Removing stains from stoneware

Holly Jones <jones.holly [at] saugov__sa__gov__au> writes

>I am working on a children's stoneware ceramic tea set, decorated by
>a pink transfer pattern.  The ceramic body was stained brown which
>had entered the body through cracks in the glaze.  The treatment
>steps included wetting out the objects with water prior to
>poulticing with Laponite.  Soaking in water alone turned the water
>tea brown and the ceramic body felt slimy.  Despite the large amount
>of staining leaching from the object, staining was only slightly
>reduced and several successive Laponite poultices were applied. The
>Laponite gel also became very brown, however the staining was only
>mildly reduced.
>...
>I am now considering what other methods I can try.  Both tea or
>coffee stains are coming out of the objects and pustules of white
>have also appeared.  I am assuming that this is milk fat?

First, it is likely that you are dealing with a refined white
earthenware, not stoneware, because 1) transfer printing was only
rarely used on stoneware, 2) stoneware is not usually covered with
clear and/or crackled glaze, and 3) stoneware is much less prone to
staining than lower fired wares (though not immune).

As long as color is coming out in the wash water this step may be
worthwhile.  Change water baths frequently to promote diffusion.
Wash the objects with a soft bristle brush and running water to
remove surface slime.  When you are convinced this is no longer
productive, you can poultice with pure cellulose paper pulp as a
drawing medium.   It is unlikely that the staining material will be
completely dissolved and drawn back out through the glaze crackle.
The only alternative I know of is bleaching.  Chemical agents that
may be helpful include solutions of reagent ammonia or hydrogen
peroxide.  Of course both of these solutions require appropriate
handling, proper personal protection, and ventilation for worker
safety.  There is no sure recipe but try alternating poultices of
increasing strength, 2%, 5%, 8%.  Neither chemical is likely to harm
the fabric of the ceramic itself.   Stay clear of sodium
hypochlorite and other chlorine bleaches, these are very likely to
damage the objects through the development of a soluble salts
problem if any residual bleach remains behind.  Good luck,

Kory Berrett
Objects Conservator
Oxford, Pennsylvania


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                  Conservation DistList Instance 14:14
                  Distributed: Friday, August 18, 2000
                       Message Id: cdl-14-14-002
                                  ***