Karl P. Platt on sat 17 aug 96
This is to clean an old item from my hard drive before I ditch CI$
>>: stresses are produced in forming that will manifest themselves as distortion
if the body were not formulated to minimize that likelihood, i.e. an open
structure with as little longitudinally aligned
particle orientation as possible (for instance using a primary kaolin -
with rounder particles).<<
Sho' nuff.
Orientation of the clay particles is an explicit function of the process and I'd
suggest the easiest way to reveal these is to freeze the tile or clay pug.
You'll see right where the stresses live in living color and have the
opportunity to make any useful changes to the process.
If wettish forming methods must be used, I would suggest that one avoid clays
having a lot of colloidal material as this tends to get carried to the surface
as drying proceeds resulting in a couple things usually manifest as a denser
outer layer or skin on the ware. Clay with a lot of soluble material, likewise,
can cause these problems. Especially if no measures are taken to mitigate
soluble salts.
In respect to drying, having a dense "skin" on the outer surface of the ware
only complicates drying by forming an impediment to the departure of the water
present.
Quite apart from peculiarities put into the ware by the forming process used,
the "skin" thus formed likewise tends to vitrify much sooner and much more
thoroughly than the clay behind it -- shrinking first and presenting stresses
within the clay article which, if severe enough, may well deform the body should
the body soften sufficiently during firing. This occurs ot simply because of
preferred orientation -- which optimized the specific amount of surface area
each clay particle has in contact with the other and its neighbors -- but to the
extent that there are any dissolved salts in the body -- and they're there --
they are carried to and deposited on the surface as it dries. These, being
alkaline in nature, accellerate vitrification.
A lot of grog isn't necessarily a good solution. It also tends to make cloddy
looking tile -- but some people like that, to be sure.
I would also suggest that poor drying practice has killed more ware than badly
formulated clay bodies. One of my first experiences with a studio ceramist was a
fellow making tile and who dried them from one side only -- resulting in
warping, cracking, etc., in both drying and firing. Providing for uniform drying
eliminated the difficulties entirely. Oh, the body was originally developed for
throwing and was used only because the local clay guy had it in regular supply
and it was pretty consistent. The point being that the highly plastic throwing
body sufficed for wet-pressing tile and could be dried and fired flat so long as
it was dried as it required.
Last, I'll mention the old Design Technics -- now a part of Sherle Wagner, Inc.
-- 18" x 18" RAM pressed tile which were always flat. I was amazed when I first
saw these in their shop and was impressed that their success depended on the
drying arrangement which permitted drying from all sides.
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