Making Urethane Molds for Paper Mache Clay

Black-Footed Ferret, Clay Model
Black-Footed Ferret Clay Model, Being Sealed

This week I made three urethane molds of clay sculptures – a black-footed ferret, shown above; a walrus; and an Arabian Oryx. I intended to make a Caspian seal, but after it was done I decided a walrus would be more fun.

Arabian Oryx and Walrus Clay Models
Arabian Oryx and Walrus Clay Models

The other two models are shown above, after being sealed. The models were sculpted using water-based pottery clay. I allowed the clay to dry to leather-hard stage before making the molds. The ears of the oryx dried faster than the head, and I now realize I should have left them off the model, since they could be added after the paper mache clay was dry. Live and learn…

After the molds were done I then messed around with the paper mache clay recipe, changing it so it would be slightly easier to use in a mold. For that part of this project, I’m definitely still in the experimental stage, so any suggestions will be appreciated. I’ll show you what I’ve done so far with the clay recipe down towards the bottom of the post.

The clay models were sealed with a product called SuperSeal, made by the Smooth-On company. I ordered the sealer, the release, and the Smooth-On 40 rubber mold compound from Farwest Materials in Walla Walla. (They have great customer service). After the models were sealed, I sprayed on the universal release, and allowed the excess to evaporate. Then I mixed up the two parts of the rubber compound and brushed it over the models.

Adding the Rubber Mold Material
Adding the Rubber Mold Material

I allowed the rubber to set up to the tacky stage between coats. After four coats were added the mold looked like this:

Rubber Mold
Rubber Mold

After drying overnight, I made a support mold with plaster of Paris:

Plaster of Paris Support Mold
Plaster of Paris Support Mold

When the plaster was hard, I removed the clay model. The mold is now done. You can see that I cut the rubber mold at the ears, to make it easier to remove the finished piece.

Rubber Mold and Support Mold
Rubber Mold and Support Mold

Now for the fun part. I tried the paper mache clay recipe just as we normally make it, pressing it into the molds in a thin layer about 1/4 inch deep. There were two problems – the clay takes a very long time to dry because air can reach only one side; and small voids mar the surface of the piece where the clay doesn’t fill the mold correctly, no matter how carefully I pressed it into the mold.

For the first problem, I tried using a fast-setting joint compound to replace the regular product we normally use, but my efforts weren’t very successful. It seemed like such a good idea, but…

In the end, I came up with a variation of the recipe that seems to work, although I can’t give exact measurements. Lots of experiments still need to be done. I mixed up a small batch of wet toilet paper, just enough water to cover the paper in a small bowl, and Elmer’s Glue-All. This made a “soup” with a lower paper content than the normal recipe. I then mixed in some corn starch and some plaster of Paris. The corn starch helped the mixture stick together so it was easier to work with, and the plaster caused the mixture to set up so it could be removed from the mold fairly quickly. Obviously, with the plaster in the mix, I needed to work a bit faster than I normally do.

I used a soft brush to paint a thin layer of the mixture into the mold, and then covered it with about 1/4 inch more.  Using the brush and a thin layer helped reduce the number of voids on the surface. As you can see from the photo, the “clay” I mixed up has a consistency like whipped cream cheese. I mixed just enough for one mold at a time, and each mixture came out a little different.

Adding the New Paper Mache Recipe to the Mold
Adding the New Paper Mache Recipe to the Mold

The ferret and the walrus have now been taken out of their molds – I’m still working on the oryx. The plaster has set up, but they are still very damp, and will need to dry for several days. Then I’ll make the walrus’ tusks and the oryx’s horns, fit everything together, and paint them. I’m still not sure how they’ll be mounted – I definitely want to avoid the hunter’s trophy look. Suggestions?

Since the first pieces have not yet dried, we still don’t know how strong they’ll be. They’ll be hung on a wall so they don’t have to be totally unbreakable, but I do need to be able to move them around without worrying too much about breakage. I hope to have at least 12 different pieces finished so I can take them to an art fair at the end of August. You can see the plastic mesh that I’m using to reinforce the pieces below, under the ferret. I might also use some traditional paper strips and paste on the insides of the pieces to make them stronger, if needed.

