Matte slumped pieces

[beva]

Hello, first happy new year to everyone!
I'm a newbie, and I made only very few experiments with fusing and slumping. While my fused pieces come out as I expected them, I have some problem with slumping. I always (3 times so far ) get a matte surface where the glass deformed. In the center of the pieces the surface stayed shiny. I fired twice these pieces, first I full fused them at the 1510 degree process temp. The second firing was for slumping, at 1200 degree. All 3 pieces had the same problem, no matter how thick it was the glass or how fast or slow was the firing speed. The matte surface is not even, but stayed glossy where the glass is "higher" - above bubbles, and waves as you can see on the photos.
I'm not sure if it's devit or not. I think it is not, because the whole slumping process was below the devit zone. The most interesting thing for me is that I put another pice for slumping while a tack fusing programme was going on. This piece was already a waste because of other reasons, and I was just curious what it's gonna do in the kiln. As for 5 layer tack fuse the program was quite slow with a 1410 degree process temperature. It slumped and its surface stayed clean and shiny.
Any idea what it can be and how to avoid this matte surface? Shall I go as high as 1400 just for slumping??
Thanks for any clue

[rosanna gusler]

Float glass? If so that is the tin layer compressing. Rosanna

[beva]

Yes, float. What's that? I've tried to google it, but nothing found.
Can I avoid that by higher process temp?
Eva

[Bert Weiss]

Yes, float. What's that? I've tried to google it, but nothing found.
Can I avoid that by higher process temp?
Eva

Float glass is formed while floating on a bed of molten tin. This tin leaves an invisible coating that has several effects when making kiln fired glass. When the tin layer gets compressed, it "blooms" (tin bloom). I have some spherically shaped molds. In that mold, the frosting is uniform over the whole piece. However any mold with a flat spot, will have areas that are not compressed by the slump. So, with float glass, you must place the tin side on the bottom, so it stretches in the slump, instead of compressing. There are numerous work arounds. An overglaze might work.

The tin can react with certain colors and turn them brown. This can be a killer, or an enhancer, depending on the color.

The tin is also a protective layer that is much less sticky to kilnwash or ceramic fibers. Glass fired tin side down comes out much smoother on the bottom than the air side of the glass. This is significant. For years, I wondered why some glass came out with smooth bottoms, and some with rough bottoms. It was the air side yielding the rough surface.

The solutions are easier when you work in the traditional configuration you have chosen, just put the tin side down. I like to reverse paint on float and fire paint side down. So, I have to work with colors that will not be turned to crap by the tin. BTW, Youghiogheny EZ fuse colors are not effected by tin. I tested them all and got no reactions.

To discover which is the tin side, you need a short wave UV light, preferably with a purple filter. The tin side will glow white from direct exposure to the light. The air side will not.

[rosanna gusler]

Search tin side, Uv lamp, tin. Bloomon this site. Basically float has a tin side and an air side. If the air side is up the piece will not do that. Tin issues are subtle and can be complex. That is why I advise spending some time searching in this site.rosanna

[beva]

Thank you for your answers!
I'll get a UV light soon to avoid this problem.
Thank you Bert for the details on other aspects of the tin side, I'll keep them in mind for the future.
Eva

[Tony Smith]

You will need a short wave UV light, also known as a germicidal UV lamp, not the typical black light (long wave) that is commonly available in stores.

Tony

[seachange]

Hi

In 2011 I did a lot of tests with float glass, and regularly removed the tin side by soaking the flat piecesfor about 30' in a flat tray filled with CRL water spot remover - a tip that I picked up here on this board. One can see a very very pale sediment in the liquid after this treatment. The liquid is reusable many times, I kept the used one in a separate bottle and added some fresh to it once in a while.

Tin was not detectable with the UV light after this. Did remember Bert's advise that the tin side can be your friend, depending on use.

At the time was making woven plates, flipping the bars, then slumping, so did not want any compression marks.

Hope this helps

Best regards, seachange

[beva]

Thank you too for answering.
getting the tin layer off the glass sounds very interesting and it would be good to try. Unfortunately in my country CRL water spot remover (just as Spray A and many more materials for fusing and casting) is not available. It would be good to know its ingredients, maybe I can find something similar here. I've already checked germicidal UV lamp, and thank God it's available, so I can get one step forward.
Eva

[rosanna gusler]

Tin can be removed with acid but then you have a whole new group of chemical costs and safety issues to deal with. Best to learn to work with the tin I think. Rosanna

[Bert Weiss]

The tin layer is very helpful, protecting the bottom of the glass from getting a rough texture. I would not advise removing it in most circumstances. Understanding how it works is very helpful though.

[beva]

This was my first thought too, but while foruming, I test fired another piece of glass. It is a single layer 3mm float. Now the outer side came out with that slight silky finish. Just the ring area where the glass bent. I'm quite confused...

