From time to time I get minor bubbles or scratches when making moulds. These don't pass as perfect but are completely usable. Nothing that can't be wiped off with a sponge after casting.
 
Plaster mould for slip casting ceramic cups. 3 sizes available. Limited stock
Important note: Moulds are 8% bigger than dimensions to allow for an average shrinkage rate. 
 
Handle moulds available in separate listing
 
Drawing with dimensions available here