Walrus and Black-Footed Ferret
Walrus and Black-Footed Ferret

I’m really looking forward to seeing the walrus and oryx with their tusks and horns. I originally intended to only design smaller items for the molds, like the ferret, that I could easily ship – but once you get your hands in clay, who knows what will happen?

As always, your suggestions are welcome. And if you’ve done similar experiments, please let us know how they turned out.

40 thoughts on “Making Urethane Molds for Paper Mache Clay”

  1. Hi Jonni,

    I *love* your site! I haven’t touched my sculpting projects in 15 years but looking at your site gives me the itch again. 🙂 I did some small hand rattles with animal faces, deer, wolf and bear, as gifts for clan members about 20 years ago. I first sculpted them in very hard oil-based sculptor’s clay, then made a urethane mold on them, backed with a plaster jacket. I was working with paper clay, as the first layer. The original sculptures had a lot of very fine detail I wanted to preserve, so I needed to thin out the paper clay to the point I could almost brush it in.

    The moods were quite deep so there was no hope of drying in this lifetime, and I couldn’t figure out how to ‘to get the detail I wanted and get the things out of the molds! Until I thought of putting the molds in the freezer!

    I’d brush in a layer and then pop the mold *into the freezer* until it was frozen solid. Then working quickly I’d brush another layer in. I continued doing this until I had a good solid layer built in. I laid a layer of cheesecloth into the last two or three layers, leaving an inch or so of cheesecloth hanging over the sides to use as “handles”.

    When I had the depth I needed I took the molds out of the freezer and left them out with a fan blowing on them. By the second morning the edges were free of the molds. At this point I laid in 1/4″ paper mache reinforcement so the masks would not deform as they dried. As soon as this layer had dried (another 48 hours) I popped the masks out of the molds. They had picked up all the detail and I was quite happy with them. I painted them, put small buttons in for rattles, closed the back with plastic cut from a cottage cheese container lid and covered that with felt and voila! Mask rattles.


    • Very clever, Deb! Will you be making some more rattles, or other sculptures? And do you happen to have some photos of the ones you made before, so we can see how they came out? I would love to see them.

      • I’ve not sculpted in a long time. I was doing primarily large pieces and it simply became too physical for me, but looking at your site I’ve been reminded of how much fun it was to do miniatures and small pieces. I also stumbled across a Dia de los Meurtos site and it took me back to fond memories of my childhood in the American SW in the Latino culture. It is of course rooted in the Central American Indigenous cultures (Aztec, Mayan and Totonac). Both my husband and I have Totonac ancestry. Next year *we* will celebrate Dia de los Meurtos, and because we live in Western Canada I have to get busy and make all the trappings for that celebration myself! Mix up some Jonni Clay! 🙂

        I didn’t take pictures of the hand rattles I made at the time, the cats “killed” my deer rattle, but I went out a few minutes ago and took a picture of my bear rattle (a little worse for wear) and a mask face which was not destined to be a rattle but was instead a portrait of an ancestor chief. He had rabbit ears before the cats ate them, and considerably more feathers. This taught me not to leave my work unattended to answer the door. I planned to put him on a backing, but never did. Over the years the paper mache has cupped around the edges. The bear had the least surface detail of the molds, but he was also the first one I did and I learned you have to use a very thinned out paint in order not to fill in the hair detail.

  2. This is great info. I am trying to do very similar things. I have a largish concrete mold made of plastic that I would like to use to cast paper mache (it’s a life sized plain face). I have been looking everywhere to see if I need a release or which one if I do. One thing I know from my reading is that you can use the shrinking properties of a clay to your advantage. If you use a water based clay that shrinks in a semi rigid plastic or silicone mold it will shrink away from the mold as it dries making it easier to remove. So besides tweaking your formula to be more liquid to get your details you probably want to try for some shrinking as the water evaporates.
    Could you say more about what kind of release you used, if it worked or would anything work better?

    • Hi Ingrid. I used a product called Universal Mold Release from Smooth-on. Since the recipe I used set up firmly as the plaster hardened, there was very little shrinkage. But the molds are flexible anyway, so there was no problem getting the items out of the mold.

      I did some more experimenting with the recipe, and came up with the one on this post.

      The original paper mache clay recipe doesn’t work well in molds, in my opinion, because there is too much paper in the mix.

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