The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 11-28-2011, 03:33 AM
tellyworth tellyworth is offline
Member
 
Join Date: Dec 2009
Posts: 2,059
Rainbow effect on a stainless steel pot

I use a stainless steel saucepan for cooking pasta. Sometimes after washing it, there's a pale rainbow sheen on the inside bottom surface. Questions:

What am I doing to make this happen? Washing it while it's still hot? Boiling salted water?

What causes the rainbow effect? Diffraction, presumably, but how and why?

Google turned up a bunch of contradictory theories.
Reply With Quote
Advertisements  
  #2  
Old 11-28-2011, 06:07 AM
AaronX AaronX is offline
Guest
 
Join Date: Feb 2011
Is it like an oil slick? The rainbow effect is similar to bubbles - caused by a layer about the same thickness as the wavelength of light (0.4-0.7 micrometers). Light is reflected from the front of the layer, and some passes through the layer, reflects off the back, passes through the layer again and combines with the light from the front. The interference cancels out some colors, leaving others.
Reply With Quote
  #3  
Old 11-28-2011, 06:58 AM
Mangetout Mangetout is offline
Charter Member
 
Join Date: May 2001
Location: England
Posts: 51,914
Quote:
Originally Posted by AaronX View Post
Is it like an oil slick? The rainbow effect is similar to bubbles - caused by a layer about the same thickness as the wavelength of light (0.4-0.7 micrometers). Light is reflected from the front of the layer, and some passes through the layer, reflects off the back, passes through the layer again and combines with the light from the front. The interference cancels out some colors, leaving others.
It's going to be something like this. The layer itself might actually be some sort of spontaneously-formed anodization
Reply With Quote
  #4  
Old 11-28-2011, 07:21 AM
Sudden Kestrel Sudden Kestrel is offline
Guest
 
Join Date: Jul 2011
I've always heard it was from overheating the pan. Most of the links that came up for me on Google were similar to this, from Stellar Cookware:
Quote:
Why do I have a bluish ‘rainbow’ marking on my stainless steel?
This is a clear sign that the pan has been over heated. Stainless steel reacts to high heat, by turning a bluey-purple colour (Similar to the colours of a rainbow).
I know that when I use my pans they're fine, but when my husband uses them the rainbows appear.
Reply With Quote
  #5  
Old 11-28-2011, 07:27 AM
Mangetout Mangetout is offline
Charter Member
 
Join Date: May 2001
Location: England
Posts: 51,914
A similar thing happens on other metal surfaces when they're heated (for example, chisels if they're allowed to overheat through friction with a rotary grindstone. The coloration is likely to be a thin, transparent oxide layer, that happens to be of a suitable thickness for optical interference to occur.
Reply With Quote
  #6  
Old 11-28-2011, 07:47 AM
JenJ JenJ is offline
Guest
 
Join Date: Nov 2011
I own a set of stainless steel cookware that always does that after I cook with it. The salesperson who sold me the cookware said that's normal for stainless steel, and it's not toxic or anything.

A popular product to remove the "rainbow" is Barkeeper's Friend.
Reply With Quote
  #7  
Old 11-28-2011, 07:49 AM
JenJ JenJ is offline
Guest
 
Join Date: Nov 2011
Quote:
Originally Posted by Sudden Kestrel View Post
I've always heard it was from overheating the pan. Most of the links that came up for me on Google were similar to this, from Stellar Cookware:


I know that when I use my pans they're fine, but when my husband uses them the rainbows appear.
Interesting! I guess that's why I was told never to use my pans on anything higher than medium heat. Although I still get those rainbows...
Reply With Quote
  #8  
Old 11-28-2011, 10:02 AM
AaronX AaronX is offline
Guest
 
Join Date: Feb 2011
Quote:
Originally Posted by Mangetout View Post
A similar thing happens on other metal surfaces when they're heated (for example, chisels if they're allowed to overheat through friction with a rotary grindstone. The coloration is likely to be a thin, transparent oxide layer, that happens to be of a suitable thickness for optical interference to occur.
I think they're also caused by high electric currents (which produce lots of heat), but can't find any pictures right now.
Reply With Quote
  #9  
Old 11-28-2011, 10:12 AM
CalMeacham CalMeacham is offline
Guest
 
Join Date: May 2000
Is the rainbow a temporary effect? And Do the colors change with angle?If so, it's probably a thin layer of water or soap causing thin-film interference, like the above-mentioned oil slicks.

If they don't go away and persist when the pan in dry, but change with angle, then it might be the formation of a thin oxide layer, as noted above.

Rainbows that remain after the pan is dry and don't alter color with angle are probably some chemical change, probably non-interference oxide layers.
Reply With Quote
  #10  
Old 11-28-2011, 03:37 PM
tellyworth tellyworth is offline
Member
 
Join Date: Dec 2009
Posts: 2,059
Quote:
Originally Posted by CalMeacham View Post
Rainbows that remain after the pan is dry and don't alter color with angle are probably some chemical change, probably non-interference oxide layers.
It's there when the pot is dry (in fact it's invisible when the surface is still even slightly wet). Scrubbing with steel wool takes it off. A non-scratch scourer doesn't. An oxide layer sounds plausible.

I wouldn't say the colours change, but it does get less visible at an acute angle of view. Hard to say for sure, there are reflections from the inside walls that confuse things.

I've heard it can be caused by overheating too, or by heating it while empty. I'm careful not to do that. But I do sometimes wash this particular pot while it's still hot.

I'm not concerned it's harmful, just curious about what's going on.
Reply With Quote
  #11  
Old 11-28-2011, 03:43 PM
tellyworth tellyworth is offline
Member
 
Join Date: Dec 2009
Posts: 2,059
Quote:
Originally Posted by Mangetout View Post
A similar thing happens on other metal surfaces when they're heated (for example, chisels if they're allowed to overheat through friction with a rotary grindstone. The coloration is likely to be a thin, transparent oxide layer, that happens to be of a suitable thickness for optical interference to occur.
Yup, now that you mention it I'd say that's the same effect.

I'm pretty sure I haven't overheated this pot; would cooling it quickly do that? Washing it in warm water while it's still hot.
Reply With Quote
  #12  
Old 11-28-2011, 04:05 PM
barbitu8 barbitu8 is offline
Guest
 
Join Date: Aug 2000
Quote:
Originally Posted by JenJ View Post
I own a set of stainless steel cookware that always does that after I cook with it. The salesperson who sold me the cookware said that's normal for stainless steel, and it's not toxic or anything.

A popular product to remove the "rainbow" is Barkeeper's Friend.
I have no financial interest in Barkeeper's Friend or Bon Ami, but I've found that Bon Ami works better.
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:40 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright © 2013 Sun-Times Media, LLC.