Always slump into ceramic, drape over steel
This myth is based on the fact that steel expands and contracts more than glass and ceramic expands and contracts less.
So, the myth goes, slumping into steel means the glass will be trapped or crushed by the contracting steel. But draping over means the steel will contract more than the glass making the removal of the glass easy.
The reverse is the expectation for ceramic. Slumping into the ceramic allows the greater contraction of the glass to be removed from the mould without sticking. But draping over means the glass traps itself against the ceramic as a result of its greater contraction.
These things are true. But….
The most important thing in considering a mould is the draft. This not about cold air, but the angles of the mould. A mould with vertical sides will not release the casting or kiln formed object even if the expansion characteristics of the two materials are identical. To release, the mould must have a slight angle from the vertical away from the glass. This applies whether a slump or a drape. This is called a positive draft, as illustrated.
If the draft is sufficient, it does not matter whether you are slumping or draping into steel. In using a stainless steel mixing bowl for draping, you can only use the lower portion where the angle is shallow. If you rest the glass on the rim, the draft will be too steep to allow the glass to slide upwards as the steel contracts on cooling.
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Even when draping over steel, you need to have a draft to aid the easy removal of the glass, as in this example:
When draping over ceramic, you need to be careful that you have sufficient draft over the whole of the mould. In the case of this ceramic draping mould you need to make sure the glass is not fully formed as the steep portion at the top will be where the glass grabs the mould.
And if you were to use this casting mould as a slumping mould, the steep straight sides would make it difficult to get the glass out of the mould.
Although the facts behind the statement “slump into ceramic, drape over steel” are established, you need to understand that the draft of the mould is as important as the way in which you use the material.
All myths have an element of truth in them otherwise they would not persist.
They also persist because people listen to the “rules” rather than thinking about the principles and applying them. It is when you understand the principles that you can successfully break the “rules”.