Earthenware ceramic

Helen Privett <helen.privett [at] ngv__vic__gov__au> writes

>I am currently treating a Tang dynasty cream coloured earthenware
>figure of a horse.  The figure has previously undergone extensive
>restoration including the insertion of approximately 4mm diameter
>ferrous dowels into each of the legs; each leg having been broken in
>at least three places.  ...
>... removal of the plaster layer has revealed ferrous staining
>to the cream coloured earthenware.  Has anyone any experience with
>removal of ferrous stains from unglazed earthenware ceramic?  Is
>this possible?  ...

While I was at the Freer, we had some success with removal of iron
stains from glazed earthenware with a stained body by using EDTA,
with alternating heating and cooling to attempt to draw the stains
up through cracks in the glaze.  If it is of interest, I can go and
get the details from old treatment records. Oxalic acid might also
be something to try.  The biological rust remover called Biox might
be another possibility.

In terms of removal of the rusty iron, the rust has probably worked
its way out into the ceramic and has formed a continuous rust phase
from the iron out into the ceramic.  About the only way I know to
get the iron rods out is to drill around them with a core drill to
get through the rust membrane.  You can make core drills to fit
exactly around the rods using stainless steel tubing.  We used
telescoping stainless steel tubing available from Saturated Steam,
Ltd. in Florida and I believe that they have a web site.  With a
hand grinder and a separating disc you can grind teeth into the end
of the tubing and then use it to drill around the rods.  Go slowly;
probably the best way is to use your hand to twist the rod and apply
pressure.  We were able to remove a totally rusted hinge pin from a
18th century horn snuff box using this method without damage.



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                  Conservation DistList Instance 16:29
                 Distributed: Friday, October 25, 2002
                       Message Id: cdl-16-29-001
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