Telescope Reviews: Ceramic mirror development begins

The pottery instructor is convinced that I can buy the same or better material in smaller quantities from her supplier. I have thus put off buying the Aluminum-Silica from McMasters until I have had an opportunity to spend time at the pottery school.

There are any number of glazes available for this or we can place a sheet of glass on top of the mirror blank, fire up the kiln and then simply go past the intermediate "slumping" phase to a less viscous stage where the glass actually flows. She says they do this often.

Dare I buy a thin plate of Pyrex for this purpose? Or, should I go with her suggestion of using a porcelain glaze which is, she says, a harder material? I believe that it's important that both the glaze and the substrate expand at the same rate or risk failure of the glaze.

I expressed concern about the glaze cracking over time. She said that is the one area where her reputation excels, Her glazes don't crack.

She did express some concern when I suggested that the blank be 1-1/2 to 2" thick. Clay made that thick may warp some during the drying stage and perhaps again when the "green" item is fired.

There are many unanswered questions. I suspect that there will be several iterations of the process before we finally come up with an acceptable blank ready for figuring.

One thing is clear: ceramic mirrors do not lend themselves to interference testing. You have to be able to see through the back of the mirror to see the interface. Obviously, you can't do that with ceramic mirrors.

The key thing to keep in mind is that the surface finish and shape of the ceramic is not critical. The sagitta only has to be approximated and left rough. The finish can be rough in much the same manner that a pregenerated blank is. In fact, it may prove beneficial to have the sagitta finish rough to enhance the mechanical bond of the glaze. I simply do not know.

It's the glaze that gets polished and figured. Given how slick glazes are coming out of the oven, that should not take as much effort (he said crossing his fingers).

I am unsure how thick a glaze can be in order to support the removal that takes place during figuring.

I am also unsure how the thickness of the glaze is affected when applied to a concave surface. When heated will it flow towards the center and be thicker there then at at the edges?

John, the temperatures are ramped up by computer, maintained during the firing process and then ramped down, so as to avoid thermal shock.

Are we having fun yet?

"And so it goes"

Art