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Registered Member #82
Joined: Thu Feb 09 2006, 12:03PM
Location:
Posts: 3
Hi guys, i wanted to make a flat, round glass plate around 33cm(13" for our friends of imperial units ) diameter and about 30mm high for grinding a telescope mirror. Getting the glass into the right shape is not my problem. Because there is quite a lot of glass involved i thought about what would be the most efficient way to get about 5kg of glass to 1200°C. My first guess would be to just use an charcoal furnace just as many do for casting metals. My concern would be how many kilograms of charcoal i have to burn and if i can even reach this temperature. My other guess would be an arc furnace. I have an 140 Amp stickwelder to use, but then again i have only a very small arc and am unsure in my design where the arc has to be set up. And of course the amount of electric i have to use(read: pay). Using electricity to heat something is the opposite of cheap. Any Ideas anyone?
Registered Member #4230
Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53
You need a clean flame. Acetylene is out. The flame is dirty and it's way too hot. Charcoal isn't going to burn hot enough unless you have forced air. Also it has to be a high grade of charcoal to get a nice clean burn. It would take a lot of charcoal. This all has to be done in some type of insulated container (kiln) because with applications this large you have to worry about all the heat escaping.
Glass blowers these days have gone to electric resistance heat for many reasons. There are a lot of good kiln designs on the net. You need a special crucible in there to hold the glass of course. Just any old ceramic crucible will not do, as it will leach the glass into it and crack. This is more of a problem if you're planning on reusing it. Then there is the opposite of course where the crucible material leaches into the glass and pollutes it.
The grade of glass is not as important with a mirror, but for lenses you need optical grade glass.
The cooling process is also important. It has to cool slowly so that it doesn't crack. This can be done fairly easily with a well insulated kiln. Just shut it off and plug any holes with insulation.
Now you know why telescope mirrors are so expensive.
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Melting Glass efficiently
) diameter and about 30mm high for grinding a telescope mirror.
