[ATM] Ultra thin mirrors, more rambling

[vorblesnak@peak.org]

Pie plates are nice in that they have the curve already there.  The big
ultra thins need to be shaped, slumped.  The first one I got pre curved
from Mel Bartels so it was much like a kit.  The second one though has
been a learning experience.

I searched for a kiln locally and finally was offered some kiln time in
the Bay Area of California.  This about 600 miles from me, but that is
what UPS is for.  His kiln is smaller than my mold, so I need to make a
new mold, which is the theme of this note.

I chose plaster for the mold as it is a good one shot slumping mold.  The
piece was cast into a plastic plant saucer 17 inches in diameter and
allowed to dry for a week.  The plastic stripped right off, though I had
some edge chipping.

The tool was also made of plaster, cast in a thicker saucer.  No chipping
when I popped this one out.

The problem is there is a lot of water in plaster.  Three weeks later and
one heating session in the oven to 300 degrees, and the stuff is finally
dry enough to shape.

I hogged the curve using an old 3 inch hole saw.  Basic strokes one would
use to hog a glass blank.  I wanted to do an f6 16 inch mirror this time. 
This required a sagitta of around 10.5/64ths or 5/32nds.  I hogged the
edge of the tool and the center of the slumping mold.  I then worked the
mold and the tool, 1/3rd stroke COC, plaster on plaster no grit.  About
every two rotations of the top element, (tool or mold), I had to stop and
brush out the plaster dust.  I ended up with about two quarts of plaster
dust.

I checked the curve by laying a short bubble level, with measurements
along the edge, across random diameters of the tool and the mold and
checking the curve depth with a dial caliper.  My logic ran, a theoretic
true curve would have a uniform sagitta from center to edge.  At some
point along the radius, i.e. 60%, the sagitta should be 40% of the total. 
So, at 60% one would expect to find a little over 4/64ths of curve depth. 
I got it spot on for the most part by swapping tool or mold into the top
position.  By checking both I was able to watch and track the curve.

Then I got sloppy and chipped a piece off of the edge of the mold about
3/4 inch wide.  However I think I can bridge this with ceramic paper and
still get a smooth slumped surface.  One more oven session and I will ship
the mold and the 16 inch glass off to be slumped.

This leaves the tool, which is 18 inches in diameter.  I am thinking of
getting another piece of glass and doing an 18 inch.

Finished mold, 16 inch diameter, f6 curve.
Target mirror, 16 inch diameter by 1/4 inch thick f6, plate glass.
Second target??,  18 inch diameter by 7/32nds inch thick f5.3, Pyrex??

David Davis
Toledo, OR  97391

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