Sunday, December 21, 2008

The mistake I avoided

As I said in my previous post, I almost made a really big mistake with my mold. Although I had expected a number of technical problems with this particular piece, everything had worked out perfectly. My first pull - the large side that comes off first - released easily without disturbing any of the other pieces. That's important since jarring the inner pieces can stress the legs, causing them to break later when the piece is demolded or cleaned. Even more important, I had good registration on the legs. Nothing had shifted during my pouring.

I began to wonder if Imp might not prove less complicated than I originally feared. Perhaps this first version would work and I would never need to make the more complicated second version. (The one where he gets cast in pieces and is then reassembled.) Here he was, looking really good with a five-piece mold! That's the really basic type of horse mold; two sides, front gusset, back gusset, poll piece. I was so excited by the idea that I might get away with making one of Sarah's horses with just those five pieces, that I almost tried it.

Which would have been a big mistake, and I would have realized it the first time I tried to demold him and ripped off his tail. The mold needed a sixth piece.



I had already seen the necessity of this when I drew my mold lines. The undersides of his tail and fanny recess too deeply for the second large side to pull away. To capture those details, I would need a tiny inner piece. Because pieces like this are small and don't extend to the outside border of the mold, they cannot be poured like the gussets and poll piece. They have to be made by hand for each plaster mold. It's fussy, time-consuming work, but the worst thing about them is that there isn't a corresponding rubber piece for them.

That means if you just replace the rubber pieces one-by-one, without thinking about it, you will forget to make them. I've done this more often than I'd like to admit, but not usually with the very first test mold.

Thankfully I did remember it before I went any further.



Here I've built up a clay barrier to contain the plaster under his tail.



I want my plaster to look like this for the next step.



Here I've spooned a small amount of plaster into the area. I'll need to add some more plaster, but I need it to set a little more before I do that.



When the plaster looks like this, I can add some more. I want to add a little height to my piece so I have something to work with when it comes time to shape it, so I need the plaster firm enough that it doesn't just spill over the barrier.



Here I've dabbled a bit of the firmer plaster onto the top of what I applied before. Now it's time to wait for the plaster to firm a little more.



When the plaster looks like this - pastey but still damp - I can shape the piece.



If the plaster is at the right stage, the clay barrier can be removed and the plaster will retain it's shape.



The plaster should retain it's shape, but still be soft enough to shape with light pressure. Here I've used a minarette to clean up my mold lines and plane the piece so it will pull later.



After I have the basic shape right, I use a soft, slightly damp brush to smooth the piece. I will also need to clean any excess plaster or plaster dust before I pour the side piece.



Here are all my finished and sealed inner pieces. To the right is the rubber master, which I have begun to reassemble as I finish with each piece. (I had not yet cleaned the last bits of plaster dust from the pieces on the left when this picture was taken.)

Now all that's left are the two big side pieces.

2 comments:

Becky Turner said...

Now isn't there also a small undercut under the cheek and chin area too? will you have to make a piece for that ? or is it more pointed there..I haven't seen one in person yet... and once you pointed this tail part out I saw it.. but I don't think I would have caught that myself. Looking good!
Rebecca Turner
www.solticeartstudio.blogspot.com

Lesli Kathman said...

No, it's not really undercut there - or at least not enough to be an issue. The clay does shrink back ever so slightly as it reaches the leatherhard stage, so you can get away with a little. But the recess under his tail, especially when the tail connects so narrowly to the body, would never wiggle free without snapping the tail.