Snail Scott on sun 23 dec 01
At 08:14 AM 12/22/01 +0100, you wrote:
Could the heat shock cause the [shelf] crack?
It's possible that it contributed.
But, full-round shelves endure more stress
than using two half-rounds, since with half-
round shelves, each shelf is held up by three
points, and is evenly supported. (Remember high
school geometry class? Three points determine a
plane.) Most full-round shelves are also loaded
with four posts in the same configuration as a
half-shelf stack, but if there's any unevenness
at all, only three of the posts are really
touching the shelf. A full half of the shelf
area, and its load of ware, is actually
cantilevered out beyond the nearest two posts!
It's no wonder these shelves crack so often.
You'd actually get a longer lifespan out of
these full-round shelves by only using three
posts, evenly spaced, than you would with a
standard four-post stack. (Draw the geometry,
or try the post options yourself. Does the
shelf rock at all, with four posts? Even if
it's just a little wobble, then one of your
posts is NOT holding up the shelf, leaving
that entire side unsupported!
It will help a lot to use a bit of kiln putty
under the short side. Just use the kilnwash
that's gotten thick along the sides of the
jar, or mix up 50/50 silica & kaolin with just
enough water to make a stiff putty. Put a
patty of it on top of the post that doesn't
reach the shelf, then put the shelf on top;
it will squish to the proper thickness.
(The four-post probelm is less pronounced with
the rectangular shelves commonly used in fuel
kilns, since the unsupported spot is only a
shelf-width away from another post, and the
stress runs the long way diagonally aross the
shelf. Kiln putty is still a good idea, though.)
Keep your shelves evenly supported, especially
with heavy loads.
-Snail
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