ATM Obsidian

[Anthony Stillman <atmer@flash.net>]

Last Saturday I slabbed off a piece of obsidian for what I hopped would be
a five inch diameter mirror.  It came from the biggest rock I could find.
Had it been any larger it wouldn't have fit into the jaws of the vice which
feeds the slabbing saw's diamond blade.  I either read or was told
(possibly by Kevin Medlock) that obsidian mirrors are prone to astigmatism
hence I was careful to orient the cut with the rocks natural planes.  This
was made easier by a layer of bubbles which could be followed around the
rough rock.  It took nearly three hours to make the two cuts.  When
complete I had a odd shaped piece of obsidian nearly one inch thick and
plane parallel.  I'd like to say I then carefully laid out the blank's
perimeter, but in fact I just tried variously sized cans, plates and
miscellaneous bric-a-brac.  I settled on a Tupperware lid as a template and
Sharpied a circle just shy of the edge.  I then used a oil cooled trim saw
to rough cut the rock into a disk.  Proceeding to the grinding wheel I
slowly changed the blank from a multi-sided polygon to nearly round.   I do
a lot of work with rocks, but most of the pieces I deal with are much
smaller.   Hence, working this piece I became impatient and moved from the
diamond wheel I was using to a coarse silicon carbide wheel.  Naturally,
within seconds I took a large clam shell out of one face and so humbly
returned to the diamond.  Ultimately I used a 180 diamond flat lap to
square up the edge, put on bevels and eye ball the disk round against an
aluminum ring. When I was done the blank measured 113mm in diameter and was
nearly 22mm thick.  Not the five inches I was hoping for, but in such
situations I try to remembered what the late Miller White, the man who
taught me how to facet, would say.  "There are no mistakes in lapidary,
only smaller stones."

Initially I decided to tolerate the clam shell and use the opposite face
for the mirror, but close inspection with a strong light revealed a fine
layer of bubbles and inclusions near that surface.  As I had planned to
grind both surfaces flat to remove the marks left by the slabbing saw and
provide a nice starting point, I decided to see if the clam shell could be
removed.  To do this I used recycled 60-80 grit on a 10.5 inch porthole.
I've used this porthole for face flattening and de-wedging several other
small blanks and hence it has started to go cave.  Toward the end of the
de-clamming process I flipped the porthole over and with 220 removed the
small amount of vex the obsidian had taking on both of its surfaces.

I started the mirror Tuesday night at the educators only pre-opening of the
new Chabot Science Center.  Though I spent more time talking than grinding,
I did manage to put in an f/6.something curve with 80 grit and then back it
off to f/7.something with 120.   I used a 4 1/4 inch Pyrex blank as the
tool.  While shaping the obsidian into a blank I developed the impression
that the material was quite hard.  However, while de-clamming and hogging I
arrived at the opposite conclusion.  Finally I checked.  Obsidian is almost
exactly the same hardness as BK7.  That is, it is softer than Pyrex, but
harder than soda-lime glass.  This runs counter to what I've heard from
others and read in various ATM books and magazines so I examined several
other pieces of obsidian know to come from a variety of geographic
locations.  Using the scratch test, there was no detectable variation in
hardness.  Perhaps those who have made obsidian mirror before me had
particularly hard material or perhaps this is another example of unexamined
ATM dogma.

Last night I worked the blank through to 3 micron and then set an old 4 1/4
inch pitch lap between it and twenty pounds.  So far this evening I've
re-channeled the lap and done enough polishing to test the mirror.  I was
anxious to see if any gross astigmatism would be visible.  Happily, I can
say no.  With only a few hours of work to go the mirror is 18.5mm thick,
has 111.5mm of clear aperture and my best guess of its radius of curvature
is 1562mm. Visually the glass is a dark plum black.  Striation are faintly
visible below the polished surface, however they are not as apparent as
those in the photo of Jackson Carle's 8" shown in Peter Manly's book
"Unusual Telescope".  The figure currently is a bit oblate and the mirror
has a down edge.  Obsidian takes an excellent polish, but so far it is not
particularly smooth.  This may improve with polish and so for now I'll
continue using cerium-oxide.  If necessary I'll switch to rouge after it's
polished out.

I enjoy working small glass as progress is so rapid.  However, it has left
me with a problem, I've got to find something else to work at the new
Chabot Science Center's grand opening this coming Saturday.

Anthony