The first photo below shows a 12.5 inch diameter, 1.5 inch thick, telescope mirror blank, made from recycled glass, that has been lightened by casting the mirror in a mold that made lots of hexagon-shaped pockets in the back of the mirror. This is the story of how I did it.
Why light-weight mirror blanks? Well, telescope mirrors are made from thick glass. Thick glass is heavy, no, really heavy. Most people don't realize how heavy thick glass actually is because they usually only deal with small pieces of thin plate glass. Handing a large telescope mirror to someone who has never seen one before, is a revelation to them about just how heavy glass really is. I like to tell them that it is as heavy as granite. So imagine the piece of granite that was cut out of a granite countertop to make a sink opening. That piece of granite is about the same size and weight as a large telescope mirror. It's heavy stuff.
In a large telescope, the primary mirror can constitute 1/3 or more of the weight of the entire instrument. Supporting the massive weight of the mirror requires that the rest of the telescope be sturdily built. The result is that large telescopes can be shockingly heavy. I know this first hand because I have thrown out my back several times moving my relatively "easily portable" 17.5 inch Dobsonian Telescope. So I decided to try making some-light-weight mirror blanks, just to see if I could do it.
Let's start this story at the beginning. About two years ago I bought a small kiln at a garage sale and began experimenting with making my own telescope mirror blanks by melting together pieces of thin scrap glass in my kiln to make the thick glass needed for telescope mirrors. The process works great. I started out small, producing 6, 8 and 10 inch diameter blanks up to 1.5 inches thick in the small kiln (Photo #2). Then I saw a much larger kiln for sale cheap on Craig's List. I bought it (Photo #3) and scaled up my mirror making process. The kiln was in rough shape. Well, to be honest, it was near the end of its useful lifetime and about ready for the landfill. But I fixed it up and gave it new life. So not only am I using recycled glass, I am also using a recycled kiln. Fixing up the kiln though is a story for another Instructable. I have now produced wonderful solid mirror blanks in the bigger kiln, up to 14.5 inches in diameter and 1 1/2 inches thick (Photo #4). Once I had the process of making solid blanks pretty much perfected, I wanted to try making light-weight mirror blanks.
Two years of trial and much error went into developing this process. This Instructable just presents the finished product. If you would like to see the whole history of my glass casting (mis)adventures, please visit my web site at http://www.mdpub.com.
Step 1: Designing the Mirror Blank
I played around with some layout ideas in Google Sketchup. I wanted to make a mold that is the negative shape of the finished mirror. I had settled on a mirror 12.5 inches in diameter and 1.5 inches thick for my initial experiments. So I tried arranging hexagonal prism shapes inside a circular wall or dam 12.5 inches inside diameter that will retain the glass. I had the idea of assembling the mold using one of my kiln shelves as a base. So I built up all the parts and played with sizes and spacing of the hexagons. The hexagonal prisms would form pockets in the back of the glass mirror that would lighten it. I settled on hexagons 1 7/8 inches in diameter (as measured across the flats) and 1 inch tall. That would leave 1/2 inch of glass on the top face of the mirror for grinding in the curve. The spacing between the hexagons would be 3/8 inch.
All the parts of the mold would need to be built of materials that could withstand glass fusing temperatures. The hexagons would be made from plaster and silica mixture. The outer wall would be 2.5 inches tall and cut from soft firebrick with a band saw and the pieces glued together with furnace cement. The kiln shelf base will easily withstand the temperatures involved.
With an initial design in hand, I set out to make it happen. I wish I could just CNC mill the mold out of a big block of refractory material. Or even use a 3D printer to print a plastic negative of the mold that I could pour castable refractory into. Oh well, maybe someday. Till then, I have to piece it all together the hard way.