ACTSNYC@cs.com on sat 6 nov 99
> ---------- Forwarded message ----------
> Date: Fri, 5 Nov 1999 16:55:33 EST
> From: David Hendley
> Reply-To: Ceramic Arts Discussion List
> To: CLAYART@LSV.UKY.EDU
> Subject: Re: Manganese in Cone 6 clay
> Resent-Subject: Re: Manganese in Cone 6 clay
> ----------------------------Original message----------------------------
> Lynne, I'll not comment on wisdom of using a clay that
> contains granular manganese.
> There has been lots of discussion about that here on
> Clayart in years past. (Access the archives!)
> >From your description, I think that your clay must contain
> some soluble salts, and that, not the manganese, is
> what is responsible for the 'brown stain on the surface
> which burns out in the bisque firing', and the 'black ring
> around the bucket' of your throwing water.
> As long as this causes no problems with applying glazes
> or the look of the finished piece, don't worry about
> the brown stain or the black ring.
David Hendley
Maydelle, Texas
---------------------------------------------
The black stuff at the surface could very well be manganese. Throw some
manganese dioxide in water and see if it does this. The fact that is stays
on the surface indicates that the manganese or whatever is in an insoluble
form. If it were soluble, it would only impart a color to the water
throughout the bucket. And the burning out in the kiln of the manganese is
just what I'd expect of a very thin layer of manganese. That means the
manganese is fuming during firing. Some always does. Its why you need quite
a bit on the surface to remain to stain the piece. And also why there is
often a kind of "fuzzy" look to the edges of manganese decoration applied to
the surface of glazes.
But soluble or insoluble doesn't matter with regard to inhalation of the fine
dust. And even by ingestion, there may be no clear relationship between
insoluble and soluble forms. That's what we have learned from frits and
other ceramic chemicals that are exposed to all the mechanisms humans have
for dissolving stuff in their lungs and guts.
Monona Rossol
ACTS
181 Thompson St., # 23
NYC NY 10012-2586 212/777-0062
ACTSNYC@compuserve.com
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