When mold-making goes wrong

As promised, here are some pictures that show how not to make a mold.

The project was the CT River Classic Live awards. I had a resin casting of Sonya’s medallion “Moose”. From that I would make a rubber master mold, and then plaster production molds. I was on a pretty tight deadline since some glaze artists had offered to finish a few pieces to auction for the show. I wanted them to have as much time as possible to work on their donations. The show also needed a number of unfinished bisques, so I wanted plenty of breathing room since I would be working in the rainy season. Because plaster molds have to dry (after they are made and each time after they are used), damp weather makes everything take longer.

My first mistake was in miscalculating the amount of rubber I would need to make the master. I didn’t have enough to make the last pour, which was the rubber positive. Not wanting to lose a few days waiting for another rubber shipment, I added some “stale” rubber to make up for the shortage. Rubber works best when the two parts are fresh, so I try to order in small quantities and only when it’s needed. When I have leftovers, I keep them to use for making mold inserts since the quality of casting doesn’t matter; I just need something to fill the space while I pour the plaster lid. That was what I added to my last batch of rubber for medallion master.

Oh, how costly those two or three saved days have been!

Here is the rubber negative, made from fresh rubber. The design surface is nice and clean – no bubbles.

And here is the positive. The bubbles are highlighted because they have been filled with light blue plasticine. That was done later when I tried to fix the problems. The extent of the problem wasn’t obvious before that, though. The rubber is a translucent amber, so bubbles are visible. It’s just hard to tell, especially when they are tiny, if they are on the design surface or just slightly below. I knew my master was very bubbly, but I hoped that the surface was smooth.

I thought the true test would be the first plaster molds. If they came away clean, then the bubbles were either below the surface or too small for the plaster to fill. I poured several and looked for any roughness in the surface, and they were perfectly smooth. I was relieved and set them aside to dry. It was a process that could take up to two weeks.

All went well until I had pulled a few castings and the molds started showing wear really fast. Worn molds develop pock marks, which in turn create bumps on the greenware. Here is one of the molds showing the problem.

And here is the casting it produced. What a mess! Yet that same mold had made several good copies in the beginning.

At first I thought the wear was from pushing the molds too hard. I was pulling two castings a day in a damp studio. I kicked myself since new molds would have to dry, adding another two weeks to the process. Then I noticed that all three molds were producing the exact same bump patterns on their castings. So it wasn’t mold abuse. It was inherent in the master. That’s when I went back and used the blue plasticine to reveal (and partially fill) the pock marks in the master.

What I realize now is that the trapped bubbles in the master were indeed too small to admit the plaster. But what they could hold was air. Air that would migrate up into the plaster as it set, but not very far before the rapidly setting plaster would trap it. When I pulled the mold, that plaster would look pristine. But just under that smooth surface were all those little trapped air bubbles. Each use of a mold wears the plaster down, so with each casting I was revealing more of those bubbles.

I did make another set of molds using the filled master. It would have been better to pour a new master using fresh rubber, but by this point I really was running into some unpleasant deadline issues. Not only would the show need the bisques, but I had my own Spring Lottery deadline looming. I settled for filling the holes with the plasticine as best I could and making a few more molds. At worst I thought they would wear like the first, though hopefully with just a few more good castings. And that was what they did. They are also showing wear much like the first ones, though I got enough castings for the show.

Now they are all cleaned, fired, boxed and ready to ship. And I’ll be glad to return to the much simpler world of glazing horses!

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