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Subject: Metal spraying

Metal spraying

From: Ebenezer Nsiah Kotei <kotei>
Date: Friday, February 13, 1998
I'll like to thank Tom Cremers for bringing up the info on spraying
metal artifacts with molten metal, and I will like to thank David
Harvey for replying so eloquently to it. I was going to write just
what David wrote but since he beat me to it I will go ahead and make
just a short comment.

The method is not new at all, and it was not developed by the guys
at the Los Alamo weapons site. It is called Thermal Spraying. It has
been around for over a hundred years and has changed dramatically
over the years. The type that Tom saw in the New Scientist is called
Arc or wire spraying. Other types of it are plasma spraying, high
velocity oxy-fuel coating (HVOF), detonation gun (D-Gun) spray,
flame spray, etc. I have once answered a similar question from a
colleague belonging the Thermal Spray Society which is a
sub-division of ASM (American Society for Metals).

It is used extensively on engineering materials to enhance corrosion
protection, especially in high temperature operations and high
gaseous environments. It is used to deposit metal on surfaces for
wear and other tribological operations. Its primary applications now
is in gas turbine engine, aeronautic and space programs, and
automotive companies are now adopting it for some of the benefits I
have listed above. If you have a Lexus, the roof of your car is
joined in part by a thermal spray process which I cannot explain
here right now. If you have a medical hip replacement the odds are
that the product was sprayed with arc thermal process. Stainless
steel coatings sprayed with this process are porous and provide good
surface porosity for body tissues to grow into.

It is a good process for engineering applications but definitely not
for conservation, the reasons already mentioned by David Harvey. Due
to the porosity of these coatings they almost always have to be
further sealed before they can be used, even in industry.

Is is reversible? Yes, by blasting it off with hard abrasives. Maybe
one day we can find some use for a modified form it but not yet.

Ebenezer Kotei
Objects Conservator
Hagley Museum & Library
P.O.Box 3630
Wilmington, DE. 19807

                  Conservation DistList Instance 11:71
                 Distributed: Friday, February 20, 1998
                       Message Id: cdl-11-71-001
Received on Friday, 13 February, 1998

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