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RE: Kiln Forming Lenses

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Posted by Robert May on May 29, 2000 21:16:06 UTC

Using a kiln to do slumping is a nice thing as it can "save" the cost of grinding glass off of a blank. The thing that you run into is that most amateur glass for mirrors really doesn`t need to be thick enough to require the weight savings that you can get by using a kiln. Once you get over about 25" or so, making up a front and rear plates with spacer walls in between will start to help and by the time you get to the 35" size, that sort of technology starts becoming quite usable. What you need to do is figure out how to fuse pyrex in the kiln without it fully slumping into a mess. Others have tried it with plate glass but that needs a lot of good annealing and then a sudden thermal shock will still destroy the mirror real fast. Sit down and figure the stresses in a hollow block of glass with rods from each corner to the center and you will see what I mean.
Now as to "Optical Glass" for lenses. There are a lot of glasses (thousands) out there for refracting purposes and they all have very accurately controlled formulas of what is in them. You can slump the glass without affecting the quality of the glass but you need to anneal them a lot better than pyrex is done because the residual stress in the glass will cause problems with the light going through a lot more than it will with even the plate glass for a mirror. Most opticians will prefer to cut the glass to the size that they need just so they don`t have to go through that process of getting the glass annealed properly again. I might also note that optical glass has different specs from each melt of the glass at the factory and can be specified even closer than that if the customer desires it. Basically, if you slump optical glass, you can add or subtract enough of some of the chemicals to the glass to change it quite a bit if you aren`t doing it exactly right and that will mean that the glass will be something else which the optician didn`t figure on when designing the optics.
As far as using pyrex for a mirror substrate is concerned, it is there for the stability under temp changes. The stuff does need to be fully annealed so that it doesn`t want to change shape under temp changes any more than it does now. Remember that when the mirror is 1/10th of a wave error, that is putting the light from the mirror no more than 5 millionths of an inch off from where is is supposed to be and that is a real small number. A 10C. temp change can easily take pyrex out of that spec even though it has a rather small amount of expansion over that temp.

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