Stains on creamware

Susan White <smwhitewhite [at] aim__com> writes

>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. ...

This sounds very much like a manufacturing fault rather than some
kind of dirt or corrosion. I can think of three origins of these
stains:

    1.  Something like sand or other mineral dust might have settled
        on the object before or during firing. Quite often, kiln
        furniture had been covered with sand before firing in order
        to prevent pots sticking on it due to running glaze,
        softening body or melting fly ash. That sand often trickled
        down onto objects on lower levels and then melted into
        glazes, causing rough and possibly discoloured areas.

    2.  The ceramic body may have contained salts (as an impurity of
        the clay or the batch water) which, upon drying, were
        transported in solution towards the edges of the object
        where the salt layer then during firing might have spoiled
        the glaze.

    3.  Fly ash may have settled down on the object during firing.
        This  seems to me the most probable explanation for the
        phenomenon you are describing. Normally, during firing the
        object would have been sheltered from direct contact with
        the flames by a saggar. Reading your description regarding
        the objects' appearance it seems likely, that the saggar
        containing the object was broken or some other accident
        happened to it during the firing process. As a result the
        glaze of the pot might have been contaminated by fly ash,
        mineral grains in the vicinity or by the saggar itself.

Depending on circumstances, fly ash melting into glazes may cause
odd effects such as small crystals like ice crystals on frozen
windows, rough or powdery layers, speckles etc and is usually dark
in colour; brownish or yellowish due to iron content.

Needless to say that this type of stain cannot be removed.

Rainer Geschke
quarzsprung restaurierung
Berlin, Germany


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                  Conservation DistList Instance 24:45
                  Distributed: Monday, March 28, 2011
                       Message Id: cdl-24-45-005
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