How to get Aurora Borealis effect on crystal beads?

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Ed. note: If the application is high volume, and the readers have PVD machines available to apply this coating, please see Jim Treglio's response below.

Carnival_glass

A. Hi, Moiz. Whether they are called rainbow coatings, carnival glass coatings, iridescence, or aurora borealis effect, such coatings rely on the same effect: Part of the light striking them reflects off the surface of the coating, and part penetrates the transparent coating and reflects off the glass. If the coating is very thin, the path the two portions of light travel differs by a very small amount, a partial wavelength. The two portions of the reflected light then interfere, amplifying some wavelengths while attenuating others.

If the coating is of non uniform thickness, rather than one particular wavelength (color) being amplified and one attenuated, different colors are amplified here and attenuated there, causing iridescence. A drop of oil on a mud puddle is a well known example of the effect.

This was traditionally done on glass with salts that were fired into the surface, and Wikipedia has a good article on it. Such salts fired into the glass made for a robust, permanent, iridescence that lasted decades.

In the case of beads, there may be not only this iridescence but an iridescence from the cut of the beads as we see in the colorful glint of cut diamonds. I would expect that any very thin transparent coating will produce this same effect, but I understand that in vapour coating applications titanium is usually used. Good luck.

Regards,

A. I used a chromium target with CO2 as the gas. You can get a thicker, more durable finish this way. You'll need to pulse the bias as the coating is insulating. Can be done with some nitrogen in the gas mix as well. Had some success with zirconium target as well.