11.5 Moulding

Today, original designs are made of modelling clay (plasticine), sometimes on a plaster-of-Paris frame. They are made about 25% larger to make up for shrinkage. The model is then cut up into appropriate pieces, ensuring undercut or complicated parts can be released. As an example, a teapot might need a 4-part mould to enable it to be removed from the mould on completion. These pieces are then used to make the plaster-of-Paris moulds, which is used in turn to make a hard “positive” mould called the “master mould”.

Elephant mould, left positive master,
right negative working mould. Image
courtesy of the Potteries Museum and Art
Gallery, Stoke-on-Trent

Store of master moulds. Image courtesy of the
Potteries Museum and Art Gallery, Stoke-on-Trent

Sevres postcard showing porcelain
modellers, 1,900-1,910 - courtesy R&G
McPherson Antiques

Figures are invariably more complicated and standard figure models may need to be cut into 20 to 30 pieces and moulds made from these, with the previously mentioned Worcester Arab Stallion needing 38 moulds. The master moulds are then used to make working moulds, but only about 25 of these can be made from a master before its definition deteriorates unacceptably. Clay in the moulds shrinks on drying and the item can then be removed. They may be allowed to dry out a little more before being luted (stuck) together with water or slip. Some spectacular models have been made in the past by the major porcelain manufacturers.

11.6 Kilns

All early kilns used an intermittent batch process, where the wares are placed in the kiln cold and heated.

Model of a Roman kiln partially
underground. Image courtesy of the
Potteries Museum and Art Gallery,
Stoke-on-Trent

During the UK industrial revolution the “bottle kiln” was developed that was used up to the middle of the 20^th century. They were relatively sophisticated and the culmination in the art of batch firing. Bottle kilns were made of a tall inner kiln incorporating a chimney, and a shorter outer skin or “hovel” to contain the fireplaces and to protect the kiln from the weather, giving it the characteristic bottle shape.

Bottle kilns in pottery. Image courtesy of
the Potteries Museum and Art Gallery,
Stoke-on-Trent

The kiln was built of firebricks and was typically 14 ft in diameter inside, having around eight fireplaces.

Bottle kiln hovel showing
fireplace/stoking hole, and upwards to a
vent. Image courtesy of the Potteries
Museum and Art Gallery, Stoke-on-Trent

It was filled with wares and then the doors would be bricked up just before firing and broken down after. For updraught firing the flames and gases passed through the kiln to the chimney at the top. For reduction firing, used for porcelain, the flames and hot gases were directed to the vault of the kiln, downwards through the contents and through holes in the floor leading to the chimney. Typically it would take 48 hours of firing to reach the maturing temperature, 2 to 3 hours to extinguish the fire and 40 hours for cooling. The fireman’s job was more of a skilful art than a science, as the only controls were the dampers, and the weather and winds could have a significant effect.