Blaine said:

> My casual observations is that the wax melts from the top of
> the tray first - always.  Which indicates to me that the melter
> works by heating the air to melt the wax.  You want the interior
> of the box dark to absorb the solar energy and heat the air.

There's another way to look at what you are seeing.
The wax is melting on top because it is exposed to the direct
radiation of the sun.  The bottom and sides of the wax are not
melting as fast because you are NOT reflecting the light onto
the sides and bottom of the wax.  If the air temp in the box was
doing the majority of the melting, the sides and bottom would melt
just as fast as the top, since air quickly reaches an "equal"
temperature throughout the fairly small volume of the box.

You could test this with thermocouples, or even some cheap
thermometers.

and Tim said:

> I paint it black to absorb light, not to reflect light. The
> absorbed light is radiated as heat.  A light-colored surface
> might reflect some of the light back out of the box. Some of
> the heating effect would be lost.

These are accurate statements if (and only if) the walls of the
box have a significant thermal mass (stove brick, thick metal, etc)
and/or were insulated on the outside.  If the walls of the box have
no significant thermal mass, and/or no insulation, it is better to
reflect than to absorb light and heat.

If the wax were in direct contact with the walls of the box, then
even a tiny amount of thermal mass (like sheet metal) would be hotter
if it were black and "absorbing" heat.  Most melters have the
wax up on a screen or mesh, hence my advice to reflect the light,
and heat the thermal mass in the box (most often, this is the
wax itself.

> When we walk across an asphalt parking lot in summer, we feel the heat
> radiated back from the pavement. Sunlight hits the asphalt; heat is
> radiated from it.

A layer of asphalt has a significant thermal mass.  It also has "insulation"
on the side not facing the sun in the form of an "infinite" mass of gravel
and dirt with lots of "dead air pockets" between the pebbles.

The exterior walls of a wax melter (wood or metal), if uninsulated,
will radiate to the outside just about everything absorbed on the inside,
since the outside temperature is lower than the inside of the box, and
heat seeks "equilibrium" across any temperature gradient.

Just to review, here is my personal explanation of the oft-mentioned
"First 3 Laws Of Thermodynamics:

  1. You can't win.
  2. You can't break even.
  3. You can't even quit the game.

The trick is to minimize the unavoidable losses.


                        jim

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