Re: What color or other surface property results in best thermal emissivity?

Black is usually considered the best surface color because it tends to give 
up most of its heat through radiation (remember the "black-body" radiation).  
The "black" color of "Black body radiation" refers to the fact that this 
theoretical material "gives up" all wavelengths of radiation equally well.  
Of course, the amount of radiation that is emitted depends on the 
temperature (see a black-body radiation curve).  

Your are absolutely right that the more surface area, the better of an 
emitter the object is.  Emission is directly proportional to surface area.  
Now you must also consider the practical local effects of the surrounding 
medium (e.g., air) and its ability to handle and "pull away" the heat 
generated by the object.  

As far as types of metals go, I'm not sure which materials are best.  I know 
that a lot of heat sinks are made of aluminum or a titanium alloy.  This may 
be due to a combination of the weight and heat-sinking ability.  The SR-71, 
for example, uses a titanium alloy skin to dissipate the extremely high 
temperatures generated by the hypersonic speeds of the aircraft, and 
titanium is often used within jet engine components for the same reason.  

Special coatings are often used on heat sinks that bond closely to the 
metals.  I don't think that these are paints, but rather an anodized 
coating.  You might look to heat sink manufacturers for some more info.  

Good luck, 

Todd Jamison
Observera, Inc.
Chief Scientist