Top Tips for Reusable Glass Fusing Molds

by Sharon
(london)

I have been making glass fusing molds from various types of plaster and managing to get 1-4 firings before they start falling to pieces....what is the best thing for me to use to make moulds that is not going to cost me an arm and a leg...and can you tell me why a mould has to be dry before firing and what happens if it is wet?

Good questions! One of the cheapest and easiest materials for glass slumping molds is clay. It's simple to mould into shape, is relatively cheap (you can use bottom of the range clay, nothing fancy!) and you can re-use it again and again.
You have to fire it first - a bisque firing - and then make sure you use a separator to prevent the glass from sticking to the glass molds. Use a kiln wash for this - dilute with water 4:1 - and paint on in four thin layers, each one dried out before adding the next. Try not to get brush strokes - they will pick up on the glass.
One word of warning; I never put that dirty brown stuff anywhere near my kiln, it will contaminate your lovely CLEAN glass kiln! If you have a friendly potter/ceramicist, get them to fire the fused glass molds for you. It's great that you're making your own moulds - it adds a uniqueness to your work that you won't find elsewhere.
Glass casting molds, or any molds that go in your kiln can damage the elements, break kiln shelves or cause devitrification if used damp. So it's good practice to dry them out beforehand. Having said that, I used to dry my glass casting molds out during the early part of the kiln firing schedule with no adverse results, but glass casting is a bit different to glass fusing or glass slumping, as the face of the glass isn't exposed.