Learning to Make Plaster Molds 1 - Santa Ornament

I would like to reproduce a few of my carvings and sculptures in ceramics.

I have worked with making silicone molds before but plaster offers the chance to make hollow cast objects. The downside is that using plaster means the figure must not have any undercuts. At least no single part of the mold should not have any.

I sculpted a little fisherman Santa in kids modeling clay and began my journey along the learning curve. I invite you to follow along.

Step 1: Preparing to Cast One Side of the Face, Below the Hat

First I added a bit of clay to the bottom of the figure to make a pouring hole.

It looked like I would need a three-piece mold to avoid any undercuts. One for the Sou-Wester hat and one for each side of the face.

I got some cardboard and made a box to roughly fit the figure (I should have measured!).

I had an idea that playdough could be used to form a base for the plaster and yet not stick to the modeling clay. I used tape I had on hand to form the cardboard into a small box and I laid the figure down on its side. The playdough supported the model and allowed making a shape that would hold the plaster.

I pushed in some circular depressions to act as registers for the final plaster when it is assembled.

When everything was ready I mixed up some hydrocal I had on hand and poured it slowly over the model. Once the figure was completely covered I tapped the box lightly on the workbench to loosen any air bubbles and let it sit overnight.

Step 2: Now for the Other Side of the Face ...

I flipped the box over onto its other side. Then I cut open the box at the edges to reveal the other side of the Santa face. The playdough needed to be removed to expose the model for the next cast.

I cleaned the plaster and built up a layer of the playdough to hold the plaster in place for casting the other side of the face. This time, only the top needs to be made. I added the register depressions on this as well.

A coating of vaseline over the plaster kept the next part of the mold from sticking to this one. I almost forgot to do this. Plaster bonds to itself and it would have meant starting over.

When everything was ready I mixed more plaster and slowly poured it over the model. I am using hydrocal I had on hand. it is old and a little lumpy but I think it will work ok. One disadvantage of this is that architectural hydrocal, although very strong, has ingredients added that reduce its tendency to absorb water. Casting depends on the plaster absorbing water. So casting plaster would be better.

Step 3: Pour the Final Piece of the Cast.

I cut open the side of the box to get at the figure and clean up the plaster. The playdough mixed a little with the plaster and proved hard to clean off. You can see the colour in the picture.

I put the box back together, this time with the top of the Sou-Wester exposed.

Mixed plaster and poured it in.

The whole process sounds pretty simple when I describe it, and it really is ... if you have done this a few times. But I was pretty unsure of what I was going to do next and had to figure things out as I went along. Deciding where the parting lines should be and how to get set up for the next plaster pour took a little thought. This is a "learn by doing" approach and it does work, but it might take several attempts.

The plaster set up in a half hour but I waited a few hours before taking things apart.

Step 4: Clean Up the Mold and Make Some Castings.

Getting the plaster apart proved to be harder than I thought it would be. Tapping with a rubber mallet, prying on the seam, controlled force, and a little persistence did the trick. I removed the model and cleaned up the plaster mold.

I could see that the end of the Sou-Wester was touching the edge of the mold. I made a little patch to cover the area to keep the slip from leaking out. You can see it in the picture. Measuring the size I would need, for a box would have been a good idea.

I didn't have casting slip and it might be a long time coming, so I made my own. I followed instructions I found, on how to make some sodium silicate and made the slip.

I held the mold together with rubber bands I cut from some old innertube.

I poured in the slip and let it sit for an hour since hydrocal would not likely work as well as casting plaster. I think 1/2 hour is more normal. In later casts using a better slip, I used 1/2 hour. I turned the mold upside down over the bucket of slip to drain out the clay and left it that way for an hour. Then I set it on a shelf and waited three days for it to dry. Three long days.

When I took it apart the clay was damp and thin. The little figure broke in half. I reasoned my casting slip was way too thin. I drained off the excess water and remixed it. The next try I used a heat gun to speed up the drying time.

It worked. I was able to cast in the morning before work and remove it from the mold that evening.

Step 5: Dry, Glaze and Fire.

I usually apply glaze to the greenware and single fire the piece. This is how I do it for most of my ceramic sculpture.

In this case, I did fire a few to bisque and will try them with the more common pottery method of glazing and firing a second time. This would allow some grandchildren a chance to paint them. The green clay is too delicate for kids to be able to paint without breaking.

I fired the Santas in my little test kiln which I set out on the back deck. I have a ramp and soak controller and a relay to control the firing temperatures and times.

Step 6: Conclusion

In my opinion, the only way to learn how to make plaster molds is to jump in with both feet and have at it.

The cardboard stood up surprisingly well. It was an easy way to try making a mold. It had a coating on one side with printed advertising. This seemed to keep the moisture at bay.

The mold is crude and I hope to do better as I gain experience, but it works.

It is harder than I thought to get detail and avoid using undercuts. Making models for reproducing is quite different from just doing the sculpture. I'll need to improve in that area as well..

I have made silicone molds before so I have a base to work from. If you are totally new to the concept it would be best to make a few one-piece molds before trying a full 3D image. I had planned to do this Santa in two pieces but the Sou-Wester has an upturned brim so I had to make it three.

I think I will make the boards used for plaster molds before try to reproduce the next model.

Just a note: The Sou-Westers I wore as a youngster were black and stiff and metallic looking. I never saw a fisherman wear the floppy yellow plastic kind you see comedians wear. In the late 60's the oil coat came with a hood and the Sou-Wester went out of style. I guess Santa would wear a hat so I'll make most of them that way.

Hope you found it informative.
Clifton

I made a video for those who prefer it ...