I have a troublesome relationship to making molds. I’ve made a few, but I really, really, really don’t like doing it. It’s very technical, stinky, and messy. It’s a skill that develops over time with much experience, so I feel like a total novice at this. When I lived in Maryland, I relied on the skills of the incomperable Patrick McEvoy to do my molds. He is perhaps the best mold maker in the entire country, but he is in Baltimore and I’m here. So I must make my own. Let’s just hope it works!
I use Tuck Langland‘s book, From Clay to Bronze, as a guide in the studio. I’ve read his chapter on rubber molds several times to mentally walk through the process. I highly recommend his book as an essential studio reference. Here is what I have done in my process, but get Tuck’s book for another perspective.
Making a rubber mold is time sensitive, so it is important to have all of your materials at hand before beginning anything.
- Rubber mold ingredients (I use Ploytek 74-30)
- mixing bowls or cups, labeled PART A and PART B with a sharpie
- a sharpie
- a disposable bucket in which to mix your PART A and PART B
- mixing sticks (Polytek will tell you not to use wooden sticks, but Tuck says it doesn’t matter and I haven’t had a problem with them. Plus, they are free when you buy paint.) Label them A, B, and AB so you don’t mix them up.
- rubber exam gloves (washing this stuff off is nearly impossible)
- drop cloths
- saran wrap
- cheap chip brushes
- a keyed mold knife
- mold release
- Vasiline (Rub this on anything you don’t want rubber sticking to.)
- Cab-o-sil (fumed silica)
- Respirator mask. Fumed silica is not something you want in your lungs.
- extra clay
- trash bin
- Spray the sculpture with mold release. Plastiline probably doesn’t need it, but just in case.
- Build a tiny wall around the sculpture out of clay to catch rubber that will run all over the place. It will look like a tiny moat.
- Rub the armature board down with vasiline.
- Cover your work surfaces with a drop cloth and any additional tables you will use.
- Lay out your materials, so they are easy to access.
- SHIMS: to make shims, put strips of clay on a board with a bit of space in between them. Line with saran wrap and fill with liquid rubber. You will then stick the shims on between layers of rubber.
Mixing and Applying the Rubber
Poytek 74-30 is a pretty straight-forward product. Part A + Part B = chemical reaction that will make Polyurethane rubber.
- Open each bucket and stir with a clean stick, clearly labeled for each bucket, A and B. Tuck recommends putting the rubber in a more pour-able container, but the new containers have to be PERFECTLY clean and dry. I didn’t think ahead well enough. The buckets are hard to pour, just be careful.
- Pour Part A into a measuring cup. Mark the level you want to use with a sharpie. Polytek 74-30 is a 1:1 ratio, but both products still have to be measured carefully.
- Pour Part B into another cup.
- Mix the two in another container. Fold the two parts into one another. Do not go crazy stirring in circles; this with just add bubbles. I fold for quite a bit of time to get a nice consistency. The color will change throughout. Do not be alarmed.
- Using a disposable chip brush, spread the first coat of rubber onto your piece. This is the hardest part for me, emotionally. If it doesn’t work, your piece will be ruined. But, honestly, it doesn’t happen very often. You’ll be fine. Just hold your breath and GO!
- The rubber will Be very gentle with the brush and get the undersides and blow into areas where bubbles are forming. You could use a straw to blow out bubbles. I had one bubble that kept forming in the corner of the eye, but otherwise not much of a problem.
- Coat the entire piece. A lot of rubber will fill the moat.
- Use the rest of your mixture to pour in the shims that you’ve set up with the strips of clay and saran wrap.
- Wait a bit before applying the second layer. The weather, heat and humidity, will determine the curing time. Tuck says about 4-6 hours.
- Apply the second layer much the same as before, filling any spots you may have missed.
Part II will cover the next several layers, including adding the cab-o-sil, and making the mother mold. Part III will be pouring the cast. Stay tuned…