How to Make a Glass Slump Mold
Slumping is the process of melting glass in a kiln until it bends to fill a mold. Glass will melt at different temperatures depending on its composition. For this reason, make certain that all of your glass is compatible. Clay molds are ideal for glass slumping because they can take the high heat and be reused dozens of times.
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Things You'll Need
- White earthenware raku clay
- Guitar string or other thin wire
- Cotton cloth sheet
- Rolling pin
- Sharp kitchen knife
- Plastic wrap
- Haike brush
- Kiln wash
- Kiln
Instructions
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1
Cut the lump of clay with an old guitar string or other thin wire. A regular knife will not cut through thick pieces of clay without binding to the clay surface. Raku clay has added frit, which is course material to absorb thermal shock. Work on an old sheet so that the clay will not stick to the table surface.
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2
Roll the clay into a slab that is 1/2-inch thick with a rolling pin. If the clay sticks to the rolling pin, lay another piece of sheeting on top of the clay; you are then rolling the clay between two pieces of cloth.
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3
Cut the contours of the shape you wish to from into a glass slumping mold. Use a sharp kitchen knife to cut through the clay and remove the outer scrap pieces so you are left with a shape larger than you wish the finished piece to be.
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4
Form the slab into the shape you desire with your hands; a low concave vessel is best for a mold.
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5
Allow the mold to dry completely. This may take a few days depending on the humidity in your environment. Cover the clay with plastic wrap as it dries to help prevent cracking.
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6
Fire the mold in a kiln to cone 6 or 2,165 degrees Fahrenheit. This temperature is higher than the average glass slumping temperature of 1,400 to 1,600 degrees, therefore the mold will withstand the repeated high heat of slumping.
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7
Brush kiln wash onto the mold in four directions to keep the glass from sticking to the clay and forming a permanent bond. Apply four coats, each brushed in the four directions to ensure complete coverage; use a haike brush to apply the kiln wash. Kiln wash or its components can be purchased from a ceramic dealer; it is made of equal parts of kaolin and alumina hydrate, with water added until a yogurt-like consistency is achieved.
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Tips & Warnings
Quartz inversion is the temperature when clay changes in volume. Avoid extreme temperature changes around this point, which is 1,050 degrees Fahrenheit. This will prolong the usage of the clay mold as it will prevent cracking.
Always heat molds slowly for slumping.
Kiln wash needs to be applied each time you slump glass into the mold.
