Black Paint Doesn't Improve Cooling

I suspect it has to do with the thermal properties of the paint or with the heat transfer between the metal and the paint. Black objects should radiate faster than white objects.

When you collect the energy of the sun at all wavelengths by not reflecting the visible band of energy there is a difference. However when you radiate heat from an object that's temperature is relatively low the color really is of no importance. The emissive qualities of an object is not related to its visible color in the way that (sometimes faulty) intuition would lead us to believe. Also, at near infrared frequencies the reflectivity of an object is more a factor of the surface finish and has little to do with color. Color is really only significant to colored light.

The color effects radiative cooling.
You are unlikely to observe a measurable difference at such low temperatures.
If you can safely do so, try 400F. to 100F.
You might see the difference if you measure carefully.
Caution, anything above 140F. or so can cause injury quite quickly.

The results of your experiment are interesting. I vividly remember carrying out an experiment with two identical copper containers full of hot water. One of the containers was shiny, the other painted black and the black container cooled considerably faster that the shiny one. I suggest trying several different colours over a greater temperature range because the experimental evidence I have suggest that black radiates heat better.

About 30 years ago, I did a fair amount of research on solar energy and eventually found that NASA, in conjunction with Sandia, had done a bunch of research on coatings, including paint, to reduce radiative cooling.

I don't remember which ones had the highest radiative losses, but I do recall that a particular anodizing (meaning color, chemical, and method -- I don't remember the details), only several microns deep, produced the then lowest heat radiation.

The drawback was that it cost an incredible amount to produce solar collectors made that way, which gives "free energy" a meaning I don't quite understand.

I guess I could have done a simple cost analysis, but without that benefit (or enough money to be just a little more efficient) I took a different tack by using cheap materials like galvanized roofing, copper pipes, 2 X 4 wooden frames, and good old flat black paint.

Anyway, now that we have the internet, maybe you can find something at Sandia National Labs, http://www.sandia.gov/ or NASA, http://www.nasa.gov/home/index.html

Is the paint really black @ 10uM wave length. It may look black at 500nM (visual), but not at the thermal wave length. It should have a significantly reduced emissivity at the thermal wave lengths compared to the bare metal.

I did a little research on thermal solar collectors as I have been thinking about building one. The folks who are building them use a special paint. It seems like it was some kind of chromium oxide or something. Anyway, they said plain black paint would cause an insulating effect. Additionally, there seemed to be a consensus amongst the solar people that anodizing was best as it was a dye rather than paint. You might look up thermal solar and see what you can find.

Have you tried a hardware store type Stove Black? we used it when I was a kid about 60 yrs ago.

Hello Everyone,

Flat black both absorbs and radiates optimally, although there may be differentials in visible vs. infra red as mentioned at different levels going in or out. But what everyone forgot here is that much of the cooling at lower temperatures is through CONVECTION to the atmosphere. Paint, in any thickness, will act as an insulator and reduce convective efficiency while improving radiated efficiency. I found this conversation while attempting to find some info on this trade-off point.