float tin side

[tonyroberts]

Does anyone know if Pilkingtons have changed their liquid metal ('tin') for floating their glass? My last two batches of Optifloat have fired with a distinct brown tinge. Multiple layers are coming out unpleasantly opaque. It's not much, but definitely there. It's most noticable where the tin side is in contact with sand: I changed my sand in case it had been adulterated, but it came out the same. And while I'm about it, I think they have altered the glass formulation: it seems the anneal point has gone up maybe 10C to the mid 560s. (I buy in my glass in stock sheets, so it's not likely that the supplier has substituted an inferior glass).

Any thoughts?

Tony

[Bert Weiss]

ugh!

All I can say is that the color tin often makes is brown. So my guess would be more tin, not less.

[Stephen Richard]

Tony,
I had to fire some optiwhite recently (10mm) for LED~ lit shelves.
I had lots of trouble with it until I fired on pre-fired fibre paper and very fast. I used a gas kiln and it came out with excellent clarity.

[tonyroberts]

Thanks Richard, bert. The casts I was making (they're done and delivered now) were formed from three layers of 10mm glass over an area about A1. A 6-day firing. Experimenting, I cast a stack this high, tin side down, over a new silica plaster mould embedded in a much-used talc and sand bed, all dammed by prefired ceramic board. The cast was imperfectly clear over the plaster, horribly stained brown (tea coloured) over the sand, beautifully clear against the board. I've ditched the old sand bed as a result of this test, so maybe next time will be better. But a necessarily long firing like this I suspect will always be less than perfectly clear.

All opinions welcome. Optiwhite is low-iron: maybe that helps?

Tony

[Bert Weiss]

One time I fired Starphire and it got white stuff stuck to the top surface. I had Jack Pollocked some blue enamel and put on blotches of platinum lustre. I threw my hands up and decided that if my client had insisted on Starphire she would have to live with the marks. I took the glass to be tempered. When it came out of the tempering oven, it was perfectly clean and shiny. I have described this incident to many people. One person said he thought the marks were from sulphur and burned off. It has always been a mystery to me.

Tony, one thing I have seen happen that made the tin side turn brown is contact with turquoise vitreous color. I believe the copper in the color reacts with the tin and turns brown. Cobalt blue will turn a darker (brownish) shade of blue when fired on the tin side. Many colors show no visible reaction. I was really surprised when I tested Youghiogheny EZ fuse colors on the tin side and none of them showed any reaction at all. I do a range of work that only works right when I put my colors on the tin side, and fire tin side down. Then when I slump in to a bowl shape, I do not get tin bloom. There are a few limitations to the technique, but I love the results I get when it works right. I can make a painted bowl in one firing, with the color on the outer surface. There are tricks to getting it to work right.