Question : How to get Rainbow type finish on polished metal - etch

If the object is steel, that finish can be applied by using a hot bluing tank with the normal bluing salt, but at a slightly lower temperature. Experiment for effect. Steve

Yes. The precise thickness (as a multiple of the wavelength of light) of the coating determines which colors are interfered with.
I know a lot of yellow dichromated steel shows this pattern... It's also why steel gains a range of different colors as it's heated (as to temper hardened metal).
Gee, don't know if that's possible. Maybe laquer, carefully applied? Doesn't really have much index or refraction though...
Would anodizing do it to some extent? If it's really thick or thin? But then, a polished anodized surface is notoriously impossible to attain as I recall.
What you're looking for is a transparent or semitransparent coating of reasonable refractivity in a controlled, even thickness. Think along those lines.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

You can get one colour with a thin coating (as others have pointed out) but to get multiple colours you must either vary the thickness of the coating - oil on water does this because is spreads outwards from the initial drop thickness; or go with what is called a diffraction-grating. This grating is produced by engraving a pattern of very finely-spaced lines. I believe that the varying colours produced are a function of the angle of the incident light; and so patterns of lines other than straight have more of this "rainbow" effect.
You can see the difraction-grating effect on the underside of a CD.
Achieving a varied-thickness surface coating might be difficult.
Engraving a diffraction-grating may well also be difficult.
Foil with diffraction-grating patterns engraved is readily available, and cheap; but you would have to find a pattern that you like, affix it to your substrate, and depending on your application determine some method of protecting the foil.

Old type lacquer can be applied thin enough to get that effect. Forget about the spray on lacquer as that goes on way too thick to work. Thin the paint and try it and you will eventually get to the point where it will work just fine.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!

We're talking three different possibilities here.....all a little different and I don't know which you are actually looking for.
1) Interference. This happens with the oil slick or soap bubbles. The light entering bounces off the top surface of the soap layer and the bottom also (same point of light, say 30% bounces off the front and 30% of the second layer while the last 40% passes) The light that bounces has traveled 2 different distances by a few light wavelengths or les sdepending on the bubble thickness so when they re-combine they add or subtract to form differing colors. With soap bubbles the material is still runny and flows so will get thinner and thinner at the higher spots. If you do an experiment with this you will see the soap get so thin that it no longer causes enough difference in the light paths for interference to happen...essentially, a "clear" spot forms in the bubble with no color reflection.
2) Diffraction grating. This is similar to above but caused by VERY fine scratches in the surface. The scratches reflect the light at angles that cause the light waves to interact and add/subtract to their wavelength to appear colored. Generally with a diffraction grating you get a "play" of color as you move your viewpoint. It's a little like looking at thousands of tiny prisims. This happens on the surface of a CD. Note that you need about 30,000 scratches per inch to get this to happen.
3) Surface color caused by heating, applied color or similar. This is probably what you are actually thinking about. I believe titanium colors very well with blue's, purples and oranges when heated and is now used as jewelry.
You might get #1 to work by coating a polished surface with something clear that reflects from the front and back surface of the coating. I'm not sure what that might be but someone mentioned old forms of laquer. Possibly it can be thinned to the point that the applied coating is thin enough to cause light interferrence.
#2 is probably not an option as it's tough to put in 30,000 scratches per inch and this means that the surface of the metal before scratching would need to be smoother than the 30,000 (in general) to get a good effect.
# 3...is there such a thing as depositing a thin layer of titanium or similar metal to the surface of the steel and then applying light localized heat to color the coating? Can titanium be lightly plated or flame sprayed on a surface? Maybe something to look into.
Koz
dgawf wrote:

Too reactive for flame spray, however, I do seem to remember seeing some irridescent bits in a rock shop last month.. guy said they took a trip through a vacuum chamber and came out titanium plated. I guess they were still warm or something because they were oxidized enough to do that.
Maybe Lerch can help you out with that ;)
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

David,
Take a look at the POR-15 Products. Here is a quote from their web page:
COLORCHROME® is an exciting new transparent color-coating system for polished metals. Beautiful, transparent colors can be applied permanently; intricate patterns may be achieved by layering one or several colors over others. For best results a spray gun is required.
COLORCHROME® works by changing the color of the light traveling through it while remaining transparent enough to maintain the reflective characteristic of the underlying surface. While a highly reflective surface is important, even a moderately reflective surface will yield excellent results. You can easily blend one color into another (gold transitioning into blue or green or red) you can even mix colors together to achieve even more hues or shades.
You can view the product page here:
I'm not involved with that company in any way, except aa a customer, that is.
-=- Steven Harris Everson, WA