Slumping means heating glass in a kiln over a mold until it takes the shape of that mold. The slumping glass process is used for making bowls, platters or decorative 3-D objects.
There's a huge variety of commercial moulds for sale, or you can make your own and create something truly unique without too much extra work.
There's three main ways of shaping glass - into, over, or through a mold.
Bowl with folded edges from being slumped over a mold
There's lots of different types of materials that can be used for slumping molds. You can buy ready-made ones, use metal bowls or even found objects.
The two key things to remember are:
Two commercial bowl moulds for slumping
- The good thing about these moulds is that they are re-useable. You can either buy a commercial one made especially for glass, use a found pottery bowl, or make one yourself.
- As long as the clay has been bisque fired, and has a small (1/8"/3mm) hole drilled in its lowest point (see photo above) it should be fine.
- Clay moulds need to be brushed with kiln wash to stop your work from sticking.
- It's best to slump into, rather than over, clay moulds, as it shrinks less than glass when it cools down. Slumping over clay can cause the piece to crack.
2 Stainless steel
- These moulds are cheap, easily available and very strong.
- As the steel contracts more than glass when it cools, it's best to drape over this type of mold.
- It will need a separator, so brush kiln wash on with a haike brush. This is easier if you heat the mold to 1200F/650C first. Use heat resistant gloves!
- As long as the sides aren't too steep, it is possible to fire into these moulds, but there's a higher chance of breakage.
Stainless steel bowls from Ikea used for slumping
3 Fibre blanket/board/paper
- This is the most flexible of the materials on this page. You can shape this material to your own designs.
- Wear latex gloves and a mask when using fiber.
- You don't need kiln wash for fiber material, as the glass comes away from it easily after firing.
- The blanket is quite thick and can be gently shaped in the kiln with the glass fired on top. If you want more dramatic shapes you can place kiln props underneath the blanket in the kiln.
- You can buy a rigidizer if you want to make more permanent, complex molds.
- This is great for slumping texture onto a fused piece.
- Using a craft or exacto knife, you can carve into the surface of 1" (25mm) fiber board. Place your fused work on top and fire.
- The board will need pre-firing to 1400F/760C before it can be used as a mould if the manufacturers haven't done so already.
- By cutting shapes out of and into 1/8" (3mm) or 1/4" (6mm) fiber paper, you can create shallow 'moulds' for your glasswork.
- These can be layered on top of each other to make a stepped mould that looks like a bas relief.
- Place your glass on top and fire.
4 Miscellaneous objects!
Lots of found items can be used as moulds. There's a couple of things to remember:
- it's best to heat them up slowly, in case they have any water in them (stones etc)
- they don't have any 'undercuts'. These are areas that the glass could flow into during firing which will prevent the mold from being released on cooling.
If you are a beginner it's best to start with a mold that is smooth and shallow with gently sloping sides.
Anything deep, with steep sides is tricky, as the glass will want to 'shrink' at the edges as it slumps.
Textures and wavy edges are difficult, so try and choose one with wide, smooth rims to start with.
The following books are great for learning about slumping. You can find them on Amazon.
- Warm Glass: A Complete Guide to Kiln-Forming Techniques: Fusing, Slumping, Casting by Philippa Beveridge
- Kiln Firing Glass – Glass Fusing Book One by Boyce Lundstrom
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