Richard Aerni on sat 9 feb 02
John,
A quick reply, as I've got a multitude of tasks at hand and you indicated
that you would be testing glazes today. For the purple, it would indeed be
cobalt you would use, rather than chrome. In the glaze you referred to in
your earlier post, the predominant color is pink, obtained through a similar
process as getting the chrome-tin pinks, but using rutile (titanium) in
place of the tin. (This is my guess...for a more complete explanation of
the chrome-tin pink, refer to Michael Bank's post which I've excerpted and
attached below.) My guess is that the addition of 1/2 of 1% of cobalt to
the glaze (the Dick Lehman pink) is what sometimes gives it a purplish hue.
Cobalt is an oxide which can give many different colors. There is the
cobalt green color which clayart has discussed in some detail. But cobalt,
in a stoneware glaze containing significant amount of magnesia, will tend
towards a lilac shade of purple. That is why I recommended replacing
whiting (or other fluxes) with equal amounts of Talc [Mg3(OH)2)Si2O5)2] and
Dolomite [Ca(Mg,Fe)(CO3)2], in order to bring the magnesia into the glaze.
Your computer glaze software could probably suggest a proper
silica:alumina:flux ratio that would put it within the limits of safety and
stability.
Good luck with the testing.
Richard Aerni
Bloomfield, NY
------------------------------------------------------------
Michael Banks' post on chrome-tin pinks:
The reason why the colours of sintered zirconium-vanadium compounds (in
turquoise stain), and chrome-tin stains differ from their single constituent
oxides, is because in entering the combined molecule one or both of the
metal ions is forced into a lower valency state than we are used to
encountering in glazes.
E.G: In chromium-tin stain, the normally green chromic (III) ion is forced
to reduce to the chromous (II) ion which is ruby red in colour. Normally
(on it's own) Cr (III) is too stable to reduce to Cr (II) and yield a red
colour in glaze. I believe a similar mechanism operates in
vanadium-zirconium stain, the turquoise colour coming vanadium reduced to a
lower valency value than the pentavalent (V5+) ion (yellow-brown) normally
seen in glazes.
Michael Banks,
Nelson,
New Zealand
----- Original Message -----
----- Original Message -----
From: "John Weber"
To:
Sent: Saturday, February 09, 2002 6:21 AM
Subject: Re: Looking for a Matte Purple and blue cone 9 and ? for Ron & John
. Would it not be Chrome rather than cobalt to mix with Tin to
> get purple? I am testing glazes today so I'll check out, but I wanted to
> ask. Thanks for your great workshop, do more and we'll attend.
>
> > I believe that John got this recipe from one of my workshops. I call it
> > Dick Lehman Pink/Purple Matte, since it came from a Dick Lehman CM
article
> > called "Steal This Glaze." Notice that the title is PINK purple
> > matte. It
> > goes more pink than purple for me. Emily's Purple, quoted earlier, is
> > really, really purple!
> > You might try taking a matte or semi-matte recipe, delete the whiting
and
> > substitute for it half talc and half dolomite. Then for
> > colorants add 1.5-2
> > % Cobalt Oxide, and 2 % Tin Oxide. Those coloring oxides, in
combination
> > with the magnesium, should give you a purple glaze.
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