>I am thinking of making a solar melter.
>
>I understand that the design is nothing particularly difficult, but I
>wonder if
>there is any necessity to use glass on top, or if plastic will work as well --
>assuming it does not melt.

        Several modern plastics, e.g. polycarbonate or modified acrylate
with glass fibres cast in, will certainly take the heat fine  -  I have
used them on solar water-heaters, which run somewhat hotter than a solar
wax-melter, and they last many years developing only slight cloudiness.
But scrap plastic of suitable area is generally harder to come by than
scrap windows.  And if it is non-flat  -  say, corrugated  -  you will have
to fit it to a frame of some sort.  I see no point.
        In many places, it will be easier to get a 'demolition' window,
complete with hinges or at least still having an entire wooden frame onto
which hinges can be screwed.
        Any wooden cross-members are a non-problem; the incoming solar
radiation is hundreds of watts per square m, and even if the box under the
glazing is just a big drawer or other simple wood container, with no
special insulation, the wax will melt within a few hours.  This is not a
case of needing to maximize efficiency; considerable losses can be
comfortably tolerated.

        Of all functions that can be subserved by solar energy, this would
be one of the most clearly ahead of all rivals.  Go solar!

R