Wednesday, October 27, 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.

Sunday, October 10, 2010

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.

Wednesday, October 6, 2010

Learning from mistakes

I tend to focus a lot on mold-making on the blog, probably because it is the area of my work where I still have so much to learn. Blogging is a lot like talking, and like most extroverts I process my thoughts best outside my own head. It has made for a messy conversation, at least for anyone else wanting to learn mold-making by reading, because it seems many of my posts are about what I should not have done.

I have been blessed throughout my adventures in ceramics with an extremely generous and patient mentor. Joan has always been available to answer any questions I have had, and to offer whatever help she could. But she does live 2,400 miles away. That means she cannot look in horror at something I am about to do, and cry out, "Oh no! Don't do that!"

Which is a big limitation when I assume I understand the next steps. I am usually a pretty decent problem-solver, but sometimes I just miss the obvious.

For some reason, when I made the second version of the Vixen master, I had it my head that I needed to end up with two separate, complete masters for the two different molds. I was used to thinking of master molds as exact - or at least almost exact - copies of the plaster molds they made. I knew the first master, which cast a whole Vixen, would need to hold the different rubber pieces (her head, her one front leg) so that separate molds could be made of those. What I didn't realize is that I didn't have to destroy that original master to do it.

I didn't need perfect masters of the eventual production molds. All I needed to do was temporarily modify the master to make the separate molds. If I could do it once, I could do it again when another production mold was needed. And if it didn't work, I could always modify it a different way the next time. (That is what I cannot do with the current Vixen master mold, now that I am unhappy with the modified design. Darn!)

This is what I should have done. I've boxed up the (unharmed!) Elsie master and just blocked off the area that will make up the separate mold for the head.

The mold sides are rough since I didn't bother to smooth the clay barrier, but a planing tool can fix that easily enough. That way this piece, and the one already poured (visible in the previous picture) and the rubber head can all be boxed and the final piece poured.

I did find that because of the angles of the cut to her neck, it was better to cast the other side of her face second since it gave me more control over the angle the plaster made with the opening of her neck. When this particular mold was finished, the edge was thin enough that I suspected it would crack before too many pieces were cast.

What I didn't remember was that when I poured the plaster for the first piece, I had inserted both the rubber head and the rubber body because there is a small gap where that inner piece meets up with the body. I didn't include the body the second time, so of course some of the plaster poured down the gap and into the body cavity, where it pooled in the tail.

Now I have two Elsie head molds and a white plaster Elsie butt. Next up - the body! I am sure it will be a learning process as well, and I'll share any of the mistakes here.