Archive | October, 2010

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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Finishing up the Elsie molds

In the last post, I had pictures of the first large side of the head mold being made. I thought it might be helpful to show the next few steps, since they might not be obvious.

These pictures might be a little confusing, though, because I normally make two copies of each mold. This one is the second copy of the mold, and the other pictures are of the making of the first copy. As I mentioned before, after I made the first mold of her head I decided to reverse the order that I poured the sides.

Elsie’s head mold has three sides: left, right and a gusset. The gusset piece runs from between her ears (like a typical hat piece) down her face and up under her mouth. That piece is designed to break so that the final mold actually has four pieces, but it pours in three. The gusset is the first to pour, and it can be seen in that first picture of the clay barrier. The second piece there is the left side of the face, but for the picture above the second pour was the right side of the face. The left side is the third and last pour.

In this picture above I have already used a planer to clean up the edges so the head can be boxed and the last side poured. Since it is a relatively small mold, I’ve used Legos. I still had to use clay on the right-hand side of the mold since I am going to be pouring directly into the opening at her neck. Since that cut is not straight (making an uneven cut helps to “key” the pieces back together properly), that side cannot be level and must be shaped with plastelina.

Here is the second mold after it was removed from the Lego box. I haven’t yet used the planing tool (left) to level out the other sides of the mold. The top of the mold will still slope along the line of the neck opening, but I will clean up as much of that as possible. It might seem like an unnecessary step, but leveling the sides makes it much easier to stack the molds in the storage cabinet, and it makes it a lot less likely that the corners will get chipped.

Here are all three of Elsie’s finished production molds. As you can see, the head mold (top left) is all cleaned and planed. The smaller mold to the right is her tail, and the large mold on the bottom is her body. All totaled, it takes 19 mold pieces to make her. All together, her molds weigh just over 30 lbs. when still damp.

Now all that is left is the waiting. It will take 2-3 weeks, depending on the weather, before the head and body molds are dry enough to use. That’s when I will know if this set works.

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