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OK. Here's what I know about it : Tropical lap has been formulated some 30 years ago by Dr. Tenukest at the Optical Department of the University of NSW (thanks, Mark!). As its name implies, it was intended to be used in extreme conditions (wild temeperature variations, humidity etc.).
The lap is prepared in the following way : take rosin, woodflower (very fine softwood sawdust), and castor oil in quantities approx 5:2:1 (by volume). First melt rosin, slowly heating it, then add castor oil, and mix well. Add woodflour slowly, constantly mixing until mixture evenly saturated. Grab a small bead, cool it and test on hardness. If too hard, add castor oil, if too soft, add more rosin. When happy, allow it to cool a little, and pour it on the tool. While hot (in liquid state), this thing flows significantly better than pitch, so be sure to make a dam around the edge of the tool. From there, work in the same way as with pitch. Another advantage of this compound is that will take relatively small amount of heat to soften - submerging the tool in hot water for only few minutes will soften it enough to press channels etc. Cutting this stuff is a bit easier than with pitch. I now use rubber mat, and it takes me but 20 minutes to get excellent tool, from start to finish.
Advantages : excellent thermal stability (recommended for machine work); it should be far more resistant to chemical attacks than pitch
Disadvantages : in use this stuff is much slower flowing than pitch, so keep your f/ratios above, say f/5. Being hard, it produces surfaces somewhat inferior to softer pitch (tad more ripples, not as smooth)
All this said, I figured my 7" f/3.2 oblate spheroid (SC=1.4) primary for Wright camera with this stuff, without any problems. Admittedly, I didn't care much about surface smoothnes, this is only photographic instrument after all. But I'd expect that 6 or 8 inch say f/6 could be easily figured to quite high standards without any problems.
Bratislav
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