Re: [ATM] beginner's questions

["Dave McCarter" <dmccarter@sympatico.ca>]

> Glass - I have a stack of old windows and would like to know the
> feasility of glueing (epoxy) several pieces, cut to size, of this glass
> to build up a stack of appropriate thickness. I was going to vacuum bag
> the assembly to eliminate air bubbles in the glue. Could it then be
> annealed in a kitchen oven (first it needs a preheat out-of-doors to
> drive off volatiles)?

Nope. Bad idea. If you were to place frit (ground and powdered glass)
between each sheet and then melt the sheets together in a kiln you might be
able to get away with it. The top layer would have to be very thick, 10mm or
so, to contain the curve. But there is something you really have to
consider:
1. The time you will spend making the mirror is certainly worth something.
2. The difficulties of making a good first mirror are significant, involve a
lot of learning and great care.
3. The difficulties are magnified enormously if you are using glass that is
not stable. Your efforts to figure a poor glass will be thwarted when the
glass changes shape because of differential thermal expansion. I worked a
piece of BVC some years ago, that was made of several layers of glass, and
as the top layer was not properly attached to the lower layers, not fritted
properly, the surface would rapidly change shape as the glass changed
temperature. I eventually gave up trying to figure it, after expending a lot
of time and effort. Don't work with crap glass.
4. Making a mirror with a properly annealed and stable piece of glass is so
much easier that the time you save by not having to fight a poor glass
greatly outweighs the cost of the blank.
5. A stable piece of glass will keep a good figure forever. A hunk of glued
glass will not.

> Grinding supplies - Got Grit and Newport Glass are two sources; are
> there any others for continental U.S.? Can I rely on their sieving or
> do I need to do my own elutriation?

They are good sources of grit and polishing compound. I mill and separate my
own rouge, but use the SiO and CeO as it comes. Newport is also a good
source of blanks and glass tools.  I prefer to make my own tools with dental
plaster and ceramic tile.

> Mirror coating - Aluminum and a protective SiO2 layer (which yield
> about 84% relectivity) I understand but are the enhanced coatings worth
> the expense (claims of 94% to 97% reflectivity)?

Enhanced are certainly worthwhile if done correctly, particularly for a
smaller mirror.

Dave
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