s probably the biggest factor. Wrecking a new pot by cooking fluorine glass only 300° over working temp? Sounds like there may be a chemical reason for this. You can cook SPB 300° over working temp all year and not have issues. If you melt a certain Swedish glass 250° over working temp you’ll get stones in one melt.

Higher levels of zinc are known to attack pots and I don’t think zinc lowers the viscosity of a melt too much if at all.

I remember reading an old paper of a detailed study on how different optical glasses attack pots. The conclusion was that barium glasses are even more corrosive than lead glasses. I’m not sure which of the two have a lower viscosity though.

As far as calcium goes, It absolutely does do its job very well in terms of resistance to water. pg 421 in chemical approach Illustrates this well in figure 99. By weight % its almost on par with zinc and magnesium. Strontium is far behind and barium doesn’t even come close. In addition to this, calcium is the stabilizer with the highest coefficient of expansion, so in our formulas you can keep the alkali % lower than if you were using zinc or magnesium, making for more durability. I test glasses for durability often and can say without doubt that replacing portions calcium with barium or strontium lowers the durability.

[Pete VanderLaan]

The observation made about viscosity was made by Frank Wooley, senior melt engineer at the Corning Glass works someone I absolutely respect.

Otherwise, you are clearly entitled to your own opinions. I am looking at bringing in the AZS pots for this specific reason.

[Marc Carmen]

Found the paper. It’s called “Attack on refractory clay pots by optical glasses” and a google search should work. A nice plain english read.

I don’t really have a formed opinion on it, just something I wonder. There could be steep viscosity curves at high temp with certain glass additives too. If you know of anything Wooley wrote on the subject then I’d love to read it.

[Pete VanderLaan]

Frank did small papers on mismatch but never addressed this issue. They were private discussions with the exception of a presentation John Croucher and I did at Corning in which Frank was a very animated member of the audience. It evolved into the three of us consuming some excellent beverages and talking shop.

I would not disagree with Barium dramatically lowering viscosity if you thought it to be not doing that. I remember a barium glass mark made at one point that actually "splashed" if you stuck a pipe in too fast. But then, consider this:
In all of Mark's melts, it's done entirely on a venturi burner and does NOT ever go above 2200F.

It's also quite true that different formulas can have profound reactions with the crucible and its temperature. I have glasses I know to not allow above 2200F ( ordinary soda limes) since they will immediately begin to throw pot stones. One learns not to do that. The difference between my melt temp and my fining temps in fluorines is based in sad experience. The only time I depart from that is if I'm including a bunch of cadmium sulfide which wants to be hotter. It gets watched like a hawk with a proof every twenty minutes until the errant angels have dissolved. The it gets slammed back down. All of my run cards for glass bodies have notes on them that warn of things to not do.

I would however continue to maintain that viscosity is in the drivers seat all the time if you want to talk pot failures. Quite a number of elements can cause the failures, lithium being a big one, so I rarely use it. Fluorine is bad enough.

[Pete VanderLaan]

I ran across the formula for SP87 today and was really surprised to see how much calcium is in the glass. Without being specific, it's almost 8% without considering other stabilizing materials., That's a lot of calcium.

And the alumina content is under 2%. Devitrify? why not?

[Marc Carmen]

Yea I believe I have the current formula for spb and that’s about what it says. Also, fluorine supposedly helps nucleate devit the same way it helps nucleation in colloidal colors and phase separation. At what % that really starts to happen I don’t know but it’s something to think about.

I really wish It were as simple as just replacing calcium to rid our glasses from devit but the more research I do, the more I see that I know nothing. It seems that things like viscosity and crystal growth are more related to the forming of balanced eutectics within the glass structure. Mixed alkalis and such, which of course practical glassmakers have known of for a while. Apparently glass scientists have been trying to form a model for predicting these things but anomalies keep popping up.

European Crystal formulas look like they do all the right things and then they go adding boron.

The food grade MgO just came in. Pure white powder and I think in the long run it ends up being cheaper than magnesium carbonate from the pottery supply.

[Pete VanderLaan]

I can only say that I've spent about 1/4 million dollars on melting studio glasses over the last fifty years, and I keep records. Beyond that, I would say that the industrial glass makers have zero interest in what you want to see.

[Eben Horton]

Flourine will etch your windows opaque grey if you don’t have a killer exhaust set up. Ask me how I know.

[Pete VanderLaan]

Fresh air changes in the studio has been well ignored for the entire studio glass movement's existence. I know Eben remembers the one we had in the Santa Fe shop. I remember the one at Uroboros. Most people seem to think a 24 inch fan in a 2000 square foot shop is fine. It's not. I preferred 3 air changes a minute when we were making color. That was about 35,000 CFM per minute.
It does depend on the amount of fluorine in the glass as well as the temperatures it is melted at. Once, Henry Summa and I bought a lot of a white from Keystone Cullet back around 1974. It smoked when it was drawn from the tank. It made Henry really sick. I think that the worst way to put it in a batch is sodium silico fluoride. Then melt it hot. When I melt mine, it's at 2050F, a very cold melt. The fans are on.

[Pete VanderLaan]

[quote=Marc Carmen;153237]Yea I believe I have the current formula for spb and that’s about what it says. Also, fluorine supposedly helps nucleate devit the same way it helps nucleation in colloidal colors and phase separation. At what % that really starts to happen I don’t know but it’s something to think about.

I really wish It were as simple as just replacing calcium to rid our glasses from devit but the more research I do, the more I see that I know nothing. It seems that things like viscosity and crystal growth are more related to the forming of balanced eutectics within the glass structure. Mixed alkalis and such, which of course practical glassmakers have known of for a while.
.[/QUOTE.

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Reading Vargin is an eye opener when you want to think about balance in glass based simply in the eutectic. Enamels, which frequently have silica content as low or lower than 50% often have alumina contents about 15%.
Calcium may be a stabilizer but it also makes glass really runny.

I don't share the experience of fluorine being a nucleation point. I think of it more as a monovalent element which readily combines with calcium and alumina to make crystals, big ones. I consider materials like Manganese , Iron and selenium as nucleators all being multi valent. I view boron as having it's uses, particularly in low expansion glasses and in soda limes in very small quantities. SP doesn't use it.

I view Volf as being notably vague about fluorine and all the halides. He's vague or at least fleeting about silver as well. I take that as a bit of an indicator that his interests again lie with industry, not so much with studio stuff.

A book that would actually be useful would be one which cautions against specific things in the small shop. "A DON'T DO THAT!" kind of book. The appeal is so small that it would be hard to justify publication.

[Josh Bernbaum]

I melted a run of fluorines 6 or 7 years ago, several melts over the course of about two months. Pete's formula from the class that I had modified only slightly. After hearing everything about etched shop windows (and actually seeing it while visiting Robin Mix's studio here in VT), I was surprised I didn't see any of it here. I even put a test piece of plate glass right under the exit of my flue exhaust outside the building and none showed up there either. I did however get a couple whiffs of that 'acrid' odor coming off of fresh gathers, and made a habit of holding my breath while gathering just out of an abundance of caution. My melts were done at 2200F, maybe that's higher than it needed to be but the alumina pot didn't look too much more beat up than usual after those dozen or so melts. Only had to cook it a couple hours, it was super quick. Also thankfully didn't splash too much when I went in there with the potato at 2200, but then again I wasn't fully filling the pot each time.

[Eben Horton]

It was my experience that windows in areas further away from my furnace etched first. It really blossomed in winter months when I would fire up the glory hole and the building would heat up quickly and form condensation which in my mind was the flourine combining with water to form hydrofluoric acid. I could be wrong ..