What’s causing the discoloured patch and thin-glazed area on the cast?

This problem is known as ‘casting spot’. It is caused by the slip constituents separating at the first point of contact of the slip on the plaster surface. This phenomenon is always going to occur to some extent, and in a correctly designed mould the area which is going to have the first contact of slip is placed in an area where it will not show. Furthermore, the more fluid (thin) the slip is, the greater the separation so the more exaggerated will be the ‘casting spot’. Treat by lowering the overall level of deflocculation by additions of clay and water to thicken the slip.