[TheForge] Re: Fire clay

[Mike Spencer]

Bruce> You ARE aware, I suppose, that fire clay is a natural mineral
Bruce> and can be dug from the ground in many areas.

Bob> I've always wondered if the refractory was mainly due to the raw
Bob> material (clay) or the way it is processed.

I have this vague recollection that "fire clay" or "refractory clay"
is made up by adding finely ground *already-fired* clay to a selected
quality of the natural stuff dug out of the ground.

I've fetched what appeared to be reasonably good clay from an
excavation site in Boston.  My father (who became a potter after ill
health prevented him from doing a regular going-to-work schedule) made
some respectable pots from clay he got from a Massachusetts brick
works.  But natural clays vary enormously in both the properties
required by potters/ceramists and in those that emerge at prolonged or
cycled high temps.  The brickyard clay was rather difficult to work
with and some of the pots fired a bit strangely at the earthenware
temps Dad used to fire it.

My understanding is that the addition of grog -- pre-fired clay ground
to varying degrees of fineness -- results in a composite structure
that resists cracking far better at high temps or when being cycled up
and down from kiln to room temps.  IIRC, the addition of ordinary
silica or quartz sand doesn't doesn't work and results in worse
properties than plain clay.

Given that, I would suppose that the addition of other refractory
materials might well be part of formulating a commercial product.

Grog is relatively cheap and you might experiment with adding it to
your favorite back-yard clay yourself.  The stuff I've seen is about
the grit  of uniform coarse sand.  I assume that it's available in
various grits.

Why don't I google it myself?  Because I'm still stuck in 9600
bps-land with an old modem resuscitated from the spare parts bin until
the telco and union settle.  If I could get my 56k modem to connect at
all, I could get a signal/noise number and maybe get some service.  As
it is, if I can make and receive voice calls, all those accountants
and Junior Assistant Executive Deputy Under-Poobahs who are out
climbing poles don't want to bother with me.  Feh.

- Mike


-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^

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