You might also look into cheap plastic or glass mirrors - a bit smaller across than the crucible - to tile the dish.  That will give you a better focus.

Mirror tiles may be available at your hardware store.

Or get some inexpensive glass hall or door mirrors totaling about the area of the dish and a glass cutter.  B-)

The point of this post is that I was left with the cylinder ends.  The bottom end was concaved.  I smoothed the surface with some fine emery paper and then polished it to a mirror finish with metal polish.  The result was an 18 inch (or so) mirror that when put in the sunlight could burn holes in piece of wood.  I never tried to melt metal with it, but I can see no reason why it shouldn't.  Not upto smelting standards I know, but great fun all the same.

What you really need is one of these..

http://en.wikipedia.org/wiki/Image:The_City_of_Nottingham-_Summer_04_088.jpg

Don't remember if they burn down the boat but there are some small scale test with great result's (burning wood).

As for melting metals, also collect all the wood as u search for those mirrors, burning that is a good plan B :).

The melting temp for aluminuim is 660C (1220 F), now a solar reflective parabola may well provide this temperature or even greater. But to really melt ali,steel etc it needs to be soaked above that temp for all the solids to dissolve. Personally unless your living in the sahara desert in mid summer, I don't think you'd get enough energy to melt anything worthwhile before a cloud came over and the lot cooled quicker and you ended up with a solid mass of junk. When I was doing my engineers diploma we did some ali casting and I remember we thru some pill into the mix and i do remember the teacher saying without this little pill forget any chance of a casting. It's been that long I can't remember what the pill consisted of but some research will surely find it.

All in all I'd luv to be proved wrong but using solar to melt ali,stell etc wont happen unless the sun visits earth I reckon.

First, view the 1400mm diameter Solar Cooker at www.sun-and-ice.de

I calculated the BTU's per hour on a typical sunny day anywhere south of a Salt Lake City latitude. in North America; spring, summer or fall seasons especially. I got about 14,000 BTU's per hour with my 4.5 ft. diameter reflector/concentrator dish.  

The focal area on that solar cooker was about as big around as a dinner plate. Maximum temperatures are in the 425-500 F range. The focus on the www.sun-and-ice.de solar food cooker is designed WEAK/DIFFUSE so that even if you leave it unattended & forget that there's liquid in the cooking pan, it won't just all boil off and burn a hole clean THROUGH the pan. This is a good safety feature for cooking, yes.

HOWEVER, if you really WANT aluminium melted down inside a crucible, the focus must be sharper .... say about the size of a teacup saucer.

All I can say at this point is that a PARABOLA design is optimal.
The larger the diameter of this parabola, the more raw BTU thermal power you'll have .... on the presumption that your Solar Micro Foundry Dish is built closer to machine shop specifications and we distance ourselves much further away from the poor quality of a garage-based weekend hobbyist.  

If you're interested in getting a business going where all manner of ordinary metals is melted-down with VERY high temperature parabola dish focused sunlight, contact me at 480.528.0632 in Phoenix, Arizona.