Making the rubber master mold

I’m probably making everyone dizzy, shifting back and forth between such differing interests as moldmaking and horse coat genetics. I need to figure out how to show subject tags to the bottom so everyone can organize my posts by subject instead of having them appear chronologically. Of course, that will only help if I get better about adding the tags in the first place. I usually forget.

But to get us back to making the rubber master mold for my new plaque, I now have my finished original ready to mold.

I’ve mounted the original to a glazed tile with a little Elmer’s glue, then sealed the edges with a little clay (the reddish-brown edge around the design). That keeps the rubber from seeping under the original. The greenish circles around the design create keys so the mold pieces fit together without shifting.

With most medallion designs, there are just two pieces to the mold: the piece with the design and the “lid” that fits over the top. As you may remember from the waste mold made from the original piece, this particular piece requires an extra piece because of the undercuts around the muzzle. If you look carefully, you can see where I’ve drawn the mold line in pencil on the bisque. My next step, now that the piece is all set up, is to clay up that piece.

It’s hard to see because the clay is dark brown, but I’ve added three natches to the insert piece. Each mold piece needs to have these so the final mold is tight and well-aligned. Now we’re ready to box it all up and pour our first rubber piece.

I’ve used marbles once again to create my mold keys. The marbles are removed when the rubber is set (but not fully cured) and then a plaster backing is poured. The rubber and the plaster backing then remain in the clamped mold box for two days to allow the rubber to fully cure. After that point, the boards are removed and the plaster and rubber are banded along with the base tile. (The base tile with the design are still sealed in the rubber at this point.)

With the rubber, plaster and tile still banded, the clay forming the insert piece is then carefully removed.

Here the clay has been completely removed from the original. This step is much trickier when the original is made of clay instead of hard bisque like we have here! The original looks a little shiny because I had to spray the area with mold release so the rubber pieces separate later. (You only forget to use mold release once…)

Now I can pour the rubber. Like the larger piece, it won’t be fully cured for another day or two so the mold must be set aside once again.

Here the cured mold pieces have been removed from the mold box. The circles with the dark edges are the keys on the back (the ones formed by the marbles) that hold the mold to it’s plaster backing. The fainter circles along the top and bottom edges, and the three faint circles to the right side are the natches in the mold itself.

So we have two of the four pieces we will need – one large one with the design cavity and the smaller muzzle insert. All that remain now are the plaque itself and the “lid”. To save ourselves a bit of hassle, we’ll actually be pouring those two as a single piece of rubber.

Here I have poured rubber to fill the mold cavity and then up and beyond for several inches. (Think of the final piece as looking like the plaque sitting atop a platform.) In this picture the rubber has not yet set enough to place the marbles, and you can faintly see the outlines of the other mold pieces below. Once it sets a little more, the marbles are added and another plaster backing is poured. Then the box must be set aside for another few days so the rubber can cure.

And here is the final master mold. (I hadn’t yet banded it when I took the photo. Normally rubber bands hold the layers in place.)

I know this post is already long, so I’ll save the process of making working plaster molds from this kind of rubber master for a later post.

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