Cindi Anderson on sat 13 apr 02
This is exactly the impression I got from Peter King who does architectural
ceramics. I really would like to understand this, if anybody out there
knows the science.
Another thing I am stumped by is the Laguna clay guy at NCECA was telling me
that absorption is also not a good indicator of whether something will hold
water. He claims they did tests of clays which had very low absorption
rates (less than 1%) but many of them still leaked. The ones that didn't
leak were somtimes ones with higher absorption rates. Can anybody explain
this?
I have learned a lot about clay, vitrification, etc. from this group, but
there seems like there are many second order effects that haven't been
discussed (or I missed it.)
Cindi
----- Original Message -----
From: "Snail Scott" >
>
> There is a theory which says that frostproofness is
> a function of vitrification. This is definitely crap,
> IMHO! Unfortunately the exact mechanism by which clays
> can resist the force of ice is rather poorly understood.
> Some very porous terra cottas have withstood freezing
> for 100 years or more, while more vitrified clays have
> cracked in a single season. The best research on this
> seems to have been done between 1900-1930, when the
> fashion for architectural ceramics was sufficient to
> support substantial research. It seems that if a clay
> is sufficiently strong, full vitrification is not
> necessary. However, it's very difficult to judge which
> clays will perform well in advance of actual testing.
>
> I test my clay by making test bars, boiling them, then
> when the water has cooled, putting them in a ziploc
> baggie while still wet. I put them in the freezer
> overnight, then take them out during the day, and repeat
> until I forget to do it. (Very scientific.) A more
> reliable check, if you use commercial clay, is to ask
> your supplier if anyone else has successfully used that
> body for outdoor work in that climate. Ask to speak to
> artists who do such work, and ask how old their oldest
> outdoor works are. It takes a lot of freezer tests to
> equal a succession of real winters. (Actually, it's the
> springs and autumns that do the damage. Freezing, when
> it stays frozen for a month, is still only one freeze/
> thaw cycle!)
>
> -Snail
Ron Roy on sun 14 apr 02
Hi Cindy,
All my experience tells me absorption rates are directly tied to
vitrification and leakage.
If the clays I monitor leak I need to make them melt more - simple as that.
If a clay has an absorption rate of 1% they either have to be cracked or
have a hole in them to leak.
Perhaps the method of testing the absorption is the problem. It is quite
common to see bars weighed while still hot - this means the water is
evaporating while the testing is being done - a sure way to get confusing
results.
RR
>Another thing I am stumped by is the Laguna clay guy at NCECA was telling
me
>that absorption is also not a good indicator of whether something will hold
>water. He claims they did tests of clays which had very low absorption
>rates (less than 1%) but many of them still leaked. The ones that didn't
>leak were somtimes ones with higher absorption rates. Can anybody explain
>this?
>
>I have learned a lot about clay, vitrification, etc. from this group, but
>there seems like there are many second order effects that haven't been
>discussed (or I missed it.)
>
>Cindi
Ron Roy
RR #4
15084 Little Lake Road
Brighton
Ontario
Canada - K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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