That's a good question. My understanding is that this is a
vapor deposition. I wish that someone would complete the
explanation.
It IS vapor deposition. The first part of the explanation is
the note that this is industrial level processing. Don’t expect
to be able to do this yourself, unless you’ve got some really
substantial engineering background to make the equipment. But
there are firms that will do such coating for you.
As to how it’s done, first imagine the old vacuum tubes used in
pre-transistor radios. Remember how they were always blackend
around the electrodes after being used for a while? What happens
is that under a vacuum, the heater filiment, which is also under
a high voltage in order to send an electron stream towards the
various anodes/cathodes of the tube itself, is also sending a
stream of it’s own atoms, propelled by that voltage. The stop
when they hit the glass, and form that black deposit. In a
vacuum tube, this eventually results in the tube burning out. In
vapor deposition, it’s harnessed. Here, in a high vacuum
chamber, a metal electrode (whatever you wish to deposit) is
heated high enough so that under a high voltage, some metal
evaporates, and the ions are projected towards the other
electrode by the voltage. In a way, its sort of like
electroplating, except there is no bath dissolving metal ions,
only heat evaporating them, and a voltage propelling them. When
they hit whatever is in the way of them, they deposit. It’s a
line of sight process, the atoms don’t turn corners, so complex
shapes may need several electrodes spaced around the chamber in
order for all surfaces to be covered. Multiple electrodes can
also be of differing metals simultaneously, with differing
voltages applied to each, in order to coat an object with a
complex coating, like an alloy, except that the process isn’t
limited to just metals. Anything that can be vaporized and
ionized can be coated this way. Those gold colored burrs coated
with titanium nitride? I’m pretty sure those are vapor
deposition coated. Buy a plastic hobby shop model of a car and
some of the plastic is chrome plated for the bumpers? Those too
are often vapor deposits on the plastic. The coatings can be
VERY precisely controlled too, for thickness. In electron
microscope use, non-conductive objects being viewed (like organic
things) must be vapor deposit coated with gold, in order for the
sample to be viewed. Since you can then still see all parts of
the object even under very high magnification, you can imagine
just how thin and precise those gold coatings are in order to
not obscure the details of the item being imaged… In optics,
vapor deposition is often the method used to make things like
multilayer interference filters, and sometimes, lens coatings.
All in all, a rather useful technology. And while, as I said,
it’s out of the reach of most artists, not all. I have seen
some folks using this technology to coat objects, such as things
being then imbedded in acrylic castings, for example. While I
suspect that those pieces were coated by a commercial firm for
the artist, you could, if you’ve access to a high vacuum pump and
suitable experience in high voltage power supplies, build such an
apparatus. just be aware that this is NOT the sort of vacuum
pump we use for investing. I really do mean HIGH vacuum.
Expensive pumps. And the voltages involved aren’t exactly easy
and safe to play around with either. Not quite a radio shack
spare parts piece of equipment to cobble together.
Hope this helps.
Peter Rowe