ATM adventure in South Africa
Another Spectacular Lesson in Mirror Making
Making plate glass blanks is fairly easy. Take thin slabs and fuse them together. Wedge them slightly part and as they soften the air between them will be forced out to the side. You do not have to melt the sandwich completely, just to the softening point so that they fuse together. Doing it right will leave very little bubles inside. The bubbles does not matter, because they are deep inside. The slightly bulged edge we just grind smooth and spherical. Our grinding machine have that function designed in. Then we decide to go one step further. Why not slump the face into a preformed curve. Saves hogging out to depth. Can save a lot of work on large fast optics. Ok, easy--make a ceramic mold from the face of a good mirror. Then use that as the bottom of the mold for the mirror to be. So we made the ceramic mold and tested it with just one thin plate glass sheet. The residual water in the ceramic exploded as steam and blew the glass sheet to bits. The ceramic mold survived. Lesson number one. First bake the seramic mold so that it is 100% dry. After that lesson we can make slumped blanks too.