Stains on creamware

I am currently working on a piece of English creamware with a lead
glaze from around 1790.  The piece has numerous brownish spot stains
on the rim of the ceramic, as well as on the foot.  At first I
thought they might be stains from food, but under the microscope the
stains do not look like they are under the glaze, but rather in the
glaze.  The glaze itself appears pebbly in the areas of the stain
and the oddest thing is that the pattern of the staining looks very
similar to the way a biological growth appears, similar to mold, as
if it spread by contagion, rather than their having formed by
accident from use due to imperfections in the glaze.

Has anyone seen this phenomenon on creamware before, or on any
lead-glazed ceramic?  I'm having difficulty even lightening the
stains, much less removing them. I've tried the usual solvent
poultices as well as 3% hydrogen peroxide poultices to no avail. I'm
wondering if perhaps there could be an ingredient in the glaze or
body of the ceramic which is degrading over time. I can't imagine
any biological organism being able to live off of the inorganic
components of either the glaze or body. I've never heard or seen
anything like this on ceramics, but I can find no other reason for
the odd pattern of its distribution.  The piece also has quite a lot
of errant dirt or sand which clearly fell into the glaze before the
piece was fired, which makes me wonder if perhaps it sat for a while
in the potter's studio before glazing.

Any suggestions for how I could lighten the stains would be greatly
appreciated.  Perhaps a higher concentration of hydrogen peroxide?

Susan White
White Conservation Services


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                  Conservation DistList Instance 24:42
                  Distributed: Tuesday, March 8, 2011
                       Message Id: cdl-24-42-024
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