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I am bending quite a few large lamp panels and I am having a problem with getting inconsistent dull areas on the surface of the glass after I fire them. In order to get the molds to be hard enough to withstand a large number of firings, I have made them by first laying down a layer of Ultracal 30 and then pouring a thicker base layer of standard plaster for support. The molds have been working like I wanted but sometimes i am getting these dull areas. My question is whether these are deposits because of the Portland in the Ultracal 30 or are they etched areas. I know that if they are etched areas the only solution is polishing with cerium oxide but if they are deposits from the Portland, what is a good solution for them?

What kind of glass?

The glass is Kokomo 145. It's an amber and white opal.

I would guess it's just good old Devitrification. I'm not sure there is much you'll be able to do about it as kokomo is not formulated to be ran through a kiln. ( I think, could be wrong) Some stained glasses will almost disintegrate when taken to fuse temps.

I'd suggest trying with either Bullseye glass or spectrum and see what happens.

Like he said but you can try rinsing with distilled water after cleaning, never using windex and venting the kiln to 1100 f during the firing. Since it is an intermittant problem this might cure. R.

I have been using this particular glass for twenty some years now and have not had the problem that I am having now. I am not taking it to fusing temperatures but rather bending temps (1350 F) and my cleaning technique isn't any different than it has been for hundreds of other panels. It may be devitrification but why?

maybe new molds and heat retention.
maybe New cullet mixture at the maker's plant.
maybe new packaging materials being used in shipping glass to you or their storage methods.

Thanks Kevin,

I realize that all of those are possibilities but I am still wondering about the possibility that the Ultracal is off gassing something different than the standard low density plaster that I usually use. After I finish this project I will conduct some experiments on smaller molds to see if I get something similar happening with the Ultracal.

I think the 1350F is your problem.. Drop bending temp to 1200 and hold for bend.. should eliminate devit.

What I do is run up to 1250F slowly and then go on high the rest of way to the temp needed to do the drop. Because of the speed of the temp increase, the actual glass temp lags a little behind the air temp that is read by the pyrometer. That being said, I did drop my top temp from 1350F to 1335F and that seemed to get the slump while eliminating the devitrification. Occam's razor strikes again.

Devit usually starts around 1300F for glass not formulated for fusing!

This is a really interesting thread to me. Thanks for the discussion. I love it how fine a line there is between success and not success. I got stumped when Terry said he has used the same glass for 20 years with no problems. Thermocouple drift can actually create a temperature difference in that range.

Many years ago, I did some Wissmach opalescent slumping and it was quite difficult to avoid the devit. It really helps when you know it can be done, and finding that narrow window is the challenge.

Thanks Bert,

It appears that one thing that is different this time is the mold mass. I thought originally that there would be more of an issue with the large molds heating up too slowly but in fact the large molds seem to heat up faster than the small ones. It is probably due to heat retention in the larger molds (as Kevin suggested) but that doesn't explain why it seemed to happen with a long cool down just as consistently as with faster cool downs. In any case, since I lowered the max temp of the slow heat up (1230F-1250F) along with max temp of the fast heat up (1335F), I seem to have eliminated the problem.