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Fused Mirrors

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Posted by Steve Campbell on February 19, 2002 05:59:02 UTC

rib mirrors were pioneered by George W. Ritchey
many years ago, he used adhesives to bond a glass front plate to a back plate with a glass rib structure sandwiched between these two plates, the problem is the adhesive(any)has a different coefficient of thermal expansion than the glass, so the structure eventually comes apart, ric rokosz of canada was i think the first to pioneer the thermal fusion bonding method in the early 80's, this involves heating the structure to the softening point and very near the working point to fuse the glass ribs/face/backplate together making a lightweight mirror having the stiffness or nearly so to a solid piece of glass the same size, i plan on experimenting on a small scale to start with maybe 4"-12" diameters, i still need to purchase a kiln, and the programmable ramping controller, a glass fusing and annealing kiln is required as well as the programmable controller, one of the main difficulties to doing this is to anneal the glass properly, a descending ramp cycle of no greater than 13-degrees Celsius per hour from 575C to 375C is required for a precision anneal, this is following the "soak" at
646C for 25-30 min to achieve fusion bonding/slumping over ceramic convex tool, then a rapid crash cool to avoid devitrification(crystal growth on glass surface)then anneal cycle following crash cool.
The all important thing im investigating is a theory that very thin glass can be used provided a complete bonding of all rib to rib joints as well as ribs to faceplate and backplate is achieved, like i mentioned before the steward mirror lab mirrors use very thin glass in proportion to the diameter, this is because the face/back and rib structure are entirely one piece, the rib architecture is another problem, i
dont know if its comparable to a conventional hex/triangle/grid geometry or not, there is some debate on which configueration is best.

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