Wednesday, March 5, 2008

Holes

I heard something yesterday that just reached right into my skull and swiveled my brain.

I was talking to my local clay supplier and at the same time fitting plugs into the bottoms of a salt and pepper set when she said, "The holes don't shrink, you know, between when you make the piece and when you finish firing it."

"Whaaaaaa?" Brain said.

"Whaaaaa?" Mouth said.

"Yes. The hole doesn't get smaller; the clay shrinks, but the hole stays the same."


"Uaaaaggggg, Whaaaa?" Brain said.

Jeanette said to Brain,
"Call up that mental image of the hole in the bottom of the salt shaker.
Run the footage of the hole in bisque firing.
See the clay shrinking.
See the hole stay the same.
Run glaze firing footage.
Ask yourself, How can the hole shrink with glaze all over it?
(Milllisecond compiling)
Huh...... She must be right."

By golly, she must be right. I can't visualize the clay gathering up in the firing and shrinking the hole. I can visualize the clay platelets solidifying and the spaces between them changing and getting compressed or with glaze flowing over them, but I can't see them binding together to draw a hole up smaller.

Message to Brain:
Keep track of holes in things and measure after each firing to see if this is true.
File under Puzzle Section in Drawer Re: Screw-on lids and other mysteries. Could have a bearing on figuring out how to make these.

Que music from "Brazil".

Note: If I ever got locked up in stir, I could easily fill my time thinking about stuff like this.

2 comments:

Anonymous said...

I seem to recall from a physics class that the hole will shrink. The teacher showed us the math to prove it, but I don't remember that part.

I admit that it doesn't make much sense when you think about how the clay particles are behaving when fired, but here's another way of looking at it. Picture a washer made of clay - just a flat disc 3 inches diameter with a 1 inch hole in the middle. And just for simplicity's sake, let's say the shrinkage rate is 50%.

Now after firing, the total diameter is 1.5". the distance from the outer edge to the edge of the hole is .5" (it used to be 1"). That leaves .5" for the hole. The hole also shrunk by 50%. Ta-da.

'Course, that's only a thought experiment. It might not work that way in the real world, but I think it does.

Anyway, I don't think you'd need to make a hole guage like this if the holes didn't shrink. http://www.potterymaking.org/pmipc/MayJune06-TipsFromThePros.pdf

Have a good one, Paul

clayartist said...

Thanks for your thoughts, Paul.

That does it. I'll HAVE to find out now. My gut tells me that the hole will shrink. With my clay, it should be about 12%.

And while I'm at it, I think I'll invent a threaded bottleneck and twist-on lid to see if I can pull the whole thing off.

A twist on lid would be a thing of beauty if I could figure out how to make a couple of plaster molds so I could make multiples. So far, I don't think anyone has invented a way to do this. I know David Hedley has tried to research this problem, but I don't think he's come up with anything yet.