Casting plaster is considered a good starting point to the discipline of casting where there are a great number of casting materials with an even greater number of possibilities to the final finish of a sculpture. A cast is the positive reproduction taken from a negative mould and can be a product heated and poured like a wax or low melt alloy, mixed with water in plaster and cement casting or when using resins chemically activated. Release agents are often required to prevent the casting material from bonding to the mould or to protect it from damage. Smaller castings are often poured by hand in a process known as gravity casting whereas larger sculptures employ the building up of layers to produce a hollow casting, mechanical casting can be achieved using centrifugal force, The C300 Centricast machine can produce large numbers of castings with improved results when casting low melt alloys and pewter. Casting materials can be coloured using pigments or replicate stone and metal finishes when combined with filler powders or when casting a modelling plaster or wax be carved and re-worked to the final form of the sculpture.