John Sankey on tue 29 may 07
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http://sankey.ws/pottery2.html
Adding extra cobalt to the mix with titania either pushed some of
the iron back out of network-forming position or prevented some
of it from fully oxidizing. The combination of added manganese
and titania has simply separated, with good black mixed with
patches of green iron and other patches of blue cobalt. The
overall cast from a distance is green for the extra cobalt mix
(left image), black for the extra manganese (right image).
More bothersome, the gloss of the green sections in both cases is
much lower than the black or blue sections. Is this metallic-type
separation? Does it mean that the quantity of colorant has to be
reduced for this base compared to the base for which Ron designed
Licorice? Might using black iron oxide (already fully oxidised)
help get rid of the green?
I'm out of my depth now :-(
(To email me privately, include 'Byrd' in the subject line.
I can only read text email, not attachments.)
Timothy Joko-Veltman on wed 30 may 07
To me, that green looks like the Ti is interacting with the Co (See
"Rutile Greens" in John Britt's books for several nice examples), and
not reduced Fe - this fits with lower gloss you're seeing, too. Ti
also has a tendency to create mottled textures. If it's a black glaze
you're looking for, I'd go with something w/o any opacifiers, and make
the glaze a semi- or satin matte instead; matte glazes are generally
opaque w/o any opacifiers due to devitrification
(micro-crystallization).
Regards,
Tim
John Sankey on thu 31 may 07
Timothy Joko-Veltman wrote,
"To me, that green looks like the Ti is interacting with the Co
(See "Rutile Greens" in John Britt's books for several nice
examples), and not reduced Fe - this fits with lower gloss you're
seeing, too."
Oh dear, an archives search found several threads confirming
this. But, note that I was getting green before I added any Ti,
so something in my base glaze may also be reacting with Co
instead of/in addition to the iron...
"Ti also has a tendency to create mottled textures."
A double whammy for Ti, then. I'll definitely have to go some
other direction.
"I'd ... make the glaze a semi- or satin matte instead"
This one is - maybe I need to control cooling better for it to
get more micro-crystallization. I've just started heat/cool rate
testing of my own kiln, and should know enough to start trials
within the week. Already have a batch of test tiles drying :-)
John
John Sankey on sat 2 jun 07
Success! The first firing in my own kiln gave exactly the results
I want, with the originally planned glaze.
They key is the slow cooling from peak temperature that a matte
glaze is supposed to have. The group kiln that was all I had
access to before is only run on a "fast glaze" cycle. I gave mine
a 45 minute even cool to 1100C before switching off. It was the
fast cooling from peak that was producing all those greens etc.
with the high calcium glaze.
On to the red overglaze...
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