Calcium oxalate on easel paintings

In answering Flavia de Souza's query about oxalate films (on
paintings) Jonathan Kemp <j.kemp [at] vam__ac__uk> writes

>With reference to Calcium oxalate, in the case of films on stone it
>has been discussed extensively as both a natural and artificial
>protective layer...

Oxalic acid has also been used by restorers as a method of removing
iron stains from stone (mostly white marble) and as a general
cleaning solution and final polishing method.  Oxalic acid is an
active ingredient in some fabric "bleaches" and cleansing powders
("Zud" and "Bar Keeper's Friend").  It might also be worth noting
that it is used in traditional stone lithography printing as a final
"chemical polish" and treatment on the limestone. In many, if not
most, of these cases surely a thin oxalate film is created.  In the
case of iron stain removal the oxalic acid does not actually remove
the iron stain but rather creates an opaque film deposit of calcium
oxalate which in effect hides the stain... or not.

Jerry Podany
Senior Conservator of Antiquities
J. Paul Getty Museum


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                  Conservation DistList Instance 22:57
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