John Sankey on fri 4 may 07
I've got my first test run of Hesselberth&Roy's High Calcium
Semimatte Base 2 with Licorice colorants. The surface is perfect
and the gloss just what I want. Unfortunately, the colour is
olive green, nowhere near black. See
http://sankey.ws/pottery2.html
at the bottom for a comparative photo.
Any suggestions for addition(s) to correct the colour?
John Sankey on sun 6 may 07
Here is the mix I used:
Wollastonite 25%
EPK Kaolin 25
Fusion Frit F2 20
Flint 15
Red Iron Oxide 8
Nepheline Syenite 3.5
Bentonite 1.8
Cobalt Carbonate 1.7
silica 3.10
calcia 0.85
alumina 0.47
boron 0.23
iron 0.19
soda 0.09
magnesia 0.04
potash 0.02
The usual black option missing is manganese dioxide, a weak brown
by itself, but a strong oxidizing agent for iron, which makes it
blacker. Although manganese is an essential human nutrient
(DRI 2 mg/day) it's toxic in overdose (TUL 10 mg/day). Overdose
is only documented via inhalation, so it seems safe once fired in
a stable glaze.
My notes from Weyl, however, suggest titania, which he says moves
iron from green to brown because iron is moved from a
network-modifying role (equivalent of interstitial atoms in
crystals) to network forming (equivalent of taking part in a
crystal lattice). With its refractive index of 2.7, it's also an
opacifier, which can ensure that absorption by the colorants is
maximised. Quantity: I've no idea, other than that it must be
significant comparted to the 8% iron.
Unless anyone with more experience has another suggestion, I'll
try adding 1,2 & 4% titania for the next test. I have to buy
everything in retail quantities, though, and my turnaround at the
kiln I have access to is up to two weeks, so I'd sure appreciate
any comments.
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