TechniquesPorcelain - An Alchemists DreamSlip casting has been used in the production of many everyday items for many hundreds of years. It lends itself to porcelain due to the almost egg shell like thinness that can be achieved, enhancing the translucency and fine quality whilst retaining strength in the material. 
Originally moulds were made from bisqued clay (low fired ware which is still porous) but for the last century plaster of Paris has been favoured for it's ability to pick up the smallest detail in the model and for its high absorbency. The original model might be made from clay or plaster, either turned on a wheel or lathe and finished or polished to a high degree of smoothness. Alternatively it can be hand modelled out of clay depending on the desired end product. A clay mould is then constructed around the model in a number of parts depending on the complexity of shape. Mould making itself requires great skill and is the most important part of the process. Once the mould is complete it has to dry out thoroughly before being used. 
The clay is then adjusted into "slip" by adding a defloculant. The resulting runny clay is then poured into the mould and left to sit for a specific number of minutes to "cast". Water is absorbed from the slip into the plaster mould forming a harder shell of clay inside the mould. The residue is then drained out of the mould and reused again. An hour or so later the clay hardens to the point where the mould can be opened and the piece removed. The seam lines are later "fettled" (scraped off) and the piece is sponged at the opening or where it was trimmed. Handles or knobs can then be added or holes cut. After drying out completely the item is bisque fired to a temperature of about 1000°C. It is now hard enough to be handled, painted and glazed without causing damage. The glaze is then sponged off the bases before loading into the kiln for its final glaze firing. |