Problems with spots on the glass
- April 16, 2010 at 7:18 am #9391koviParticipant
I would like to ask if you can help me with the following problem. I get a lot of some eneven spots after firing the glass. I think as the temperature increases the spots are getting more. I’m firing up to 675 degrees Celsius I can also send you the whole firing schedule if you think that might be the problem.
I’m attaching photo to see it.April 16, 2010 at 7:32 am #11608Stephen RichardParticipant
It looks like you are bending this piece of glass. Is that right?
Are the marks on the top or bottom? You are bending over a mould, yes?
It sounds to me like you are bending over a form because the marks increase with temperture. Therefore the marks must be from contact with the mould.
Most glass bending takes place as a slump. This takes advantage of the weight to pull the centre down. It means you can bend your glass at as little as 570C with sufficient time. This avoids marks from the mould. If it is a free curve, it can even be done without a mould, just supports along each edge, about 50mm in from the edges. You need to cut off the extra after firing, of course.
You can get some more information from the two blogs listed below.
Stephen RichardApril 16, 2010 at 7:39 am #11607koviParticipant
Well, you have guessed much of the job. I’m bending over a mould of metal.The marks are at side of the mould, like they occured from the solution that I put on the mould to avoid sticking metal to glass. I have made curved glass at much lower temperature, but here is the tricky situation, because I’m also trying to fuse the paint on the glass – paint is something like ink and needs higher temperature. So I have to find the balance between needed temperature for the ink and the lowest possible for curving the glass – something like that. I was wondering if these spots appear if I’m firing too rapidly – but the steps are – 1 – 1.30 h to reach 300 C, 1.20 h to reach 550C, 1 hour to reach 675 C.
Thanks for the quick answer. I will also check your blog for additional info.April 16, 2010 at 7:29 pm #11606Stephen RichardParticipant
If you want unmarked glass, you need to do the process in two stages. First fire the paint, then bend the glass. It will be easier to bend the glass by dropping it into the mould rather than over, because you can use less heat and so avoid marking the glass.
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