Separator is most commonly used when floating appliances off of a life cast. Sometimes referred to as liquid tin foil and a similar product to the brand Alcote. Brush on to a plaster cast and dry before applying plastiline clay.
Once your sculpture is complete immerse the cast in water and this separator will dissolve allowing the plastiline to come loose from the cast to be transferred to a new positive for moulding.
Separator can also be used as a release for silicone gel appliances when making conforming 3D transfer moulds and as a sealer for plaster when moulding with fibreglass.
Some brands have a tendency to go mouldy and bad once opened but this product seems to have a very good shelf life.