White scum on open face cast

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Andyr
Posts: 2
Joined: Fri Apr 26, 2013 10:35 am

White scum on open face cast

Post by Andyr »

Hi: I have been doing some 10" open face casts about 5/8" thick and have been getting some white scum on the surface that is difficult to get rid of. From what I have read here and elsewhere, it seems that it is devit caused from being too hot for too long. My molds are a 50/50 silica plaster mix and the glass are 96 billets and flat glass. Currently I take it up slowly to 1525F and hold for 2 hours. I tried a shorter time/lower temp but the glass did not flow to fill in the jagged edges of the mold. I also do not crack the kiln open when going down from peak temp but rather let it cool afap with the lid closed.
So my questions are:
- is this devit or something to do with the mold mix?
- what variable -peak heat or length of time - should I change?
- does venting to cool down the kiln faster help?
- once I have this infernal scum how do I get rid of it besides sandblasting?

Any help would be appreciated.
Andy
Morganica
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Joined: Mon May 19, 2003 6:19 pm
Location: Portland, OR
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Re: White scum on open face cast

Post by Morganica »

Please post pictures, the full schedule, and tell us where the scum is in relation to glass and mold. How are you making your molds? Do you allow them to cure or put them in the kiln right away? How are you venting the kiln? Are you filling the mold with frit, especially next to the plaster?

Also, you're allowing the kiln to freefall-cool from the process temp? Seems fast for a 5/8 inch casting with mold plaster around it.

It sounds like a mild devit, but hard to tell. Once the glass is scummed up, though, it usually takes coldworking to get it off. Try using a pressure washer to remove as much as possible. The remainder of the scum can be sandblasted, ground off with tools, or hand-sanded with wet-dry sandpaper and a little water.
Cynthia Morgan
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Andyr
Posts: 2
Joined: Fri Apr 26, 2013 10:35 am

Re: White scum on open face cast

Post by Andyr »

Thanks for responding so quickly. I can not get a pic right now but here are some answers to your questions.
My mold was handbuilt over a clay positive with a 50/50 pottery plaster and fine silica mix. Its about 1.5 inches thick all around. After air drying for about a week, I put it in the kiln and dryed it for about 3 hours at 212F. The mold was filled with a combo of a) coloured flat Spectrum 96 glass cut into pieces about 1 inch square then B) over laid with broken Uroboros 96 casting billets to make up the bulk of the glass. No frit. All told, about 5 lbs of glass.

For the firing schedule I used a combo of what was on the Uroboros and Spectrum websites.
My firing schedule:
1) 0 to 1225 at 212 per hr and hold for 90 min (kept the vent hole open until 1000 in case of any more escaping moisture)
2) 1225 to 1525 at 250 per hr and hold for 2 hrs
3) 1525 to 960 at AFAP (but no extra venting) and hold for 2 hrs
4) 960 to 800 at 75 and hold for 0
5) 800 to 600 at 150 per hr and hold for 0
6) 600 to room at 200 per hr

I noticed two types of devit. On the side touching the mold I found alot of the whitish scum. On the open side, at some of the extremities around the edges I found abit of glass distortion that looked like the surface of the glass was abit puckered or deformed. I can work with the latter but the scum is abit of a problem.
I have tried wet silicon carbide sand paper and it seems to take off alot of the surface look and only some of the scum. I will try a power sprayer tomorrow. If If I do sandblast it, is there any type of protectant that will guard against the elements or greasy fingers?
Once again many thanks for your help on this one.
Andy
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