Saturday, July 4, 2009
Real world physics - or, will the hole shrink?
posted at 12:17 PM |
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My jewelry mentor-teacher yesterday said we were going to work with precious metal clay (an amazing medium, by the way) to start drafing a template for a piece with a cut-out area for a gemstone. Since we know that pmc shrinks about 8-10% when it's fired, we needed to figure out what size to make the hole and that led to an interesting question: would the hole become larger or smaller when the piece was fired? We realized variations of depth and density might change the answer a bit but we also thought there would be a basic principle as to whether the hole would enlarge or shrink.
We've made and/or been around plenty of pmc toggle clasps getting made where one side has an opening for a bar that goes through the other. All was well after firing, considering that we've never had toggles that didn't work from a size point of view. But then, again, we'd never paid specific attention to changes in the holes' dimensions. So we discussed and thought about it, and decided we anticipated one of these possibilities would turn out to be the case: (1) the hole would become larger as the solid areas around the hole shrank, or (2) the hole would shrink along with the whole piece. As it happened, she had a small piece on hand with a hole cut out of the middle, so we traced the outside and inside edges and fired it. Our results — well, what do you think?? — Spoiler alert! — do not read the next paragraph until you want to know the answer.
The first answer was as expected in that the whole piece did shrink about 9 or 10%. But what about the hole? Well, it turned out that the hole shrank just barely perceptibly, almost not at all, not even the width of a .05mm pencil line. Amazing. Neither of our pre-suppositions were right or, putting it another way, they both were! Perhaps the surface tension around the space, which I expected to be pulled at and therefore enlarged by the surrounding material as it heated and shrank, is trumped by the surface tension on the entire unit, to the effect that the space in fact resists the shrinkage. Anyway, we were fascinated, pleased and educated by our experiment and I promised I would write about it here. Now any rare readers know, too.
We've made and/or been around plenty of pmc toggle clasps getting made where one side has an opening for a bar that goes through the other. All was well after firing, considering that we've never had toggles that didn't work from a size point of view. But then, again, we'd never paid specific attention to changes in the holes' dimensions. So we discussed and thought about it, and decided we anticipated one of these possibilities would turn out to be the case: (1) the hole would become larger as the solid areas around the hole shrank, or (2) the hole would shrink along with the whole piece. As it happened, she had a small piece on hand with a hole cut out of the middle, so we traced the outside and inside edges and fired it. Our results — well, what do you think?? — Spoiler alert! — do not read the next paragraph until you want to know the answer.
The first answer was as expected in that the whole piece did shrink about 9 or 10%. But what about the hole? Well, it turned out that the hole shrank just barely perceptibly, almost not at all, not even the width of a .05mm pencil line. Amazing. Neither of our pre-suppositions were right or, putting it another way, they both were! Perhaps the surface tension around the space, which I expected to be pulled at and therefore enlarged by the surrounding material as it heated and shrank, is trumped by the surface tension on the entire unit, to the effect that the space in fact resists the shrinkage. Anyway, we were fascinated, pleased and educated by our experiment and I promised I would write about it here. Now any rare readers know, too.
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