Sodium bicarbonate as a blast media

Ed McManus <ed.mcmanus [at] nasm1__si__edu> writes

>I am interested in hearing from conservators who have had experience
>with or know of examples where sodium bicarbonate has been used as a
>blast media for cleaning metals such as aluminum and copper alloys
>and whether the results were favorable or not.  The proposed
>application is the cleaning of a very large aluminum rocket
>assembly.

At the Swiss National Museum we have been using sodium bicarbonate
for some years now. It has proved to be very effective for
investigative cleaning of corroded archaeological copper alloys.
With the right pressure and amount of powder it removes effectively
soft powdery corrosion on uneven surfaces that could be damaged by
scalpel cleaning (ingots, slag, artefacts).

In case an object is too mineralized to sustain the pressure applied
by scalpel, it can also be a very effective method to remove copper
carbonates down to the original surface when located in the cuprite
layer. Of course one needs to be careful when the original surface
is located withing changing corrosion layers and products. I
wouldn't advise it then.

Objects cleaned with sodium carbonate have to be thoroughly washed
in alcohol afterwards, as remaining powder will encourage corrosion
in case left on the surface. This was confirmed by Oddy Tests.

Sometimes glass beads will also yield good results on copper alloys
with solid patinas. Applied with the correct pressure and amount it
can be a careful method removing superficial corrosion products
without affecting tool marks of the original surface.

In outdoor bronze conservation the soft nutshell powder is often
used to remove unstable corrosion products and even out the color
difference between the green brochantite and the black conglomerate
of copper sulfides and soot particles. Nutshell will be the softest
abrasive and could be the one to start with in your case.

Valentin Boissonnas
Conservator
Swiss National Museum


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                  Conservation DistList Instance 14:31
               Distributed: Wednesday, November 29, 2000
                       Message Id: cdl-14-31-003
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