Many of my ceramic bowls are press moulded inside plaster casts, and I also seem to need an endless supply of moulds for my workshops. Luckily, making moulds using of plaster of Paris is pretty simple and here are the steps;
Place your model bowl upside down on a slab of clay and fix it to the base with a thin fillet of clay. This is important as otherwise the bowl will start to float as you pour in the plaster. I’m using a pyrex bowl because I like its shape, but plastic or ceramic bowls are fine, too.
Start building clay walls around the bowl.
Remove the clay walls when the plaster has cooled down…I tend to re-use the clay walls as I make several casts in one sitting. Remove the glass bowl and there you have your mould. Smooth down rough edges with a sureform and sand paper. It will take a week or so to dry and then it is ready to use.