Karl P. Platt on sun 1 jun 97
In response to a question I was asked privately about the effects one
might anticipate in replacing lime with dolomite in a whiteware body I
wrote:
>Dolomite, which adds MgO, when put into a clay body would tend to make
>its firing range much shorter than a similar lime bearing body. Consider Cordio
>porcelain") bodies show similar behavior.
> As the body could contain a lot of glassy material, one would want to
>test the ware for thermal shock resistance for applications requiring
>it.
Ron replied:
>Dolomite adds calcium as well with a similar effect on the melt. I >would cauti
I probably should have put more of the original post in to be more
clear, but sinci I've bitched about huge quotes, I'm sensitive as to how
much of any post I utilize.
I only want to point out that CaO and MgO have been extensively and
successfully used as body fluxes since antiquity. CaO, as was present in
the existing composition, has also ben used in Cone 15 electrical
porcelain to great advantage. MgO has been less popular in clay bodies
(when anyone had a choice) for the reasons I described in my original
post -- i.e. it makes lots of glassy material and it does so quickly.
Any of the "RO" materials will behave in this way -- BaO, for example,
is more well known to do not much of anything until some certain
temperature at which it all goes into fusion at once.
The suggestion that RO body fluxes force the conversion of quartz to
crystobalite is new and should be examined carefully. There is extensive
literature on the effects of the alkalies urging this sort of reaction.
I only know this through a hard experience with quartz turning into
crystobalite in 1983. As a lot of expensive production was lost and I
was obliged to do a lot of research on the matter.
We were making an acid vat tile with a new body formulated to consume
this enormous pile of "sand" that was generated as a byproduct of our
mining operations. While lab results and initial test production looked
quite good, winter came and "all of a sudden" kiln car loads (30,000 lbs
of material) came out of the kiln and shortly thereafter started making
a racket that sounded like wind chimes -- pretty noise, bad
consequences. The beancounters were not pleased. Turned out that the
"sand" had a good bit of feldspathic material in it and while the quartz
it also contained was in the beta form -- which is more open than the
alpha form -- the alkalies (mostly soda) inserted themselves into the
lattice and on cooling the quartz had little choice other than to turn
into crystobalite. In this case a _small_ addition of talc (MgO)
improved the situation.
KPP -- Watching a magnificent sunset over the North Bay
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