Re: ATM Porcelain Primary

[Anthony Stillman <atmer@flash.net>]

After the mollusk event, like removing a  bandage stuck to hair,  I edged
the blank round and ground down the face.  I then hogged at 360 targeting
the previous ROC.  At each finer grit I ground for much too long.

Regular examination of the surface through a microscope showed that the
coarser grits created cracks into the ceramic, but unlike glass, crack
girdled columns of porcelain were often not freed from the sub-straight.  I
was hoping to fine grind through the surface and sub-surface fractures, but
the finer grits were ineffective at removing material.  The fine grits did
create tiny conchoidal pits in the vitrified surface, just as I've seen in
glass, but these were twenty to fifty times shallower than the deep
fractures.  After many hours of fine grinding, I relented.  One curious
note, at times individual grains of porcelain, and clumps of grains, popped
out of the surface leaving smooth hemispherical divots.

I cleaned the blank with ultra-sound, though cursorily, after 17u then
proceeded to 9u before polishing. Consequentially there was residual fine
grit lodged in the fissures when I started polishing.  It took several
hours pushing pitch (TOT) to develop a reasonable polished.

By examining at 200x, the area around an identifiable inclusion in the
porcelain, I was able to track the removal and re-deposition of
sub-straight.  Through holes and transparency in the specular surface I
could see that polishing compound and 9u polluted enclosed and
semi-enclosed volumes below a surface of vitrified ceramic.  As the
polishing continued, wear exposed voids previously closed while
re-deposition closed voids previous pocked with openings.  The openings,
long and thin in places, bunched and speck like elsewhere, followed the
swirling patterns in the clay.

The re-deposited sub-straight over contaminates was thin.  At the start of
polish, these flakes would grow and break free ever one or two sessions (30
min. per).  Occasional their breaking free would leave a scar across the
mirror.  As the large pockets of contaminates were polished away, the
flaking behavior stopped and oddly, re-deposition also slowed.

I've now been polishing for nearly twenty hour, the last few figuring.
Currently, I'm working in five minute sessions using very little pressure,
testing and testing again after some thermal equilibration, and then
pressing for one hour or one day depending on the time of night.

There are material defects, low spots and stig.  The ultra-hard pitch I'm
using has helped hide these.  I may try to remove the stig, but then again
I may not.  The plan is to use this 60mm f/34 mirror for a solar
herschelian.  A trace of stig, properly oriented, will improve the
performance.

Also notable, at f/34 the foucault test is so sensitive to error that even
1/20 wave deviations stick out like a sore thumb.  At this point the mirror
is plenty good enough, so I'm just going to work it a little more.

Anthony

ex malaris victoriae cladem avello
>From the jaws of victory, defeat I snatch.