[Bert Weiss]

This was my first thought too, but while foruming, I test fired another piece of glass. It is a single layer 3mm float. Now the outer side came out with that slight silky finish. Just the ring area where the glass bent. I'm quite confused...

There are 2 different causes for fogginess. One is tin bloom. The other is devitrification. Devitrification is pretty complex. It happens from some combination of uncleanliness and temperature. I am able to avoid it on most all first firings. Each time you fire float glass, you increase the chances for devit to show up. I am pretty conscientious to fire my float as low as I possibly can get away with. I seldom fire hotter than 1480ºF (805ºC).

On the top layer, a borax overglaze can prevent devit, if you fire to at least 1400ºF (760ºC). On the bottom, I am not used to having this happen.

Focus on cleanliness. Use an alcohol based glass cleaner, not an ammonia one. I don't know what you have available, locally.

[Rick Wilton]

in my experience 3mm float by it's self is a nightmare for devit. I don't fire anything less than 6mm and 90% is 10mm +

[Lauri Levanto]

If you happen to have old fashioned iron fillings in your teeth, you can test the tin side by licking. If you lick the tin side, you felel slight electricity. With air side not.
Remember to reclean the glass after that.

[Bert Weiss]

in my experience 3mm float by it's self is a nightmare for devit. I don't fire anything less than 6mm and 90% is 10mm +

I have always shared that opinion with Rick. If I had to choose just one glass to use, it would be 10mm float.

That said, oddly enough, I cast some 3mm broken tempered, and then slumped it. Result was perfectly clear and shiny.

[beva]

This was my first thought too, but while foruming, I test fired another piece of glass. It is a single layer 3mm float. Now the outer side came out with that slight silky finish. Just the ring area where the glass bent. I'm quite confused...

There are 2 different causes for fogginess. One is tin bloom. The other is devitrification. Devitrification is pretty complex. It happens from some combination of uncleanliness and temperature. I am able to avoid it on most all first firings. Each time you fire float glass, you increase the chances for devit to show up. I am pretty conscientious to fire my float as low as I possibly can get away with. I seldom fire hotter than 1480ºF (805ºC).

On the top layer, a borax overglaze can prevent devit, if you fire to at least 1400ºF (760ºC). On the bottom, I am not used to having this happen.

Focus on cleanliness. Use an alcohol based glass cleaner, not an ammonia one. I don't know what you have available, locally.

I clean the glass very carefully. It's a mystery. But thank you anyway.

[beva]

in my experience 3mm float by it's self is a nightmare for devit. I don't fire anything less than 6mm and 90% is 10mm +

You mean that devit depends - among others - on thickness??

[beva]

If you happen to have old fashioned iron fillings in your teeth, you can test the tin side by licking. If you lick the tin side, you felel slight electricity. With air side not.
Remember to reclean the glass after that.

Fortunately I don't have any. But I can check my neighbourhood

[Morganica]

Devit is simply crystallization of the glass. Glass is, well, glass, because the molecules in molten glass don't have any bits to clump onto (nucleation sites) while cooling, which would start them forming orderly arrays (crystals), and glassmakers ensure that the glass cools too quickly to allow the molecules to move into those orderly arrays. The cause of devit can be mechanical, chemical OR thermal--there really isn't a set temperature at which glass is guaranteed to devit, but if you hold it in a soft-not-flowing state long enough you're certainly increasing the probability that it'll devit like crazy. Presumably you can prevent devit by doing the opposite, but I'm finding that in float, devit tendency is very glass-dependent. The schedule that eliminates devit in one glass has absolutely no effect on another.

I've had 6mm float that doesn't show any devit at all, but it's rare. It could be that the float formulated especially for fusing is more devit-resistant, dunno, never tried it. I can sometimes predict the tendency to devit by the shade of the glass on edge--aquamarine glass seems less devit-prone than strong iron greens. Unfortunately, because my float is all salvage, I have no idea who makes the "good" glass or its composition.

Borax glazes may prevent devit but they can also cause more problems than they solve. Uneven application can create blotches that look worse than the devit, and if the solution lands on a kilnshelf or mold it can fuse to it--I tore divots out of one of my favorite slumping molds that way (and put divots in the glass where it fused to the mold through the kilnwash).

I wanted devit for this piece to ensure strong outlines of the rows of glass. (And actually got more than I bargained for--this was a beveled glass shelf in a gorgeous, very green tempered glass, and I arranged the crumbles in very precise rows--it's devitted so far into those rows that it's lost a lot of transparency). I set up a casting schedule that included a 6-hour hold at 1600F, and a ramped anneal over 20 hours that started at 1050F and went down to 960F (the piece is 4 inches thick at the nose). Tempered crumbles make for lots of sharp edges and surface area (good nucleation opportunities). The front shows how the devit followed the rows of glass; the back shows the crystallization on the tempered crumbles, which was extreme. I very much like the effect, but I'm going to coldwork down the sides to see how far it takes to get to the transparent glass.