Lili Krakowski on wed 14 mar 07
Dear Vaishali!
What a gorgeous name! I have been saying it aloud to myself all morning!
Cornwall Stone is in the feldspar family. It is a wonderful material with
two drawbacks.Apparently it no longer is mined, or not mined much,
or like that, AND it always has come in 4 varieties, which made it
unpredictable.
There are recipes using it if you check out the books. If not, test it as
replacement for a spar in a glaze.
Magnesium Carbonate is--guess what?-- magnesium! Fraser speaks of it
as a useful secondary flux at over 1150/2100 degrees, and it
lowers the coefficiant of expansion. I do not think I ever
have used it.
Jordan clay, as M. le docteur Bastarache told you, is no more. As so
often "they" had to build rinky dinky, tacky little houses, or malls over
the mine. If you have some either slake it and use it for fine small things
(buttons, earrings) or use it up in slips.
Potassium Dichromate is considered toxic, harmful. It was used to produce
chrome effects. If I remember correctly there was an article in
ClayTimes not too long ago about chrome and its dangers.
A potash feldspar has potash as its primary flux, and a soda feldspar
has primarily soda as prime flux. Within the potash and soda families they
are pretty interchangeable. Which is GOOD because every fifteen minutes
another spar becomes unavailable and one has to find a substitute. I would
just make a test with each new spar and go from there. By the way: the
custom is that when a recipe calls for FELDSPAR, potash spar is
meant...Sodaspar customarily is listed in recipes as SODA FELDSPAR.
BONE ASH is calcium phosphate. The phosphate part makes it do
fun things in a glaze, and adds "interest" to a surface. It is recommended
one use it carefully, with decent ventilation in the firing, as the
phosphate part makes bad fumes.
So you see you have a lot of amusement ahead, using
all those nifty ashes.
Yes, yellow ochre contains iron, yes it can be used instead of iron, but
it has only about 25% iron, and it has silica and alumina. Think of it as
you would of a red clay with --for a red clay--A LOT of iron. It gives
subtle colors, it is wonderful in slips, and I like to use it where I want
just a tidge of iron. Also, unlike Red Iron Oxide it does not stain
everything red!
Whiting is whiting....again, it is not consistently exactly the same...but
enough for our needs.
As to books: Try to get ahold of Harry Fraser's GLAZES FOR THE CRAFT
POTTER, and or Emmanuel Cooper & Derek Royle's GLAZES FOR THE
STUDIO POTTER. As far as I know both area available in the second-hand/used
book market. Either of these books will give you a
great deal of information about materials.
As to glazes with Cornish Stone
Shapiro Sung Yellow (from one of Cooper's books)
CORNWAL (CORNISH) Stone 73
Gerstley Borate, Colemanite or calcium borate frit 7
Whiting 16
Neph Sy 4
Bentonite 8
4 % yellow stain and 3% tin
No, you do NOT have to make it
yellow...It is very nice plain
semi trans. as I recall...so opacify it.
And, from Cooper's "The Potter's Book of Glaze Recipes"
# 287 semi clear c.7
feldspar 40
CORNISH STONE 20
Whiting 20
Zinc Oxide 3
China Clay [Kaolin] 10
Ball Clay 9
semi-opaque, bluish, bubbly in oxidation
291 Clear runny glaze
Feldspar 65
CORNISH STONE 10
Whiting 20
Zinc Oxide 5
clear on porcelain, semi-matt on stoneware.
Do not let the cone 7 worry you. Just add a bit more whiting to lower temp
Lili Krakowski
Be of good courage
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