iandol on wed 27 jun 01
Dear Forest Weaver,
Sitting a shelf on three or four props are both legitimate ways of =
providing support. But the two styles should not be mixed in any one =
firing. When four props are used it is not unusual to place a fireclay =
or high alumina clay wad under the shelf on the prop which shows a gap.
Supporting on four props does not prevent heat fatigue in refractory =
materials. So shelves eventually set if always fired the same way up. =
When I am firing plates and those things on four feet which would =
distort and most certainly rock if not fired on a true flat surface I =
turn the shelves over for the next firing. As I only use Alumina Sand on =
my shelves they are dusted between firings as well. If I used a =
refractory wash to adsorb glaze drips this would have to be cleaned away =
between firings and reapplied to the setting surface to prevent kiln =
dirt on a glazed ssurface.
It is usual to support on three props since this gives the most stable =
configuration. Props should be positioned under shelves so that they =
have the minimum distance possible between each. If you have rectangular =
shelves, this means one just inside each end of a long side and one in =
the middle of the opposite long side. Be certain to align props =
vertically above each other between each level. Square shelves have less =
tendency to warp than rectangular shelves.
I hope that clears any confusion.