I have been working on learning some plaster mold making techniques and my last project really challenged me. Above you can see my first four part mold. That's four pours of plaster which equates to four chances to screw up, and boy did I! At the same time, even though it certainly isn't pretty, and even though my mentor could have made this mold in a third of the time and have it come out ten times better, I must say that I am pretty proud to see it complete.
Now to dry it out and see how the casts look coming out of it.
Here is a brief write up of the steps taken to make this piece. For more information please read the previous posts on mold making. I am currently working with a master mold maker here in Denver, trying to soak in some of his knowledge. I am also using the books Slipcasting by Sasha Wardell and Mold Making and Slipcasting by Andrew Martin if you are looking for a good reference.
The model was made from clay slabs of about 3/8" thick. The model was bisque fired to ^017 so that it was still soft enough to be altered a bit after bisque firing. I spent a fair bit of time working with model when the clay was leather hard and after bisque firing to make sure the form was exactly the way I wanted the final vase (making sure corners were square and sides were straight). You can use your regular clay tools, but I find that I cabinet scraper makes planing sides so much easier. These are available at a woodworking store or on Amazon (you can get anything on Amazon!).
After preparing the base, half the model was "clayed up". This is the dark gray area on the photo. After the plaster has been poured, this clay will be removed, creating a void for the second half of the body part of the mold. The model and base plate are now treated with parting compound (diluted Murphy's Oil Soap is a readily available option for parting compound).
After the above piece was cast, the clay was removed and the model rotated 180 degrees, the cottle boards were reset and the second body part was cast in plaster.(Note that the model was in place during this second body casting.
Once everything had been coated with parting compound the final pour of plaster was done. After the last pour had cured a bit the edges of the mold were rounded over to reduce the risk of chipping and the exterior of the mold was wet sanded starting with a 220 grit wet/dry paper and ending with a 600 grit paper. You will need to careful while sanding not to alter the interior edges as this will affect your final slip casting.