Tag Archives | casting

Small breakthroughs

One day when my youngest son was a toddler, he decided he was too old for baths and requested a shower. I set him inside the shower stall in our master bedroom, and went to get extra towels just in case things got especially wet. I was only gone for a minute – just long enough to walk down the hall to the linen closet. When I came back I found him sitting on the floor of the shower, oblivious to the water falling on his head, with all the pieces of the drain scattered around him. In just that brief time he had taken it apart. I didn’t even know it came apart.

He is one of those people who are just seem born with an instinctive understanding of how things work. He gets this from his father, because I am most assuredly not one of those people. I often struggle with relatively simple machinery.

Which brings me to the item in the picture. That is the fastener on a mold strap. Mold straps hold the pieces of a mold tight while the slip is poured. I haven’t needed mold straps in the past because I have always dealt with molds smaller molds that could be held together with wide rubber bands. This has been a good thing, because I never could figure out how the fasteners worked. What is sad is that I have seen them used at Pour Horse. I’d even unfastened and refastened them, so I know how they are supposed to feel when they lock. I just couldn’t seem to make mine work.

I thought I could avoid dealing with them at all by simply using the same kind of large black rubber bands that I had used on the rubber master. They actually came off a set of “moon shoes” that my kids got for Christmas one year. When he first saw them, my friend Joe insisted that the shoes were the best job security he had seen in a while. Joe is a emergency room doctor. Shortly after that the shoes went missing (funny, that!), all except those useful-looking black bands.

I became skeptical though, when I had the completed mold. The rubber master tends to stick together a bit all on its own, so it doesn’t need to be cranked closed quite like the plaster one. I wasn’t sure the rubber bands were up to holding the large side pieces tightly enough.

As this picture of the first pour shows, they were not. The extra clay around the leg is where the liquid slip leaked between the pieces. (The white areas are from the mold soap that is present on the sides of the mold pieces.) This wouldn’t work. Not only does that slight gap distort the casting, but the clay between the pieces effectively glues the whole thing shut. It is almost impossible to remove a horse in this kind of situation without tearing it.

So I had to figure out the mold straps. I felt a little better when even my husband was at a loss. They looked simple enough, and he works in an engineering field. We must not have been the only ones, because in my search for a picture online of how they looked like closed, I found this online tutorial. Suddenly it all made sense, and now I have a tightly strapped mold. I thought it might be worthwhile to share the link, in case others were having similar trouble.


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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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