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Telescope mirrors

Posted: Wed Sep 21, 2016 2:16 pm
by brucesdad13
Hello all! I am relatively new to kilns, glass, and etc and it was suggested I join your forum. My primary interest is in casting/slumping/fusing glass to produce suitable strain-free blanks that can be cold worked into telescope optics. I bought a kiln last year from a retiring pate de vere artist in Vermont. I pulled 6-3 AWG wire to my garage and built my own ramp/soak controller. My first experiments haven't turned out so well. I'm having trouble hitting high enough temps for boro glass. Hoping to write out my trials and tribulations in under this thread and get your advise and encouragement ;)

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Bubbles and lines oh my! Might work out better as an artistic representation of Jupiter than as a telescope mirror. I'll see if I can rescue it...

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Re: Telescope mirrors

Posted: Wed Sep 21, 2016 2:19 pm
by brucesdad13
Forgot to show the other side...

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I don't remember my firing schedule offhand but it was basically heat slowly over many hours and try to hit working temp for Schott boro and hold for an hour or two and then back off to annealing temp for several hours and then allow to cool gently... I think it all was done in around 24 hours...

Re: Telescope mirrors

Posted: Thu Sep 22, 2016 5:49 am
by Tony Smith
While borosilicate glass softens at around 1500F, its melting point is around 2200F, and that's what you'll need to cast a billet. The elements in a commercial, electric kiln won't get you to that temperature, and standard bricks will break down above 2300F. If your kiln is made of high temperature refractory, you may be able to change to high temperature elements to get where you want to be.

Tony

Re: Telescope mirrors

Posted: Thu Sep 22, 2016 11:14 am
by Kevin Midgley
this will get you into the general area and you will need to join to post.
http://talk.craftweb.com/showthread.php?t=61

Re: Telescope mirrors

Posted: Thu Sep 22, 2016 2:38 pm
by Tom Fuhrman
casting in boro may not be possible in an electric kiln powered with wire elements. The elements will only go to about 2175 before having serious failures. The boro usually has to be melted to about 2400 min. and some boros need to go hotter. That's why there are few people that do boro furnace work. Most commercial kilns are constructed with firebrick that is rated at 2300 and the same is true for most fiber, although both can be purchased in 2600 degree versions. trying to kiln cast boro may require that you hold it at very high temperature for an extended period of time to get rid of all the strains. If you try to melt the boro in a crucible, it is difficult find crucibles that will handle the high temps required. Why does it have to be boro? How about Schott optical crystal?

Re: Telescope mirrors

Posted: Thu Sep 22, 2016 7:08 pm
by Tony Smith
Boro is used because of its low coefficient of thermal expansion. In this case, the shape is critical over temperature.

Tony

Re: Telescope mirrors

Posted: Fri Sep 23, 2016 9:39 am
by linn keller
casting glass for telescope mirrors have been discussed on this forum several times. don't know if using boro was discussed but suggest you do search (upper right of page) for "telescope". might help save you some time and effort.
linn

Re: Telescope mirrors

Posted: Thu Dec 08, 2016 10:14 pm
by Joe Pfeifer
Spectrum Studios in Salt Lake City has some experience in casting pretty large borosilicate mirror blanks.