Archive | Slipcasting

Listing to starboard

The first casting from a new mold is where you make the big mistakes. I knew that was particularly likely with Oliver because of the placement of his feet. Legs often get shifted during the demolding process. There is also the problem of how drying clay can effect the balance of the horse. Because this is particularly a problem with standing horses, and because Oliver is “tripoded” (that is, he balances on three points), I expected the first casting to be a little off. As the picture above shows, he was more than a little off! He is listing to starboard in a pretty obvious way.

We usually adjust for a weight-bearing leg “pulling up” by shifting it ever so slightly in the opposite direction. The idea is that as it dries, it will pull into the proper place. But to know what constitutes a small shift, I need to have the legs lined up properly.

Since the change is usually too subtle to see even when placing the original next to the casting, I needed another way to check.

Here I’ve painted the hoof bottoms of the resin Oliver with underglaze. I could have used any paint, but the underglaze was handy and it washes off the resin easily. While the feet were still wet, I set him down on a sheet of white cardstock. Now I have a guide for proper foot placement.

It is not, however, the right size. My final clay shrinkage is around 6%, with most of that happening during the drying process. Usually there is a tiny amount of shrinkage while the horse is still in the mold; that’s what enables us to wiggle the casting free of the plaster. That is the point at which I’m usually adjusting the legs. So I needed my placement guide to be just a hair smaller. To do that, I scanned the card stock with the footprints and then printed it out at a 1% reduction. I printed a second page out at a 6% reduction so that I could check it again once the piece was dry. I can’t change the legs after that point, but at least it gives me a chance to check that casting isn’t hopeless before I invest the time in cleaning the greenware.

So far this has worked and all the subsequent Olivers have stood level. I also have Elsie’s pattern ready for when her molds are working, just in case!

You might wonder, looking at the tilting Oliver, why I bothered to clean and fire him. I’ve found out the hard way that no matter what goes wrong with the first casting early, it is best to keep working because there are sometimes more discoveries. It is better to find them on a casting that is already a loss than to lose one casting for each lesson. In the case of Oliver, I also found that I needed to cast him thicker than usual so I could clean out the clay from the gap between his front legs. Otherwise he ends up with an oddly placed post hole between his front legs. We are used to seeing a belly hole where the horse is posted in the kiln, but a hole in the chest area looked a bit disturbing.

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Supersize me!

When I first converted my studio for full-time work in ceramics, I imagined that like my previous efforts with resin-cast figures, most of my work would be in the smaller “mini” scale. (For readers that are not involved in equine collectibles, that makes an adult horse just a little under 3″ tall.) Most ceramic horses were either minis (often called “thumb” scale by ceramics collectors) or the slightly larger “curio” scale, with few horses going more than “classic” scale (the standard dollhouse scale). So I got a Skutt 614, which is a hexagonal kiln that measures around 12″ across the diagonal. That seemed a safe bet for almost anything I wanted to fire.

Later I added the smaller AIM 88-D, which is a square 8″. It’s the workhorse in the studio that does most of the bisque firing. It is rare that I have anything that won’t fit in the AIM, nevermind that larger Skutt. But I knew that with ceramic horses getting bigger, the time would come when I would need to replace the Skutt with something a little bigger. Since that will also mean rewiring the studio, I have been putting it off until I had a horse that no longer fit. Although it was a squeeze, so far even Stormwatch fit. But while he is a larger horse than Elsie, she takes up a lot of horizontal space so I wondered if she might be the one. I was relieved when she fit.

But what I didn’t realize back when she first arrived was just how many other items would be too small. As can be seen from the first picture, my largest mold boards proved just barely long enough to hold her master mold. I’ve included the master for Imp, a mini-scale foal, and my foot for scale. I tried to hold my hand up for scale first, but I didn’t have enough arm length to get back enough to get the whole mold in the frame. (I guess by now my liking for flowers painted on my toes is obvious.) The Imp mold is 3″ across its length, whereas Elsie is 12″.

The mold boards did fit, but I realized as I assembled my supplies that I didn’t have a container large enough for mixing that much rubber. I usually use small throw-away rubbermaid containers, but the largest ones only hold a gallon. I thought I would need around 5 lbs of rubber. I ended up purchasing a few plastic dishwashing bins that could hold 3 gallons. I got them home and realized that they were too large to sit on my scale. So I constructed a platform that would hold the bin (and still let me read the weight).

That is when I was reminded of my inability to estimate volume. I’d already done that with the Plastalina, which is the brown clay used to block out the areas that don’t yet get rubber. Not once, but twice, I had to run out to get still more of it. I probably should have known better with the rubber, but it spoils quickly and is quite expensive. I order it as it is needed for each step. So now I have an Elsie that is covered in rubber, but not really deeply enough to be stable. Luckily, additional rubber can be added after the first pour cures, so it’s just a matter of waiting for the next delivery.

I almost forgot the “damp box”. When horses are cast in separate pieces, those pieces stay in a damp box while they are cleaned and assembled. That’s my mini-scale damp box on top: a tupperware sandwich container with a thin layer of plaster on the bottom. Not even Elsie’s tail will fit in there, so I had to find a container that would work. It was surprisingly difficult to find a box deep enough for her (with the attached tail) that wasn’t also really large, but this 12 quart tote looks like it will work out well. I liked that it had flip-down handles, rather than a pull-off lid, since it will keep jostling to a minimum when I have to open the container.

I can see that going larger is going to take some adjustment when it comes to tools and materials!

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