Help with casting telescope mirrors

[Wrecker_0]

I am new here. I have 3 kilns. I have been working with assorted soda lime glass. I have been mindful to keep like glass together. I have tried 30 experiments to make mirror blanks 2in x18 in generally
I have used plaster silica molds. I have about 1 years expeience w no real mentor. My results are frustrating. I have worked in the range of 1400 to 1800degrees. Broken up glass generally winds up with lines and streaks inside the interior. I have had one blank turn completely white. Most are no longer clear
If I need to go hotter or build additional equipment I can do so. My immediate goals are to cast a 32 in blank and then slump it over a precision ground mold to make a meniscus blank. The slumping I have a handle on. But I am really lost on the melting and casting end of it. Can you guys get a dummy pointed in the right direction? I need to know how or if it is possible to melt 30 to 50 pounds of sodalime and end up with nearly clear ,not opaque, and nearly bulbble free glass
Ron

[istandalone24/7]

w/o a crucible i don't think it is possible (the 30 to 50lbs of clear, clear), but someone with more knowledge will surely come by at some point.
out of curiosity, what are you using to make the glass reflective? silver nitrate?

[Wrecker_0]

I will have it coated by professionals using aluminum materials in a vacuum setting all after tedious grinding and polishing

[somewhere]

You didn't say how thick you need the lens. For clear glass I would start with clear float glass. If you don't have the correct thickness you will have two fuse sheets together before slumping.

I bought a couple of tons of 3/4" and 1" float in large squares. (gov surplus) A lot of this glass went to lens makers that ground these into shape.

[Wrecker_0]

I would be looking at 2 in thick to start with possibly migrating to thinner in the future. I also am not confident in my skills of cutting something thicker than 1/2 in thick in a circle. I have done fusing.

[somewhere]

I'd gather your float glass and if it's to thick to manage yourself with a circle cutter take it to a water jet.

You will have better success fusing 1/4 rounds then casting in a kiln. Remember this blank when fused as thick as your suggesting will take at least a week if not two of annealing.

If you planing on grinding and polishing cutting should be included in the cold working lineup. Denver sells a nice commercial quality bandsaw.

Do you have a kiln big enough to do what your suggesting? Pictures please.

[Wrecker_0]

I have a 40 by 40 by 18 kiln and an electronic controller that I can move around kilns all built with Dudley Gibersons help
I am unable to copy and paste to this forum if you can tell me how to add a photo I will do so. I also have a 23 in I'd round kiln that I acquired along with a 28 by 40 oval both for a song

[menty666]

You might have better luck with this one over on warmglass.com, there are more casters there.

Having said that, look for info on a bubble squeeze hold in your fusing schedule, this helps work the bubbles out of the glass. Your next headache after that is avoiding devitrification as it cools, that's your next potential hurdle.

One thing that might help is taking the furnace route where you could rake the slag off the top leaving you with a better glass in the pot.

I've never made a lens, but if you're going to have it coated, why does the clarity matter?

[BORO]

soda lime has poor optical properties no? I know they use "optical glass" for certain applications . ( stump suckers? )

Most on this forum use borosilicate (coe33)

this may help you

http://www.us.schott.com/advanced_optics/english/products/optical-materials/optical-glass/low-tg-glass-for-precision-molding/index.html

or

http://www.us.schott.com/advanced_optics/english/products/optical-materials/optical-glass/index.html?WTS=1&WTC=Advanced-Optics_us_google_Advanced-Optics-New_General-1---SCHOTT-Adv-Optical-Glass-115-Types-For-Camera-to-Medical-Eng-Use-ISO-9001-14001-Get-Quality

I know soda lime melts at a lower temp , but is prone to bubbles , and haze. ( I know almost nothing about "soda lime " )

[Wrecker_0]

Clarity does not really matter to me. I just don't know what condition the glass is in when it starts to become opaque.
I have considered the furnace route and actually have components to make one. I just lack a lot of information when it comes to vessels to contain molten glass for pouring. Raking off the slag is a thought that has entered my mind. And I will take your advise on warmglass.com

[Wrecker_0]

Optical properties is a small consideration as the reflective surface is on top. No light passes thru the glass.
Borosilicate is something I do want to learn also. I just don't know where to get useable amount of materials at an affordable price. And I have yet to work at temperatures necessary for borosilicate

[Greymatter Glass]

if the glass is going from clear to opaque that's a sure sign of phase separation. You're holding it at a specific temperature for too long.

I am 100% into the DIY thing, but i am curious, why not just buy a blank? Or a 2" thick slab of float glass and have it cut?

[Greymatter Glass]

Borosilicate is something I do want to learn also. I just don't know where to get useable amount of materials at an affordable price. And I have yet to work at temperatures necessary for borosilicate

Borosilicate is great for telescope mirrors, but it takes a lot more heat to fuse it all the way.

Also, cost wise, it's not much more than quality soda-lime, and much cheaper than most optical crystal. You can buy thick rod for about $5/lb and break that up, or cut it into slugs and fuse them.

For heat, you'll need a kiln that can easily reach 2400º or more, which happens to be right around the thermal limits of Kanthal A1 wire... better to use an oxygen enriched natural gas flame (not cheap) or Moly-D or Silicon Carbide elements, also not cheap. But there are plenty of small commercial electric boro melting furnaces out there in the $2500-5000 range.

-Doug