Make sure its level

When making a mould for glass casting your mould needs to be poured onto a level surface. When heated up in a kiln glass will naturally fill the space it is in and form a flat level surface when cast, if the mould is not level then the glass won't be either. This is especially important when making flat relief open casts. 

  Make your model taller
As you are going to put cold chunks of glass or glass frit into the mould you make, the space that you allow for the glass to go in needs to have more depth than you intend the final glass piece to be. Glass Chunks and frit have lots of air gaps between them, when the glass melts and becomes liquid these gaps will disperse through the material.

 Learn to love the bubbles

The space between the glass chunks, frit or panes will become bubbles, as the glass is liquid it is very thick similar to treacle so the bubbles take a while to rise to the top surface and pop.  Some bubbles will stay trapped in the glass when it cools and turns solid. Some bubbles may pop on the surface at the end of the firing cycle, If you don't love this, it may be easier to re-fire the cast than to grind back the surface to a flat.

  Larger glass pieces = better clarity
When filling the moulds, the bigger pieces of glass you use the less bubbles will be trapped between them. If you decide to use fine frit for example this will trap lots of little bubbles in the glass and will make the glass less transparent. The bubbles interrupt and distort the light that is able to travel through the glass, hindering the view through.

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For more hints and tips Amy Whittingham has written a book called Glass Casting which is available via her shop. Amy also teaches in person glass casting courses from her studio click below to sign up.