Why do my slipcast items have small pinholes on the outer surface?

This is caused by air bubbles becoming trapped between the slip and the mould surface. This may be due to an excessively worn mould surface (in whcih case, replace the mould), or more likely by insufficient water in the slip. Remember that casting slips do of course become progressively thicker during use because the casting process is entirely dependant upon water being removed by the plaster, leaving behind a deposit of clay. Periodically this water will need to be partly replaced.