I can't believe it's taken me all of two weeks to get this far but here it is!
This is by no means a scientific tutorial, more like making a favourite recipe. A pinch of that and a delicious glug of this.
This particular kind of mould is only for the outside shape and you'll have to finish the inside of your vessel off by hand.
Step 1: Pick the vessel you would like to make a mould of and place it upside down on a baton about one and half inches wider (old fashioned term I know). If it is not of wet clay you might want to cover it with a thin layer of Vaseline to prevent the plaster from sticking to it.
Step 2: Cut a piece of corrugated cardboard (it bends fairly easily) in a strip a bit higher than the combined height of your baton and vessel. Then 'build' a wall with it around the outside of the baton edge using brown packaging tape to stick down the edges. Make them as smooth as possible on the inside.
Step 3: Seal the joint with a clay coil by pressing it down firmly into the join.
Step 4: Now for the plaster mix. Lace a mixing bowl or bucket inside with a plastic packet (this makes cleaning afterward much easier). Take a rough guess at how much water you will need to fill your 'moat' and plus it by some. Plaster - you need about the same amount in volume as the amount of water you are going to use. Start by shaking the plaster into the water gently. Turn the bucket often, so the spread is even - no mixing at this stage!
Step 5: Initially the plaster will just sort of dissolve, but gradually it will start building up and when it starts making 'islands' like the one shown above you know your mixture is reaching saturation.
Step 6: You can then quickly but gently start mixing it into a smooth drippy consistency using your hands (the mixture will start to activate and warm up so work fairly smartly). Make sure there are no clots.
Step 7: Now using your hands, gently pour over a thin layer covering the whole vessel. This is to make sure that the plaster goes into all the detail.
Step 8: The mix will start to thicken quickly now, so pour the rest over gently and 'bump' it a little every now and again to release any trapped air.
Step 9: Viola! Whipped cream cake. :)
Step 10: Tidy up the edges and scrape the top (soon to be the bottom) into a nice flat standing surface.
Step 11: Let it rest until really firmly set before you try and remove the mould.
Step 12: Start your next project.
I have no idea whether this will be of any use to anyone but good luck and let me know if you have any suggestions that might make it better.
A big thank you to Belinda and Susan for letting me document their work in progress.