After completing the carving of the tiki mug, we decided to make it a four-part mold. I then checked the carving for undercuts because the mold has to release smoothly off of the clay surface. Then using a level work surface make sure your sculpture is level. So after you slip cast your mugs they're not crooked. ...so just imagine all that while looking at this picture of the finished carving. (That hat on his head is called a spare. It sets in from the rim so your mug has a drinking edge. When the mold is made this stays an open hole to pour your casting slip into the mold, and pour it back out after is sets.)
Now the fun part. Prepare to have a day sucked out of your life as you build the mold.
There are different ways you can build it, this is what we did. (By we I mean I suckered my husband into this mess.) We have some boards with "edges" screwed into them so we can clamp them at any size "mold box" we need. (also, make sure they're square. Even though we do this its still a crapshoot sometimes.)
Then figure out how thick we want the plaster mold. We are making the mold 3.5" thick from the center of the carving. So that is about 1.75" to 2" of plaster deep. Mark the inside of the box you've formed and begin to fill it with clay or I use a mixture of foam packing, cardboard, foam core and clay. (I've used wood scraps too.)
After I get it as high as I need "cradling" the carving. I pack clay in all around being sure to make it seamless against the carving. I packed clay on each side and bottom making it smooth up to the carving. Then I built the center wall so we could pour the first quarter of the mold. This is where you need to get all the seams water (plaster) tight or it will drip and run out. Run a bead of clay against the wood, (2nd picture) the clay even up the walls of the mold box where they intersect. Think: "this has to float" it MUST be water tight!
Next spray some mold release on your carving and in the box. Cover EVERYTHING that the plaster will touch. (You can't see it, but its covered in butter cooking spray.) Using a measuring bucket estimate how much mass you have to fill and mix the plaster accordingly. (2 parts plaster to 1 part water. Its also better to mix a little more than you need than be too short! FYI) Mix is slowly to avoid air bubbles, make it lump free and well mixed. Then pour it in the mold slowly letting in cover evenly and smoothly. Some people brush it on, but we didn't need to ...we think.
After the plaster heats and hardens it will cool a bit. then you remove the clay or filler to the next section of the mold to be filled with plaster. Spray with mold release, mix and pour your plaster. Do this part four glorious times and you have a mold.
We started this endeavor at 11:30 in the morning and finished at 7:30 at night. Told ya, a day will be missing from your life! Hopefully, it will be useable!
I'll do a 2nd post with pics of the mold and pouring slip.