Brass

Michiel Langeveld <info [at] metaalrestaurator__nl> writes

>During washing of brass objects in water, the brass stains from gold
>to brown. Can anyone advise me on the use of inhibitors or other
>methods to avoid this staining?
>...
>... the brass is cleaned by dipping in a bath of
>0.5M EDTA (ethylenediaminotetraacidicacid) at pH=10, it is shortly
>rinsed with water, brushed off with non-ionic soap in water and then
>put in an ultrasonic bath to remove loosened corrosion and dirt).

It seems to me that the pH of the solution is too high (10), and
this is dissolving the zinc out of the alloy.  Depending on the
composition of the metal, the pH and concentration of the solution
and the length of the treatment, the surface could become brown,
black or even coppery.  In the past, I have used a 5% w/v solution
(approimately 0.18 moles/litre) of disodium EDTA, which has a lower
pH, for cleaning brass, and have not had any problems with surface
colour change.

Dr Barry Knight
Head of Conservation Research
The British Library
96 Euston Road
London NW1 2DB
+44 20 7412 7229
Fax: +44 20 7412 7658


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                  Conservation DistList Instance 18:14
                 Distributed: Sunday, September 5, 2004
                       Message Id: cdl-18-14-001
                                  ***