Making fire from ice .. someone sent this link about fashioning a lens from ice...
The technique shown will work, but a couple of us devised better ways as teenagers in tropical Montana. You can make a casting out of packed snow or (better yet) sand using old car headlights to shape the mold. If you go for sand make sure it is slightly damp before freezing. As noted in the article, aluminum foil makes a dandy mold release. Plastic wrap would probably work well.
If it is really cold outside use chilled water just above freezing so you don't go through the expansion phase non-uniformly. If you use warmer water (above 5 degrees C), stir it as it chills. It also helps to start with clean distilled water taking care to avoid bubbles.
Many themes and variations are possible. A nice trick is to mount the lens. Burner rings from electric stoves work well or you can cut one with a jigsaw.
The most interesting ice optics I've built was a parabolic mirror. Using a old record player I spun a shallow metal cylinder that was about 20cm in diameter (you can easily derive the relation f = g/(2*omega^2) where f is focal length - at 33.3rpm you get f/D ~ 2 .. a fast mirror, but that's fine). With water chilled just above freezing I was able to make a nice parabola in the water and let it freeze. The resulting mirror had a very nice surface (the aluminum foil was tried on the backside to release the ice from the pan).
I painted a frozen mirror with silver spray paint, but the quality of the cold paint surface was awful (at the time I was also thinking about spinning and chilling a mercury mirror - fortunately I was never able to locate enough of the metal. My excuse is that I was probably 14 years old).
I managed to use a uncoated f/8 ice mirror (using the 16rpm setting on the turntable) as a solar and lunar telescope. It probably only reflected a few percent of the light and formed a nice image of the Sun used as a Newtonian focus with a small eyepiece. I also tried one on the moon, and could make out an image, but the light gathering power was marginal. It is amazing that, with care, you could make a reasonable parabola.
Something I never got around to was making an ice fresnel lens. With very fine sand it should be possible. Alternatively you could use a CNC tool to work the ice.
If you don't like the cold weather, you can make a serviceable mirror with motor oil. Keeping vibrations out is difficult and a reasonably fun challenge. It is possible to make very large transit telescopes that only look up using this techique.
6:45:02 AM 
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