Cleaning turquoise

Eileen Procter <eileen.procter<-a t->awm< . >gov< . >au> writes

>I have a piece of turquoise mounted in sterling silver. It has been
>previously worn and now the turquoise has yellowed but the silver
>has not tarnished.  The silver is not coated.
>[I] would like to know if the
>discolouration can be removed and if so, what would be the best way
>to go about it.

Some two years ago I had some similar problems to resolve, and the
issue was found as follow:

    1.  The turquoise CuAl6(PO4)4(OH)8.5H2O is an hydrated copper
        aluminium phosphate.

    2.  Because It has been previously worn, there is a great chance
        that the surface of the turquoise is impregnated with wax
        and oil and the yellowing is either dirt or oxidation of
        those molecules, if not both.

The solution I will suggest you to test, is to wash the surface with
acetone in order to clean the greases due to worn, and then only to
rehydrate the turquoise by simple immersion.

If you feel you can do a better water penetration under vacuum,
watch the stability of your turquoise under a good optical
microscope before. But remember that turquoise is soluble in water.

Watch carefully the effect of temperature and time of immersion. As
the silver is in a good condition, just use deionised water or even
better slightly carbonated water to decrease the solubility rate of
the turquoise. Remember to monitor this treatment at all time under
a good optical microscope.

Drying is done by simply removing the excess and several attempt may
be necessary to achieve the final result.

Watch also if the turquoise is set with the silver or if some resin
were used to set the stone, if so please think again the hole
treatment. Wish you full success and happy New Year,

David Cottier-Angeli
Jeweller and metals conservator.
Route des Jeunes 5C 5eme. 1227 Geneva
+41 22 300 19 55
Fax: +41-22-300 3 700

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                  Conservation DistList Instance 11:57
                Distributed: Tuesday, December 30, 1997
                       Message Id: cdl-11-57-003
                                  ***