Modification Date: 2013-03-25 08:46:26
Member of Group: Crystal
High Lithium, Low Titanium Cone 7-10 Glaze
|Ferro Frit 3110||50.0|
|Rate (C)||Temp (C)||Hold (Min)||Step|
Purpose: To create a glaze which would give strong, dark, true turquoises from copper through copper's interaction with lithium.
I noticed that in at least two ceramics supply catalogs, lithium carbonate
and the lithium-bearing feldspars spodumene and petalite were suggested
for obtaining copper turquoises. So I decided to give lithium a chance.
The result of several experiments is Celestite. Most glazes with very little
titanium oxide will give turquoises when only one or two percent copper
carbonate is used. With Celestite I find I can go as high as 3.5% and still
get a genuine turquoise/cyan color. Over that, however, the color does
begin to turn green.
Celestite is a somewhat fussy glaze that rewards you with infinite
wonders if you treat it right. I named it "celestite" because the typical color
of the crystals, with no colorant added to the glaze, is the same as that
of Michigan celestite crystals (which are another mineral altogether). It is,
actually, my glaze of choice when I desire to make a "white on white"
piece with no colorants at all. The crystals are particularly refractive of
light and are the easiest to see when no colorants are used to make them
Either by itself, or with cobalt, copper and especially manganese, it
gives good results as low as 1255° and I even used it once at 1250°. With
nickel, vanadium or any of the stains, you'll need to fire Celestite at the
higher end of its range; at the lower end the glaze is either a nice matte
without crystals, or is just plain underfired. Celestite accepts all colorants
with aplomb and can even give lovely greens and golds with chrome if the
glaze is used immediately it's mixed.
Actually, Celestite is really the best bet for many copper formulations
because its crystals are opaque. In all my other transparent-ground glazes,
the crystals, when used with copper, also come out transparent and are
sometimes nearly invisible.
Firing schedule above is an example. Tweak it to find the best schedule for your kiln, glaze application and clay. Fires between cones 6 and 9. When used immediately after mixing, crystals are often slightly aquamarine without the addition of any colorant. Crystals are quick growing. Crazing is moderate when fired to cone 9. Best on porcelain although will accept some very white stonewares.
Glaze Color - Pale Aquamarine Blue
When Celestite crystalline glaze is used immediately, crystals are often a pale aquamarine even when no actual colorant is added. Copper in small amounts is influenced toward turquoise or cyan.
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By Tony Hansen+