My first attempt at making plaster molds was a success. I was able to reproduce the little fisherman Santa in good detail without too much finish work on the castings.
But the mold was crude and it leaked ceramic slip unless I plugged the seams. In this, my second try at mold making, I hope to improve my skill and the methods I use.
Step 1: Prepare the Box for the First Pouring of Plaster.
My figure is intended to be a ceramic candle holder. The theme is an evil pumpkin having a lobster boil.
My first step was to make some boards for casting. You have probably seen these before. Many videos and instructions for pottery will show them. The boards are designed to make any size box by simply clamping them together. I made two different size boards from shelving material I had left over.
I used the larger boards to form a box that would allow about an inch of plaster on all sides and used water based ceramic clay to support the model. Originally I thought I would only need a two-piece mold for this round model, but it will need three pieces to work properly.
Once the clay was in place around the model I spent some time smoothing it and making a close seal to the model. This time I decided to cut a groove around the edge so the plaster would not leak. Later I saw this as overkill. The smooth surface seemed to be effective at sealing the joint and avoiding leaks. Next time I think I will use registration keys instead of the groove.
Step 2: Pour the First Side.
Clay now covers the bottom two-thirds of the shape and the top one-third will be the first pour of plaster.
I try to guess how much water to put in the pail and add plaster until it mounds above the water. Then it is mixed relatively slowly by hand. A mixer can cause the plaster to set up very quickly and I need time to get it all poured in and the air bubbles removed. (I am slowing down in my old age, after all.) If I don't mix enough I simply mix more and if I mix too much I pour it out in the neighbour's flower garden. Just kidding.
Brushing the plaster with a soft bristle brush before, the figure is covered, and lifting the box to thump it on the table are methods that will cause the trapped air to float away from the figure. This is similar to casting a figure in plaster.
I left it to set overnight and took the box apart the next day. The clay was a little too wet and stuck to the boards but the plaster did not stick. I later coated the boards with Vaseline(tm), or petroleum jelly, where the plaster would go.
I was really impressed with how I could take the box all apart and clean up the figure before going on to the next step. I wasn't sure just how this contraption was going to work but it turned out to be quite simple.
Step 3: Prepare and Pour the Second Part of the Plaster Mold.
The first pour of plaster is now on the bottom and I assemble the box around it, taking care to position the boards for a snug fit to the plaster. Clamps are put in place until a good fit is achieved and then tightened.
I build a wall of clay and carefully smooth it as before.
The first layer of plaster needs a good coat of Vaseline to keep the next layer from sticking.
Mix, pour and set until hard. I like to give it several hours in a situation like this.
Step 4: Prepare and Pour the Final Part of the Mold.
Third verse same as the first. Remove the boards, clean everything up, smooth out the plaster, coat with vaseline, reform the box, and pour.
Step 5: Take the Mold Apart, Remove the Model and Clean Up the Plaster.
Now to get the mold apart without breaking it. The plaster is pretty strong but I tend to be wary of using too much force. The last piece I poured came loose with a little prying at the seem. The second piece was a little more stubborn and required wiggling, tapping with a rubber mallet, and pulling, before it finally gave way.
Now to remove the model and clean up the mold. I needed to remove a few humps that were trapping clay but it looked pretty good. The detail seemed to have been captured and the texture looked like it should.
Step 6: Make the First Casting.
I assembled the mold and compared it to my first mold of the little Santa ornament. You have to admit it looks better. At least it does on the outside.
I wrapped the rubber bands in place, filled it with casting slip and let it set about 1/2 hour. Then I tipped it upside down and let it drain for an hour or more.
Step 7: Remove the Cast Reproduction and Fire It to Create the Finished Piece.
Set the mold aside to dry. At first, I used heat to speed things up but this easily caused problems with cracking so I went to a small fan that circulates air into the mold. This way I could cast one in the morning, take it out of the mold that night and do another at night.
I cleaned up the figures, cut out the mouth and eyes and shaped a few spots that needed touching up. I set them aside to dry on a shelf, applied ceramic glaze, and fired them.
I liked the look of the finished piece and I hoped you enjoyed the read.
I made a video for those who prefer it ... although I probably should not be in front of a camera :)