Copper alloy objects with zinc sulfate efflorescence

Marianne Nuij <m.h.nuij<-at->gmail<.>com> writes

>Currently I am working on my master thesis, the subject being the
>growth of white crystals on an outdoor sculpture.  The objective of
>my thesis is to investigate the causes and growth mechanisms of the
>crystals.
>...
Through X-ray diffraction and energy dispersive
>X-ray spectroscopy analyses that have been performed on samples it
>could be concluded that the fibrous crystals consist of hydrated
>zinc sulfate and the globular crystals consist mostly of calcium
>sulfate with some calcium carbonate.
>
>I am interested in knowing if these types of crystals are a common
>problem.  I would very much like to know what the alloy of the
>object is and in what conditions the object was when the crystals
>started growing.  ...

The formation of globular calcium sulphates and carbonates is
typical on new outdoor copper alloy sculptures.  What is basically
happening is the migration of casting investment residues from the
interior to the exterior via pores and flaws in the casting.  With
daily cycles of warming and cooling, moisture condenses inside the
sculpture, dissolving the investment, which then seeps out through
tiny pores and casting flaws in the metal.  The sulphates and
carbonates then precipitate on the outside of the sculpture.  It's
much like stalactites forming in a cave.  To prevent this from
happening, you need to remove all of the investment residues from
inside the sculpture--not always a simple task.  You could try
hot-waxing both the inside and outside of the sculpture in an effort
to seal the investment and pores, but this will just delay the
problem until the wax coating breaks down.  The third option is to
simply remove the accretions periodically when routine maintenance
and cleaning is carried out.  In my experience, the calcium
sulphates are easily removed mechanically, with no damage to the
sculpture.  Eventually, the sculpture will reach equilibrium with
its environment and the precipitation will stop.

With respect to the zinc sulphate crystals, I've not seen those on a
sculpture before, so can offer little advice.  I can only assume
that there is some sort of corrosion process happening, whereby the
zinc is being release from the copper alloy and precipitating as
zinc sulphate.  Perhaps a benzotriazole (BTA) treatment might be
warranted?

George Bailey
Senior Objects Conservator
Collection Services
Australian War Memorial


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                  Conservation DistList Instance 30:5
                  Distributed: Saturday, June 18, 2016
                        Message Id: cdl-30-5-003
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