Step 2: Making a mould
using my non-drying modelling clay I made a ball of roughly the right size, then made a rough cone which will form my sprue.
I cut it in half and lay one half on my desk.
Make a small ball and cut that in half and place the 2 halves on either side of the part to be cast, make them staggered, these will form our key for the mould making sure it will only fit together in the correct way and aligned correctly.
using the modelling clay build walls all the way round the part but make sure the bottom of the cone/sprue is butted up to one of the walls.
mix up your plaster to the consistency of thick cream, only mix up enough for this half of the mould, pour this in to the mould all the way to the top of the walls.
Agitate (shake gently) the mix to dispel any bubbles.
let your plaster harden then remove the walls and turn the mould over.
remove the small ball parts we used for the key, and then using a hobby knife carve off the edges of the key points so there's no overlap.
now place the other half of the part on top of the one in the plaster, line it up and push down lightly to make it stick.
now build walls again high enough so they come above the highest point of the part.
coat the plaster and the 2nd half of the part in a thin coat of petroleum jelly, this will act as a release agent and stop the plaster sticking to its self so we can separate the 2 halves.
now mix up some more plaster and pour it in to the 2nd half, agitate again and then wait for it to harden.
once its hard remove the walls and separate the mould, remove the clay from the mould.
now you can wait for a couple of days for the mould to dry out or you can cheat, pop the 2 halves on a baking tray separately, pop in a cold oven then put the oven on low and leave for a few hours to get the water out the moulds.
do not cast with wet / damp moulds, they will crack/crumble or explode.