Making the second half of the sandcast moulds was more successful. After remaking the first half of the Venus mould, I put more CO2 gas into the sand than before and it successfully hardened so that sand next to the form didn’t break away when I took it apart. I then brushed the second half of the moulds with graphite and made the other half of the moulds. The pieces were quite hard to get out as they were completely surrounded by sand so were damaged a bit from the removal, but the surfaces of the forms were okay and they can be repaired with a bit of car body filler. Once all the mould were made, I drilled the pour hole and the risers, made a sandcast pouring cup and finally glued the mould together using specially formulated sand glue which turns into a resin when It come in contact with heat. See below for a video of the process.