Neal on fri 15 may 09
I made a sample batch of this glaze and it turned out
unexpectedly.
Dolomite 19.0
Gerstley Borate 12.0
Kona F4 Feldspar 37.0
Silica 32.0
Zinc Oxide, calcined 7.0
Copper Carbonate 0.5
Cobalt Carbonate 0.5
The arts center that I received the base glaze recipe
from uses more copper carbonate and no cobalt carbonate
in its version. And of course that turns out green.
I was aiming for a light blue with the slightest hint
of green (similar to a Restoration Hardware shade). What
I got was lavender. It's pretty, just unexpected.
Is there something in the base glaze recipe that would
make a mix of copper and cobalt carbonates go to the
purple side of blue instead of the green side?
There is a similar looking lavender glaze on this
pottery website:
http://muddymountainpottery.com/mugs-and-cups.html
I use Highwater Clay's Red Stone. The firing was cone 6
electric with a slow cool-down.
Neal W. O'Briant
Raleigh, N.C.
Eric Hansen on mon 18 may 09
Neal: Don't give up on designing your glaze just yet. If you start from the
premise that you have a look and a color that you want, a firing range that
you want, surface, oxidation, reduction, what kind of kiln and fire you use=
;
THEN you work towards designing the base that supports those things. I had =
a
discussion with Jon Singer the other day and we both agreed that the old
idea of taking a base and making it suitable for different colors as oppose=
d
to designing a glaze where they are both implicit is often not really
successful. Perhaps with the exception of where you want to see many colors
under or in the same glaze, but that is a problem in and unto itself.
Dolomite could be substituted with whiting or another source of calcium in
many cases, although the nice satin feel may not be there. A different kind
of matte might be possible by adding wood ash or perhaps barium.
There is a very basic akaline blue green that is one of the oldest glazes i=
n
the world and fires in oxidation at mid-fire. Also there is a zinc turquois=
e
that is very powerful.
H A N S E N
On Sun, May 17, 2009 at 8:02 AM, Neal wrote:
> Thanks to all the responses to my glaze question. The cobalt
> reacts with magnesium in the dolomite to make lavender. Now I
> know. I may have to give up on getting the blue-green color
> I want from this base recipe. But I may try another test with
> just copper carbonate--and then on to some line blends with
> the two batches to see what comes out.
>
> I had another thought this morning about trying some of the
> Mason stains I have on hand--but found out that the ones I
> have are not recommended for glazes containing magnesium
> and/or zinc, which rules them out for this glaze.
>
> On another note, the Triangle Potters Guild is participating
> in Raleigh's arts festival this weekend. We have a huge tent
> with 23 potters set up. Sales were good yesterday. Today's
> forecast isn't hopeful--but maybe a few folks will still come
> out and buy. It's fun just hanging out with other potters. We
> each take 2-hour shifts at either the kids' table, throwing
> demos, or sales table each day.
>
> Neal W. O'Briant
> Raleigh, N.C.
>
douglas fur on mon 18 may 09
Hank
I think Neal's curiosty about zinc's roll is worth looling at. And boron
always raises sucpicion.
What this reminded me of was a flesh pink glaze of Bev's I tried once.
Could its materials list add insight?
DRB
Seattle
On Fri, May 15, 2009 at 6:04 PM, Hank Murrow wrote:
> Dear Neal;
>
> The dolomite is a suspect in the lavender color which you cite. High
> magnesia glazes send the cobalt towards lavender. Sometimes a cobalt
> wash with magnesia will turn out lavender to purple.
>
> Cheers, Hank
>
>
>
> On May 15, 2009, at 1:26 PM, Neal wrote:
>
> I made a sample batch of this glaze and it turned out
>> unexpectedly.
>>
>> Dolomite 19.0
>> Gerstley Borate 12.0
>> Kona F4 Feldspar 37.0
>> Silica 32.0
>> Zinc Oxide, calcined 7.0
>> Copper Carbonate 0.5
>> Cobalt Carbonate 0.5
>>
>> The arts center that I received the base glaze recipe
>> from uses more copper carbonate and no cobalt carbonate
>> in its version. And of course that turns out green.
>>
>> I was aiming for a light blue with the slightest hint
>> of green (similar to a Restoration Hardware shade). What
>> I got was lavender. It's pretty, just unexpected.
>>
>> Is there something in the base glaze recipe that would
>> make a mix of copper and cobalt carbonates go to the
>> purple side of blue instead of the green side?
>>
>
douglas fur on mon 18 may 09
Hmm
This makes sense. The cobalt pinks I've seen have been at higher levels
than 0.5% and I've seen Boron in Cu Red recipes but what about the Mg+Co=3D
pink connection?
On Fri, May 15, 2009 at 10:14 PM, ivor & olive lewis
wrote:
> Dear Neal,
>
> This is typical of reduction chemistry, whereby something in your kiln ha=
s
> created a low oxygen level, allowing for the reduction of the copper oxid=
e.
> If you have no ventilation on the kiln and it is well sealed it is
> possible
> that wax, if you are using it, was not completely burned leaving a residu=
e
> of hydrocarbon gases in the chamber This may be responsible.
>
>
>
> Remember that auto reduction happens to Iron oxide at temperatures above
> about 1250 deg C. (2282 F) but my reading of the Ellingham Diagram seems =
to
> tell me that this would not be so with Copper oxide.
>
>
>
> Best regards,
>
> Ivor Lewis,
> Redhill,
> South Australia
>
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