Glass can be trapped in or on moulds in various circumstances. These usually relate to the relative expansion and contraction characteristics of the glass and mould. The two materials most usually concerned are steel and ceramic.
Releasing glass from steel
Frequently when using steel as dams around glass, the glass becomes stuck inside the steel. The cause of this is the greater contraction of the steel than the glass. On cooling, the steel compresses the glass tightly.
Another circumstance where glass is trapped is while slumping glass into a steel vessel. If the draft of the sides of the vessel is steep, the glass cannot slip upwards as the steel contracts against the glass, so trapping the glass.
Most successful attempts to remove the glass from the steel are like removing a metal lid from a glass jar. Heat the metal and try to keep the glass cool. You can run hot water on the steel while keeping the glass cool. This will most often allow the glass to be pulled from the steel surround, assuming there was a glass separator applied to the steel.
Putting the whole assembly in the freezer will only increase the grip of the steel as it will contract even more than the glass.
Prevention of trapping the glass involves placing a cushion between the steel and glass. This is usually 3mm fibre paper. Sometimes this has a layer of Thinfire added to give a smoother edge to the glass. Other times, the fibre paper is coated with boron nitride. There is no need to use both Thinfire and boron nitride, of course.
Releasing glass from ceramic
The difficulty of glass trapping ceramic occurs during draping. Ceramic expands and contracts less than glass. This means that the glass will trap a kiln washed ceramic shape with a steep draft. The glass on cooling, contracts more than the ceramic which means the glass is tightly encasing the ceramic.
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| A ceramic draping mould from which it may be difficult to remove the glass. |
Most successful attempts to remove the glass from the ceramic form include either gently warming the glass or freezing the whole assembly. You could place the glass in a bath of warm water. This encourages the glass to expand, but does not heat the ceramic. This usually provides enough gap to ease the glass from the ceramic form.
The other approach is to put the whole into the freezer. This is utilising the greater contraction of the ceramic to release the glass. This is less immediate than the warming of the glass, of course.
Prevention of trapping glass on ceramic with shallow drafts involves covering the form with 3mm fibre paper to provide a cushion during the contraction. The fibre paper may need to be attached to the form by binding with high temperature wire, as glues will not survive the heat of draping.
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