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"Iridescence with patinas and metal restoration"


All of us have seen a rainbow divide the white light of the sun into its spectrum of colors/wavelengths. Many of us have seen a triangular glass prism, do the same thing.


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Iridescence is a related phenomena which can on the one hand make birds & animals or heat-tinted artwork beautiful ... or in other cases impart undesired colors like a child's "toy quality" binoculars. A drop of motor oil on a mud puddle is the classic example or iridescence.

attribution: John (Wiki ex-user Guinnog)

When a drop of oil lands on a puddle of water, gravity tries to flatten the oil as thin and level as possible, but the oil molecules fight back, trying to cling together (surface tension). The result is a very thin layer of oil of slightly varying thickness.

When light hits the puddle, some of it bounces off the top of the oil, and some passes through the translucent oil layer and bounces off the top of the water. As the reflected light comes back to your eyes, the portion that traveled through the oil and bounced off the water has traveled slightly further, so the two portions are out of phase and interfere. Depending on the exact thickness of the oil film at that point, the red (or orange, yellow, blue, indigo, or violet) wavelengths may be amplified or attenuated.

Current question:

July 30, 2021

Hello Everyone,

Maybe someone can give advice...

I use clear gloss Lacquer to dip sterling silver to prevent tarnish. I have discovered that under bright florescent lighting there is a rainbow color that shows up when you hold it up to the light. I called the company and they said i thinned my Lacquer to much with thinner, so I mixed a new formula very thick but it doesn't seem to make a difference! Does anyone have advise?


Leo Eis
- Brooklyn NY

August 2021

A. Hi Leo. I can tell you exactly what is causing the problem, but have to leave it to you to solve it for your individual case.

When light hits a clear-coated surface, a portion of the light bounces off the top of the clearcoat and reflects back at you, a portion of the light passes through the clearcoat and bounces off the metal (some would say it actually bounces off the bottom of the clearcoat, but that's not an important distinction for practical purposes) before reflecting back to you. The portion of the light which passes through the clearcoat travels further by a small distance (a partial wavelength), so the two halves are out of sync and interfere, causing amplification of some wavelengths (colors) and attenuation of others. The more consistent the thickness, the less the iridescence. Multiple layers like a clearcoat followed by a light sanding followed by a wax might create a hodgepodge of reflection to limit the iridescence.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

(affil. link)

August 8, 2021

Q. Thank you for responding.

I'm confused, however, why it's only showing up in fluorescent lighting and not in any other lighting. I tried many different mixtures -- thicker, thinner, etc -- nothing seems to help.

Thanks again.

Your help is much appreciated

leo Eis [returning]
metal repair - Brooklyn NY

August 2021

A. Hi again, Leo.

If the sun were a giant fluorescent bulb, this is what a rainbow would look like:

(Courtesy of http://www.bealecorner.org/best/measure/cf-spectrum/)

Fluorescent light is not "white" light like from the sun. It's not the full "rainbow of colors", the full solar 'spectrum'; rather it's just some pieces of the spectrum, some selective wavelengths. Look at the shiny side of a compact disc or DVD under fluorescent light and this is the kind of "rainbow" you'll see.

All I can say for your case is that the iridescent effect is apparently more obvious where you have this limited spectrum. Please try my idea of very lightly sanding your clearcoat, followed by waxing to fill in the 'divots', and see if the hodgepodge of different reflection and refraction angles helps mask the phenomenon.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

August 9, 2021

A. Explanation given by Ted is correct. This happens when film thickness is comparable to wavelength of light. As the thickness increases, light rays undergoing multiple reflections lose their coherence and interference can not take place. Thus increasing the film thickness should solve the problem.

Those who are interested in the physics of this phenomenon may refer to our educational blog

H.R. Prabhakara - Consultant
bangaloreplasmatek.com - Bangalore Karnataka India

August 12, 2021

thumbs up sign Thanks a lot I appreciate your help.

leo Eis [returning]
- Brooklyn NY

Closely related historical postings, oldest first:

(affil. link)


Q. I've been trying to learn how to re-patinate old copper (c.1900) using Liver of Sulphur [affil. link to info/product on Amazon] and ferric nitrate. I've used "Contemporary Patination" [affil. link to book on Amazon] (Young) and "Colouring, Bronzing & Patination of Metals" [affil. link to book on Amazon] (Hughes) as references.

Now and then, things progress exactly as I'd like, but I don't know whether these intermittent successes are more related to the actual alloy or something that I've done (or not). I'm trying to be as scrupulous as I can in preparing the surface using a commercially prepared cleaner that contains phosphoric acid.

My main problem is that the application of liver (either hot or cold) degenerates into a rather opaque coating of iridescent, metallic appearance. Typically, after the initial warm, coppery deposit, a bright gold forms followed by a magenta and then cyan coating.

For those not familiar with the original brown finish used on Roycroft copper, imagine the patina on a 100 yr-old hammered copper lamp...very heavy, almost black deposits in the recesses of the hammer marks with a warm, lighter brown revealed on their high points. I've been trying to build up most of the density I'd like with the liver to avoid having the entire effect become too red through the use of too much ferric.

In short, is this iridescence that I'm encountering typical of the process and, if so, how do I avoid having it happen?


Steve Pollock
- Collingswood, NJ

(affil. link)


Q. I am similarly trying to restore the patina of a Stickley hammered copper bowl. I have ordered the books you mentioned and have had some success with online research. I am presently acquiring and testing all known techniques and chemicals. I have not tried ferric nitrate yet, but it is on the list of tests. I will test everything and then decide whether or not to go with Michael Adams' Aurora Studios repatination services. If you really want to have it done "right," his company is knowledgeable of the subtle differences between early and late finishes of Van Erp, Stickley and Roycroft metalwork. [Ed. note: this posting is from 2001; the internet changed a lot since, and we've learned a lot since; we can no longer post commercial suggestions ( huh? why?).

Regarding your iridescence, I have a similar problem with some of my blackening chemical reactions- such as a "bluing" like gun barrels. I can knock that down a bit if I abrade the surface with ultra fine scotch-brite. I am also going to try Rottenstone [affil. link to info/product at Rockler] and olive or Linseed Oil [affil. link to info/product at Rockler] on a felt covered block. Though I have not tried it, one other idea is that perhaps the final step of wax polish (turpentine and bee's wax) will reduce such colorations.

Good luck and keep posting on your progress or with any tips you have for my restoration.

J. McGowan
- Huntington, New York

thumbs up sign Hi Steve, J., and readers ...

We have added a Tutorial for Newbies to his page which may not solve your problem but will explain it so that you can better anticipate what changes might help.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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