Glass Mould Making

by Chris
(Saco, Maine, USA)

Where would I find the type of flint needed to make glass slumping moulds? I am new at this and would like to learn the technique so my students can attempt the process in my sculpture class.
I am right in understanding that using plain plaster would not work?
I believe that I can use regular clay - what cone is best?
Thanks for your help.
Milly's reply: Your students are going to have fun! I buy all my materials for glass mould making from a pottery supplier - flint, grog and plaster.
You could just use plaster on its own, but the glass mold won't be very strong and is likely to crack at the temperatures needed to slump glass. The flint and grog strengthen the molds. You could risk it though, if it's more about process than finished result.
You're right about using regular clay - I use the cheapest available! If you're biscuit firing it, don't forget to brush batt wash on it after the clay is fired and before firing the glass. It works as a separator, otherwise the glass will stick to the mould.
Cones - you need numbers 14,15 or 16, depending on what your top temperature is (14=1540F/138C, 15=1479F/805C, 16=1458F/792C). Have a look at this Cone Firing answer for information on how to use them.
Good luck Chris, hope that helps. Let me know how your students get on?

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