The method of producing ceramic objects differs depending on the clay’s properties - its plasticity or liquidity. Throwing clay on a potter’s wheel requires a thick and plastic clay with a mushy consistency.
For slipcasting, the clay used needs to have completely different properties. This process requires a smooth, liquid mass with a milky consistency. This kind of clay is called “slip” and was transported to the casting workshop in large crates. About 60 people were employed in the workshop. They produced objects that could not be made by throwing clay, such as pitchers, coffeepots, washstands, figurines, oval trays, etc. Two narrow railways went through the workshop, where the plaster moulds stood on trolleys.
Plaster moulds absorb water from the slip poured into it. The clay closest to the mould’s walls will therefore start to dry within 10-15 minutes, while the rest of the slip remains liquid.
Sketch showing how ceramic objects can be slipcast in moulds. From the left – 1: The parts of the plaster mould are bound together and filled with liquid clay. After a while, water is absorbed into the mould and the slip starts to sink. The mould therefore has to be replenished with more slip. 2: When so much water has been absorbed by the mould that the clay starts to solidify, the mould is turned upside-down so that the remaining slip is poured out. 3: When the object is so dry that it is releasable from the mould, the mould is opened and the object removed.
The potter would cut the clay mass with a knife to check if it had reached the desired thickness. When the object’s walls were sufficiently thick, the remaining slip was poured out and returned to its original storage crate. The workers transported the slip in pitchers. When the superfluous slip had been poured out, the plaster mould kept absorbing water. This made the cast object shrink and release from the mould, so that it could be removed. When the object had been removed from the mould, the hole left from the pouring process was sealed and the object smoothed and sanded. After this, a female worker would clean the mould, dry it, and then return it to the workshop to be reused. Slipcasting and the removal of objects from moulds was men’s work. Sealing, smoothing and sanding was women’s work. Women were also tasked with casting handles for cups, pitchers, etc. The handles were cast in two-part moulds. After being cast, the handles were covered with a damp cloth and brought to the potter’s workshop. There, the handles were attached. It was important to make sure that both handle and cup had the right level of moisture. If not, one risked the handle not attaching properly and falling off.
How many times a mould could be used depended on what type of object was being made. If it was for objects to be smoothed, the mould could be used for years. If the mould leaked, the crack could be sealed with a lump of clay. This was not the case for objects to be sanded. The seams became to big and expanded when the object was fired, so that it became ruined.
The basis of slipcasting is as mentioned plaster’s ability to absorb water. There are two different types of casting moulds, both used at the factory. This is the single and double mould. Single casting is done in a mould with only an outer wall. Double casting is done in a mould with both outer walls and an inner core, where the core is made to reflect the cast object’s inner shape.
When double casting, both the inner and outer shape of the object is decided by the plaster mould. When single casting, the inner form is decided by how long the slip is left to be absorbed by the mould. This makes object thickness vary, and leaves the inside of the object less smooth, often with marks from the slip mass. Double casting produces objects with an even thickness and a smooth surface. This is because water is absorbed from the slip from both sides at once.
A mould is composed of several parts, as a kit. The more complicated the object, the more parts. This is necessary to be able to remove the mould after casting, without damaging the object. A good deal of expert knowledge is thereby required to make moulds with the fewest parts possible. When objects had been removed from their moulds, they were put in crates to be finished. One man was fully employed carrying such crates to the female workers who would smoothen and sand the objects, and controlling their work when they were done.
Each of the women were given a blackboard and a slate pencil. The foreman handed these out in the morning. In the evening, the boards were collected and delivered to the factory manager’s office.
Work in the casting workshop became piecework following the wage agreement of 1935. Hourly wages were set to between 105 and 110 Norwegian øre for men and between 55 and 60 Norwegian øre for women. Several of the work operations required in the casting workshop required a great deal of physical strength. The moulds for toilets and washstands could weigh well above 50 kilos. The potter had to be able to carry these moulds back and forth himself.
When the finished objects were ready to leave the casting workshop, they were transported downstairs by a lift and carried into the drying room.