How a plaster piece is cast, from design to finished ornament

A plaster piece is cast by first making a master model of the design, then taking a flexible mould from it, usually in silicone rubber. Gypsum plaster powder is mixed with water into a smooth liquid, poured into the mould and left to set. The piece is then demoulded, dried thoroughly and finished by hand to clean the edges and sharpen the detail.

The principle is centuries old. The materials have changed, and silicone has made moulds far more faithful and durable, but the basic sequence would be familiar to an eighteenth-century plasterer.

Design and the master model

Every piece starts as an idea and a drawing: its outline, its depth, how light will fall across it. Plaster ornament is seen in shadow, so depth matters as much as pattern. A shallow design that looks good on paper can disappear on a ceiling.

The design is then turned into a master model, sometimes called the pattern. Traditionally this was modelled in clay or carved, and run profiles were shaped with a metal template. Today it may also be carved, built up by hand, or produced digitally and refined by hand. The master must be perfect, because every flaw will be copied into every cast. It is sealed so the mould material does not stick to it.

Making the mould

Historically, plasterers used gelatine moulds or rigid piece moulds made of many interlocking parts. Modern studios mostly use silicone rubber, which captures very fine detail, flexes to release undercuts and lasts for many casts.

• Box the master. The model is fixed down and surrounded by a wall to hold the liquid rubber.
• Pour or brush the silicone. It is mixed with a catalyst, degassed or poured carefully to avoid bubbles, and left to cure.
• Add a case if needed. Larger or floppier moulds are given a rigid jacket, often of plaster or resin, so they hold their shape during a pour.
• Demould the master. The cured mould is peeled away, ready for casting.

The master is made once. Every cast after it is only as good as that first piece of work.

Mixing the plaster

Casting plaster is made from gypsum, a natural mineral, which is heated to drive off most of its water and ground to a fine powder. Add water back and it recrystallises and sets hard. That reaction gives off gentle heat, which you can feel as a cast sets.

The rule every plasterer learns first is plaster into water, never water into plaster. Measured water goes into a clean bowl, the powder is sprinkled evenly over the surface and left for a short time to soak, then mixed smoothly without whipping in air. The ratio of water to powder affects strength: too much water makes a weaker, softer cast.

Pouring, setting and demoulding

The first thin layer is often brushed or swilled into the mould so it reaches every recess without trapping air. The rest is poured in, and the mould is tapped or vibrated to bring bubbles to the surface. Larger pieces may be reinforced with fibre or hessian scrim laid into the wet plaster, the technique behind traditional fibrous plasterwork, which became widespread in Victorian Britain.

When the plaster has set firm, the back is levelled and the piece is eased out of the mould. Silicone peels away cleanly, but deep undercuts still need care. Plaster is at its most fragile at this stage.

Drying and finishing

A freshly demoulded cast is still full of water. It needs to dry thoroughly in a warm, airy space before it is handled hard or painted. Drying too fast with direct heat can cause stress and cracking.

Finishing is handwork. Seams and flashing are trimmed, small bubbles are filled, edges are cleaned with fine tools and abrasive, and the face is checked under raking light. This is where a cast becomes a finished piece.

• Fettling. The plasterer's word for cleaning up a cast.
• Raking light. A low light across the surface, which shows every flaw.
• Natural finish. Raw plaster is left unpainted, ready for a mist coat and emulsion.

From the studio to your wall

Every Flare piece is hand-cast to order in our Shrewsbury studio and arrives as natural plaster, paintable with ordinary emulsion. Our wall lights come as a shade only, with a battery LED and remote, or for UK mains hardwiring, which should be fitted by a qualified electrician. For the longer story, read our short history of plaster, see our history page, or browse the wall light collection.