Lili Krakowski on wed 6 jun 12
Sharon:
I have used Tucker's white c 6 stonewares, made under Ron Roy's watchful =
=3D
eyes! As I have not bought any in a few years purchaser should read =3D
catalog to see which is most suitable for you. The bodies may have =3D
changed a bit.
And a caution: Wherever you buy, check that a white mid-range body has =3D
NO talc in it!! I was lied to by a supplier years ago, TOLD that the =3D
body he made had no talc...discovered the lie when my cobalt blue =3D
decoration fired purple! (never bought from him again!)
And I have no commercial interest in Tucker's etc etc
Lili Krakowski
Be of good courage
Johanna San Inocencio on thu 7 jun 12
the presence of talc makes the cobalt fire purplish?
Johanna
On Jun 6, 2012, at 8:46 AM, Lili Krakowski wrote:
> Sharon:
>=3D20
> I have used Tucker's white c 6 stonewares, made under Ron Roy's =3D
watchful eyes! As I have not bought any in a few years purchaser should =
=3D
read catalog to see which is most suitable for you. The bodies may have =
=3D
changed a bit.
>=3D20
> And a caution: Wherever you buy, check that a white mid-range body =3D
has NO talc in it!! I was lied to by a supplier years ago, TOLD that =3D
the body he made had no talc...discovered the lie when my cobalt blue =3D
decoration fired purple! (never bought from him again!)
>=3D20
> And I have no commercial interest in Tucker's etc etc
>=3D20
>=3D20
>=3D20
> Lili Krakowski
> Be of good courage
Vinod Chettur on thu 7 jun 12
Tucker's Mid Smooth Stone is very good and can take a lot of abuse. Can g=
=3D
laze=3D20
bone dry with it.=3D20
V
Ben Morrison on thu 7 jun 12
-Ben
=3D
The magnesium in Talc could make a blue turn purple yes.=3D0A=3D0A-Ben=3D0A=
=3D0A=3D0A=3D
=3D0A________________________________=3D0A From: Johanna San Inocencio anna=3D
san@FRONTIERNET.NET>=3D0ATo: Clayart@LSV.CERAMICS.ORG =3D0ASent: Thursday, =
June=3D
7, 2012 4:47 AM=3D0ASubject: Re: C 6 white body=3D0A =3D0Athe presence of =
talc m=3D
akes the cobalt fire purplish?=3D0AJohanna=3D0AOn Jun 6, 2012, at 8:46 AM, =
Lili=3D
Krakowski wrote:=3D0A=3D0A> Sharon:=3D0A> =3D0A> I have used Tucker's whit=
e c 6 st=3D
onewares, made under Ron Roy's watchful eyes!=3DA0 As I have not bought any=
i=3D
n a few years purchaser should read catalog to see which is most suitable f=
=3D
or you.=3DA0 The bodies may have changed a bit.=3D0A> =3D0A> And a caution:=
Where=3D
ver you buy,=3DA0 check that a white mid-range body has NO talc in it!!=3DA=
0 I =3D
was lied to by a supplier years ago, TOLD that the body=3DA0 he made had no=
t=3D
alc...discovered the lie when my cobalt blue decoration fired purple!=3DA0 =
(n=3D
ever bought from him=3DA0 again!)=3D0A> =3D0A> And I have no commercial int=
erest =3D
in Tucker's etc etc=3D0A> =3D0A> =3D0A> =3D0A> Lili Krakowski=3D0A> Be of g=
ood courag=3D
e
Dick White on thu 7 jun 12
Yes, cobalt turns purple in the presence of magnesium. Talc is high in
magnesium. Talc is sometimes used in white stoneware bodies for its fluxi=
=3D
ng
properties to help the non-iron-bearing white clays mature at mid and low=
=3D
er
temperatures. In a glaze recipe, you can control the presence/absence of =
=3D
the
magnesium and thus control whether the cobalt is blue or purple. But, as
Lili points out, if there is hidden talc in the clay body leaching magnes=
=3D
ium
into the glaze melt, your cobalt-bearing glaze will tend towards purple n=
=3D
o
matter what the recipe has in it.
(And as an aside to the question of what color is cobalt, if your glaze
recipe is high in alumina and titania, the cobalt will turn green. Have f=
=3D
un
with that one.)
dw
On Thu, 7 Jun 2012 06:47:27 -0500, Johanna San Inocencio
wrote:
>the presence of talc makes the cobalt fire purplish?
>Johanna
James Freeman on sat 9 jun 12
On Thu, Jun 7, 2012 at 4:53 PM, Dick White wrote:
(And as an aside to the question of what color is cobalt, if your glaze
recipe is high in alumina and titania, the cobalt will turn green. Have fun
with that one.)
In this case you are artificially forming a compound called cobalt
titanate, which is green in color. This is the basis for "Reitz Green" and
related glazes. Curiously, if one pushes the amount of titanium in the
glaze mix past a certain point, the glaze will often revert to blue. I
found this phenomenon while running line blends when playing with cobalt
greens, and I am not certain as to why this occurs. It is also possible to
create a green color by employing cobalt and zinc, or by the creation of
cobalt chromite using cobalt and chromium oxides. Many of the traditional
camouflage colors are tinted with various cobalt greens.
There are many magnesium-cobalt compounds which are purple in color, and it
is hard to say which one (or ones) we are creating in our glaze melt. My
money would be on magnesium-cobalt aluminate or magnesium-cobalt silicate,
most likely the aluminate. There are other magnesium-cobalt purples
involving boron, arsenic, and some of the alkaline earths.
The cobalt blue in our glazes is almost certainly cobalt aluminate, though
cobalt silicate also exists. Artificially prepared cobalt silicate (ground
cobalt glass) is the colorant used in very old Chinese blue-and-white
porcelain. Cobalt silicate on its own is not color stable, fading over
time. When fired into a ceramic glaze, it does not fade, indicating to me
that it is no longer cobalt silicate, but is now, after reacting with the
other glaze constituents, probably some flavor of cobalt aluminate.
That's all I know. Chemistry and mineralogy, spiced with a soupcon of
educated conjecture. Enjoy your explorations.
...James
James Freeman
"Talk sense to a fool, and he calls you foolish."
-Euripides
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
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