Slip casting terms

Slip casting terms:

Ball clay: used in casting bodies to make the slip more plastic. Generally coarse particle sized ball clays are used because they release water more readily than fine particle ball clays.

Block and Case models: made from the original mold to reproduce multiple working molds. The original mold is the block and the mold for reproducing is the case.

Blunger: a stationary shaft and propeller mixer used to mix casting slip.

Casting rate: a deflocculated clay body formulated for slip casting and generally consisting of 50% non plastics. Slip is mixed with 40 parts water to 100 parts clay, plus deflocculant.

Cottle boards: boards which are clamped together to contain liquid plaster.

Deflocculation: with the addition of a sodium solution, clay particles repel each other as the sodium solution changes the electrical charge on the edge of the clay particles to negative. Thus the negatively charged surfaces and the edges repel each other like two negatively charged magnets.

Fettle: the seam on the casting where two molds meet.

Flocculation: the electrical property in clay particles. Like magnets, the positively charged surfaces of clay particles bond with the negatively charged edges of the other clay particles.

Flux: a material that melts the silica in the clay body or glaze.

Frit: a manufactured glaze containing toxic or soluble materials which is melted into glass, then ground into powder and used like feldspar, flux or sometimes glaze as well.

Keys: the male and female fittings in a mold that make two or more pieces of the mold registry exactly.

Kiln Wash: for coating kiln shelves to reduce sticking of ceramic pieces if glaze runs. Generally, it consists of 50% kaolin and 50% alumina.

Master Mold: a plaster positive, generally cast from a plaster negative and used to produce working molds.

Mold soap: a sealant used to prevent a new pour of plaster from adhering to a previous pour.

Refractory: resistance to melting or heat. The most refractory materials in the ceramic cupboard are alumina and silica; the least are fluxes such as frits.

Rubber master: a master mold made of rubber. Generally they are used to accommodate undercuts, but also, rubber is a more tolerant and forgiving material than using a plaster positive. They are able to yield more production molds.

Slumping: the unfortunate deformation of a ceramic piece during the firing process.

Sodium Silicate: used as a deflocculant, also known as water glass.

Specific Gravity: the viscosity of casting slip is measured by its weight, expressed as a ratio heavier than water.

Thixotropic: a property in casting slip, plastic clay, and plaster. It is marked by the ability of the material to hold its own weight but when the energy is applied, it will begin to run liquid and will not support its own liquid.

Vent: a tube or scratch in a mold to vent air that would otherwise be trapped in the mold as it is filled with slip. This often occurs under the flange of a lid or where the reservoir is lower than the spouts or handles of the piece. Vitrification: the clay or glaze is mature or vitrified, when the heat of the firing melts these components to the point that they do not loose their stability but are dense enough to be impervious to water absorption.

Working mold: a mold used for production either made from a prototype or from a master mold.

Source: Michael Lamar