James L Bowen on fri 25 aug 00
After I mixed up a batch of Malcom's Shino based on the recipe in the =
handout from the Laguna workshop at NCECA I found another recipe in the =
handout of Steve Hill that had the following comment "Tends to crawl, =
which can be controlled by calcining the Red Art and kaolin. My question =
is: Should I measure out the dry ingredients and then calcine the two =
clays or calcine a quantity of each and set them aside for mixing =
glazes. If the latter then how do i calculate the quantity needed to =
replace the uncalcined quantity needed? Would I just calculate the LOI =
based on the figure in the Lagunas catalog of 4.925. Also I assume it is =
an earthenware clay, so what temperature to calcine? Low red heat? =
Thanks for all the help
Craig Martell on sat 26 aug 00
Hi:
You can weigh out the correct amounts of raw clay, calcine that and add it
to your glaze mix but you need to get every bit of it out of the calcine
crucible or whatever type of vessel you use. It's easier and more accurate
to calcine more than you need and then weigh out the needed amount after
calculating this from the LOI.
Info on calcining is not that easy to come across but I think I'm
remembering from my reading that kaolin hydroxyls are lost at about 1500 F
so cone 010 should do fine with the Redart. Cone 010 is about 1629F. I
usually calcine kaolins to about cone 06. The plan is to get the bound
water out but not fire hot enough to sinter the mass which then make a lot
of trouble with breaking up and screening. At 06, the kaolins are still
nice and fluffy and screen easily when mixed into the glaze.
Another method that works pretty well with shinos if you want to go to the
trouble of using glaze calculation, is to make an adjustment in the
kaolin-silica additions by adding some calcined alumina oxide. This
requires some trial and error in finding the right amount of calcined
material to make application successful and with the glaze calc, you will
lower the raw clay and increase the silica to attain the same molecular
balance in the glaze. Sounds like a lot of mucking about but it works
pretty well.
I've been studying shinos for a couple of years and they are a wonderful
family of glazes. One of the characteristics of this type of glaze is an
unusually high amount of alumina. One of the suggestions for alumina in
glaze limits is it should be at about .5 moles max. Shinos usually have no
less than .9 and a lot of them are above 1.0. The reason I'm mentioning
this is that high alumina glazes will have a lot of surface tension which
can cause crawling. So sometimes you can calcine clays to avoid cracking
in the raw glaze but still get crawling in the fire.
later, Craig Martell in Oregon
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