One of the fundamental elements in kiln forming is gravity. When glass is hot it moves according to the effects of gravity and you have to remember that gravity has a big effect on all your firings.
The effects mainly cause:
- Uneven thickness on shelves that are not level.
- Uneven slumps into moulds which are not level or the glass is not levelled.
- Uneven forming due to varying viscosities. Gravity acts on the softest parts of the glass first.
- Faster or slower forming due to span width. With greater span, gravity pulls the glass into the mould more quickly than with a small span.
- Gravity acts on things of greater thickness more quickly than those of lighter weight. So a thick piece will form more quickly than the same sized thin piece.
- Surface tension (affected by viscosity and heat) is affected by gravity also. The glass will attempt to draw up or spread out to about 7 mm if there is enough heat, time, and low viscosity.
- The effect of gravity causes upper pieces to thin lower ones, as it presses down while the glass is plastic. This has the effect of making the colour of the lower piece less strong.
More information on each of these effects can be found throughout this blog.