14-10-2023, 08:11 PM
Jamie: i made my own green sand from a bag of play sand and a bag of bentonite clay (also known as cat litter) I bought from ebay, total cost less that £15 if I recall and of course can be reused. I have used plaster of Paris in glass casting but it is a pig to dry and can shatter under the first bit of extreme heat if not 100% dry. For glass casting you also have to mix with silica sand and wire mesh for strength.
Regarding shrinkage, I didn't have to worry too much as I was making 'art' rather than something that needed to be of engineering performance or size but certainly I saw a dipping of the top flat surface of the block I was casting. I think that shrinkage is also a more complicated subject causing porosity and/or dipping of flat surfaces, rather than simply making the mould 5% bigger for example. If yours is a large dish shape then you will have to decide if a concave finish is OK or if you make it thicker and machine it back to flat.
Regarding shrinkage, I didn't have to worry too much as I was making 'art' rather than something that needed to be of engineering performance or size but certainly I saw a dipping of the top flat surface of the block I was casting. I think that shrinkage is also a more complicated subject causing porosity and/or dipping of flat surfaces, rather than simply making the mould 5% bigger for example. If yours is a large dish shape then you will have to decide if a concave finish is OK or if you make it thicker and machine it back to flat.
Enjoy yourself, it's later than you think!