The color of the material gives you a very good
sense of how good it is at absorbing heat.
Specifically, if the material is black, it is
not reflecting any (visible) light and all of that
light is being absorbed. You could do this
just by coating the surface with black paint. (You
can do even better if you find material that also
absorbs non-visible wavelengths of sunlight,
mostly ultraviolet and infrared light.) But keep
in mind that the best you can do is absorb all the
sunlight that hits the surface--that means having
more surface to work with can be more important
than a 5% increase to your absorption spectrum.
This is why reflective surfaces that can send more
light to an absorbing surface are usually so
important to solar cookers.
The thermal paste situation is interesting.
Without using commercial-quality materials,
toothpaste and Vaseline seems like a fine
substitute. But I'm not clear on why you need it
for a solar cooker. The purpose of thermal
paste is to improve heat conduction from one
surface to another (like from the
light-absorbing surface to something you want to
heat). I would guess that you could accomplish
this just by having the light-absorbing surface as
a wall of the solar cooker.
That being said, I'm not an expert on solar
cookers at all--I would not be surprised to find
out that you already know all of this. Just in
case, I want to make sure you leverage as many
resources as you can: read about building solar
cookers using a search engine. For example, I
found some of the basics on
this site . They
mention a strategy called glazing that might help
you: by putting your entire solar cooker in a
transparent box or bag, you can trap some
additional heat. This works similarly to a
greenhouse, where heat has an easier time entering
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