Because I want to make a flexible casting, it is necessary to make a hard mould (when mouldmaking and casting, the general rule-of-thumb is essentially that opposites attract - if you want to make a rigid casting, make and use a flexible mould). Now, for various reason which I'll go into later, I need to make a latex (rubber) skin, and for that the ideal mould material is plaster. This is because the liquid latex required to make the skin dries partly by evaporation and partly through some of the moisture being absorbed by the mould, and plaster is ideal for this.
Again, Tiranti came to the rescue for plaster - in this case, I used Prestia Classic (Dental) Plaster. In retrospect, this was a mistake, but more on that later...
Now, the mould must be made in two pieces. This means making one half first, 'blanking off' the rest of the sculpture beforehand by making a dividing wall.
First, lay the sculpture (master, or pattern) down flat on a bed of clay - this bed allows the sculpture to settle into it so it doesn't move. Note the clingfilm, used so as to prevent the clay sculpture sticking to the clay bed:
The sculpture has been sprayed with an acrylic clear spray. This is vitally important! Water-based plaster will not take to drying when poured over water-based clay. The acrylic spray therefore seals in the moist clay, giving a dry surface for the plaster to dry on.
Blocks of 2x4 wood have been laid around the sculpture - these will act as a support for the sheets of clay which will be added to create the dividing wall, as follows...
Make sure the clay wall seals the edges of the sculpture by using soft tools made of wood, or your fingers.
Next, build another clay wall around the dividing wall - this is simply to stop the plaster slopping about all over the place, and gives a clearly defined edge to the mould. One final, important note, at this stage - it's not clear in the photos (there's one on the left, between the sculpture's 'hair' and the corner of the clay wall), but around the sculpture are four clay 'keys', wedge-shaped bits of clay which will form depressions or holes in the face of the mould. When the second half is poured later, it will form corresponding raised sections which will help the two halves of the mould fit, or 'key', together accurately.