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avoiding sag in a mold
Posted: Wed Jul 03, 2013 1:54 pm
by Lisa C
Hi everyone!
I did this marvelous little dish for a friend, and I'd like to replicate it... without the saggy sides. The mold is pretty steep. What recommendations can you make to reduce this problem. Should I cut the glass any differently, or is it all in the firing schedule? And can you suggest a schedule to use? I'm assuming slower and less heat.
Thanks!
Lisa
Re: avoiding sag in a mold
Posted: Wed Jul 03, 2013 2:55 pm
by lorimendenhall
I am guessing so wait for a more experienced person to answer... lol. I am thinking it's sagging because it's too hot for too long. I bet your cutting is fine. can you slump at a lower temp than you used here?
Re: avoiding sag in a mold
Posted: Wed Jul 03, 2013 2:58 pm
by Lisa C
Hi Lori,
Yes, I think I can do a lower temp. This one, I believe went to 1250. I don't have my notes with me right now, so it's off the top of my head.
Re: avoiding sag in a mold
Posted: Wed Jul 03, 2013 6:54 pm
by The Hobbyist
A picture of the mold would be helpful.
Jim
Re: avoiding sag in a mold
Posted: Thu Jul 04, 2013 11:49 am
by Lisa C
Hello Jim,
I have attached pics of 2 angles of the mold.
Re: avoiding sag in a mold
Posted: Thu Jul 04, 2013 1:30 pm
by The Hobbyist
I don't think you can do better than you did.
Just for fun I would try slumping a circle in that mold.
Jim
Re: avoiding sag in a mold
Posted: Thu Jul 04, 2013 2:01 pm
by Bert Weiss
Shapes like that in glass are caused by 2 factors. Glass thinnner than 6mm, when it gets hot enough, will shrink in on itself. This will happen on a flat shelf. Search on this site for "dog boning" . The corners don't shrink like the center of the square. The other reason this happens is when the glass slumps and stretches. It will first pull in on the center of the square as it is easier to move than the corners.
To get some shapes, you have to use a series of molds, each one steeper than the last, and precisely sized and drafted. Karl Harron has been teaching how to work with a set of molds he designed for this purpose. He took years of R&D to come up with the shapes and sizes. You can buy the molds from Bullseye. They are round, not square though. Might be awfully tough to get much better with a square shape. It is all about the relationship between shape, surface tension, and viscosity.
Re: avoiding sag in a mold
Posted: Sun Jul 07, 2013 2:20 am
by seachange
Hi Lisa C,
Have you tried BE mold 8934? I am guessing that this mold should give a similar nice little dish shape, but be easier to use, because of the way the glass would sit on the sloped sides of the mold, instead of just on the edge.
Just got the mold, but haven't used it yet.
Best wishes, seachange