Cameron Harman on tue 24 nov 98
Hi John,
Some interesting background information for you: not long ago it
was common to market ware that was vitrified, but not fully
vir\trified. It was, naturall, call semi-vitrified or 'semivit'.
However, it has always been known that a porcelain or china body
(which is, by definition, 0% absorption) is stronger than its semi
vit stage. We have proven this in the laboratory with MOR tests.
On the ther had, a clay bidy at full vitrification may actually
over fire, so it would be weaker, and probably would have some
porosity as well.
Glass formation, by itself does not cause a ceramic to be weaker.
The strength of a ceramic body is a very complex mechanism having
to do with thermal expansions of non-plastic grains and glassy
bond as well ass the thickness of the glass bond and the radius
ration between them.
That is certainly a whole lot more information than most of us
would care to know, so let us just say that in some bodies the
reactions are such that taking them to full vitrification is
harmful, yet in others it is beneficial. Most clay/talc bodies
should not be taken to full vitrification and only a few all clay
bodies can withstand going to full vitrification.
Sorry to go on so, I was interested in giving you all some
background as to why the genralization made by Hamer has to be
kept in context, not that anyone is wrong.
Cameron Harman
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