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When I fire float and pre-float old plate glass over 1250F for any length of time, I sometimes get clouding. I thought it was tin bloom at first, not knowing what tin bloom is, but I figured since it was float, that I probably was enjoying tin bloom. Now that I've fired non-float old window glass and gotten the same result, I'm wondering if it's just ordinary devitrification, or should I consider blaming the kiln atmosphere and the paints I'm including. I fired some of the old plate glass at 600 dph to 1250F with no hold (barely enough to mature the paint I like to use) and the stuff didn't cloud up. It clouds up at 1275F though, and worse at 1325 which I like to use to firepolish. I guess I have a reasonable workaround, but I'd like to know what causes it. Can you see the cloudiness in the picture? Whaddaya think?

Don, is it occurring on one face or both faces? That would be the first clue.

I use a lot of salvage plate and float, and the older, thinner glasses (i.e,. 4mm or less), especially the older ones, will exhibit this fairly often. When it occurs for me it's spread evenly but not perfectly across the face, and on both faces. It's interesting because where devit usually starts at corners or edges, or around divots, chips, and other abrasions in the glass, this type of scumming is spread evenly but not perfectly across the face of the flat surfaces. In fact, if I get the entire piece of glass, it's more likely to be in the middle and not on the edges...the opposite of what you see in kilnformed devit unless you're really terrible at cleaning your glass before firing.

I've also found that glass pieces are far more likely to show surface clouding if they are untempered and deeper green/yellow-green, and less likely to show it when they are more aqua in color. (In fact, I've gotten so I avoid obvious green glass at salvage yards now unless I'm looking for pieces that I want to devit). I'm assuming the green color means it has a higher iron content, but haven't verified that so no idea if it's true.

What I suspect is that these older, thinner, greener glasses were more likely to be used in bulk construction, i.e., for glazing and cheap cabinetry. Once installed, they were probably washed with fairly harsh chemicals for years and years that abraded the surface. It could be that the greener glass is a bit softer than the aqua (again, I'd need to verify this because I have no idea if that's true right now), and between the washing, grit blowing against the windows, peoples' fingers, etc., the glass was etched enough to expose uniform nucleation sites across the surface so it would exhibit fairly even devit and look cloudy.

Where it was held in the frame/puttied/etc., around the edges it would stay clear. Or at least that's my theory. Bit romantic, no plans really to test and see if it's true. I just know that when I play around with float and plate glass, I get a much more transparent, devit-free result if I use bluer, thicker (and presumably newer) glass.

Interesting observations. Thank you. I'll do some experiments suggested by your thoughts and post results

That stuff doesn't happen to me. First, I never fire thinner glass than 6mm, and usually 10mm. I do routinely paint vitreous colors (mixed with Micas) on the tin side, then fire to 1420' paint side down.

I put some of the old plate glass back in the kiln with the program I used in the past: 600 dph to 1275°F. It didn't fog. It didn't have any paint on it though. I had cleaned one piece of it pretty well with Glass Plus, and noted which little end of it had been under putty in it's life. The other piece I cleaned with glass plus, then with lacquer thinner, then with isopropyl. But neither piece fogged. I'm going to put some paint on the pieces and try again.

The painted pieces didn't fog. They should have fogged. Maybe I cleaned them too well. No conclusions yet. Guess I'll try a piece that has only been cleaned with dish soap. If it doesn't fog, I'll find something more productive to worry about.

Don Burt wrote:The painted pieces didn't fog. They should have fogged. Maybe I cleaned them too well. No conclusions yet. Guess I'll try a piece that has only been cleaned with dish soap. If it doesn't fog, I'll find something more productive to worry about.

Paint is an overglaze which provides a different surface that is much less prone to fogging up. The key is what happens where there is no fresh glass coating.

Bert Weiss wrote:

Don Burt wrote:The painted pieces didn't fog. They should have fogged. Maybe I cleaned them too well. No conclusions yet. Guess I'll try a piece that has only been cleaned with dish soap. If it doesn't fog, I'll find something more productive to worry about.

Paint is an overglaze which provides a different surface that is much less prone to fogging up. The key is what happens where there is no fresh glass coating.

The test was actually to see if adjacent unpainted areas fogged. It was just a few lines of glass stainer's paint in clove oil medium. I was expecting to see a halo of fog around the paint, on the unpainted areas.. I did not, although I did on my first batch of suncatchers...not so much a halo, though, as it was a overall fog. Next and final test will be with unpainted, not-scrupulously cleaned glass. (final test until it annoys me again)

There is a good possibility some glass is shitty, others not so much...