Hard spots in some moulds are the result of the method of creating the moulds. Most of the ceramic moulds we use in kilnforming are slip cast.
|This diagram shows the main stages of slip casting|
Slip casting is a way of quickly producing multiples from a mould. The original shape is surrounded by a one - or multiple - part plaster mould. This mould is used to contain the clay slip which is poured in.
The plaster absorbs water from the slip, stiffening the clay in contact with the plaster. After a defined time, the remaining slip is poured out of the mould. The clay remains in the mould a short time until it begins to contract from the plaster mould and is described as leather hard.
It is then de-moulded, trimmed and cleaned before it is further dried. When appropriately dry, it is fired.
Some moulds we receive show a spot where the kiln wash does not cover the surface in the same way as the rest. This is a result of the method of pouring the slip into the mould. Slip that is hand poured does not fall in the same place for long. But industrially poured slip often falls in the same place for the whole of the pour. This creates a hard spot - an area where the slip is more compacted than the rest of the object.
This hard spot does not affect the appearance or performance of the object. However, it does not absorb the water from the kiln wash as well as the other areas. And this is when the hard spot becomes apparent. It will still have enough separator to keep the glass from sticking, although visually it appears bare. If concerned, you can coat that area more than the rest after the kiln wash has dried a little. You need to be careful that you do not introduce an unevenness into the kiln washed surface, as that might appear on the slumped surface of the glass.