Making the plaster production mold

In the last post, I described the making of a rubber master mold. This time I'll show how that master mold is used to make a working plaster mold. That is the purpose of the master mold; it becomes the mold for making a mold. One by one the rubber mold pieces are removed and replaced with plaster ones until the whole thing is replaced with a plaster version. So that's what we'll be doing here.

When I made the master mold, you might remember that I poured one of the large flat sides first. Once I begin making a plaster mold, that process goes backward. I will actually start with the smallest, innermost pieces and work my way outward. In the case of this mold, there is only one inner piece so I'll start there.



Here I've removed the inner piece and banded the mold back together. I don't need to box the mold at this point, because the rubber will contain the plaster. You can also see that the plaster backing on each side of the mold allows me to band the mold firmly without risking distortion of the inner image.



Here I have started pouring the plaster into the cavity.



As soon as the plaster begins to firm, I scrape it flush with the outer edges using a straight edge. Flush edges make it much easier to box the mold when the larger pieces are poured.



I only have to wait until the plaster has set and is solid. It won't be dry for another few days, but solid is all I need for now. Here I've taken off one side of the master mold (the one with the negative of the design) to reveal the inner piece. This piece will get a few coats of mold soap to keep it from sticking to the next piece, and then it's time to box it all up again.

(Oh, and it's better not to remove the plaster pieces after they have been poured if you can avoid it, since the seal helps prevent later pours from leaking inside.)



Here I am pouring plaster into the mold box containing the bottom half half of the mold and the inner plaster piece.



Here's everything so far, just released from the mold box. The thicker piece of plaster at the bottom is the plaster mold. The amber area in the middle is the rubber piece with the design (the piece we just poured plaster on to) and the thinner piece of plaster at the top is the backing. I'll be taking that backing off for the moment so the rubber has a little more give to it when I pull it from the production mold.



You can see from this picture that while the rubber does have some give, it's still pretty rigid. (You can also see that I missed my curing window for using the marbles on the back of this piece, and had to cut keys in by hand. Hate it when that happens!)



So here are the first two pieces of the production mold. All that's left now is the "lid". That's the piece that will fit over the top and will have a holes drilled for pouring the slip. Tile molds are typically open-faced so that moist clay can be pressed into the design cavity. Slipcasting molds need lids so the cavity can be filled and then drained. (That's why medallions and plaques are hollow, and tiles are solid.)

To make my lid, I'll need to fill the design cavity. Normally when I make my rubber master I pour an insert - a rubber version of the design - that I use for this. Unfortunately for this project, I ran out of rubber with the last piece of the master mold. So until the box arrives with more, I'm stuck filling it with moist clay.



I don't have to worry about getting a good casting - just with filling the cavity so I don't get plaster inside the design area.



Here I'm scraping the clay level with mold.



Now I'm ready to soap the mold and box it up so I can pour the lid. You'll also notice in these last few pictures that I cleaned up and rounded the edges of the mold a bit. That helps the mold fit in the mold box a little better.



These are the finished lid and the bottom half of the mold. The moist clay insert has been removed, but it has left a bit of a stain on the mold. Of course, the mold is going to look used soon enough, so it doesn't really matter.



The finished mold has had pour holes drilled into the lid (facing away from the camera) and then been reassembled, banded and left to dry for a week or so. I usually place the drying molds near the kiln, though not quite as close as in this picture. (The kiln was not running when this picture was taken, so I moved the molds closer to it so I didn't keep tripping over them.)

Now comes the hard part - resisting the temptation to pour slip into the mold until it is well and truly dry!

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.