Why Make a Hump Mould for Bowls?

A hump mould is one of the most useful tools in a handbuilder's studio. It is a smooth, convex plaster form that sits on your work surface, and you press a rolled clay slab over it to make a bowl shape. Because the plaster is rigid and sealed, every bowl you press comes out with the same profile, curve and diameter. You can make a run of matching bowls for a dinner service, or use the same hump for small dishes and planters.

Hump moulds are also excellent for UK makers who work in small spaces. They need no wheel, no kiln until the final firing, and no complicated equipment. A bowl, a bag of pottery plaster and a few hours of drying time are enough to create a reusable form that will last for years if you seal and store it well.

Choosing the Right Plaster and Bowl Form

Use a good-quality pottery plaster or casting plaster, not the quick-setting DIY filler sold for wall repairs. Pottery plaster sets hard, absorbs moisture slowly and has a low expansion rate, so your mould will not warp or crumble under repeated use.

For the original shape, choose a smooth-sided bowl with the profile you want to repeat. The inside surface must be free from scratches, because every mark will be transferred to your plaster hump and then to your clay. Deep bowls give a deeper hump, but a gentle curve is easier to press than a sharp shoulder.

You will also need a level surface, a bucket, scales, a stirring stick and a thin release agent such as washing-up liquid.

Casting the Hump

Work in a well-ventilated area and wear a dust mask when mixing plaster. Weigh 1 litre of clean water into your bucket, then sprinkle in about 1.4 kg of pottery plaster. Let it slake for two to three minutes, then stir gently for another two to three minutes. Tap the bucket to bring bubbles to the surface.

Coat the inside of your chosen bowl with a thin film of release agent. Pour the mixed plaster into the bowl in a steady stream, filling it to the rim. Tap the bowl gently to release trapped air. Leave the plaster to set for 30 to 45 minutes. It will warm up as it cures; wait until it feels cool before demoulding.

Turn the bowl upside down and flex it gently. The plaster hump should slide out. The convex surface that was against the bowl is now the top of your hump, and the flat surface from the pour becomes the base. Let the hump air-dry for at least three to five days, away from direct heat. It is ready when it feels dry, looks pale and sounds hollow when tapped.

Sealing and Finishing the Mould

A plaster hump will grab damp clay unless you seal it. Sand the base flat and smooth the rim with fine abrasive paper, then dust off all plaster residue. Apply two or three coats of shellac, letting each coat dry completely. Follow with a thin coat of soft wax or beeswax, buffed to a low sheen. This gives you a smooth surface that clay will release from easily.

Do not use cooking oil or oily furniture polish. They stay tacky, mark the clay and can interfere with glazes. If you prefer a water-based sealer, check that it is suitable for plaster and let it cure fully. Once sealed, store the hump on a dry shelf. Never soak it or leave it in a damp cupboard.

Pressing Slabs Over the Hump

Roll a slab of clay to about 6–8 mm thick. Leave it on a batt until it has firmed up to soft leather hard. It should hold its shape when lifted but still bend without cracking. If the slab is too wet, it will slump; if it is too dry, it will crack at the rim.

• Place the slab centrally over the sealed hump.
• Press from the centre outwards with a damp sponge.
• Ease the clay down the sides with the flat of your hand. Avoid pulling or stretching.
• Smooth with a rib, then trim the rim with a needle tool.
• Leave the bowl on the hump until leather hard.
• Run a palette knife around the rim, then lift the bowl away. Support it with a batt or foam pad.

If you want a foot ring, turn the bowl upside down once it is leather hard and attach a coil or a thrown ring. Cover it loosely with plastic and dry it slowly. Rushing the drying stage is the most common cause of cracked rims and warped bases.

Troubleshooting and Repeatable Results

Bubbles in the plaster? Mix more slowly and tap the bowl. Clay sticking? Reseal with shellac and wax. Rim cracking? Your slab was too wet or you stretched it. Let the clay firm up before pressing. Mould feeling soft? Your plaster mix was too wet; weigh accurately next time.

Label the base of each hump with the date and bowl diameter. Keep a notebook of slab thickness, clay type and drying times. In a cool UK workshop, a bowl pressed in the morning may need until the next day to reach leather hard. With a well-sealed hump and a little patience, you will build a set of bowls that match beautifully.