From: "Thomas" 
To: "Anthony Stillman" 
Cc: "atm" 
Reply-To: "Thomas" 


Hi Anthony,

I have a porcelain secondary that I ground for a schifspigler, and yes I
too had the pitted looking surface, very hard to polish (88mm diameter)
took about 16hrs by hand with cerium. I just have to (oneday) core it out
and finsh the scope.

The material is a blue fully vitrified ceramic that a "material
science student mate of mine" cooked up at uni, so the pedigree isn't
all that clear.

But yeah I found it to be hard to get to move, but also it resisted edge
turning effects well too.

Well that's my $0.02....

Clear skies, Thomas.
www.tjanstrom.com
www.norsewines.com.au
----- Original Message -----
From: "Anthony Stillman" 
To: 
Sent: Sunday, June 29, 2003 9:50 AM Subject: ATM Porcelain Primary


>
> Thirty years ago I cast and fired several small porcelain mirror blanks.
> This past Fall I found the smallest and started working it.
>
> I faced and de-wedged it against a port hole glass.  In the process I fine
> grown the back.
>
> To hog, a broken bit of marble slab was cut octagonal with a hack saw,
then
> rasped round.  My first marble tool.
>
> Using 220 I put a very shallow curve into the 60 mm blank.  I tried and
> failed to measure the ROC by straight edge.  My finest wire stuck.  Giving
> a lower bound of f/60.  A little long.
>
> I worked through the grits all MOT, and after 5 micron I polished for a
bit.
>
> In the early afternoon, an overhang makes star testing nearly on axis,
> ideal.  I climbed up on a chair with note pad in hand to see the Sun's
> image two meters from the porcelain.  It showed no Sun spots, but it did
> come to a decent focus and limb darkening was evident.
>
> Polishing has gone slowly.  Much slower than glass.   And then there's the
> surface.  To the unaided eye, pits and crevices are clearly visible, and
> barely visible.  Just as they are under 50 power.  What constitutes
> polished out isn't quite clear.
>
> The mirror has! developed a polish.  Before I cleaned it in the
> ultra-sound, Sun spots were visible in the image.  Cleaning it shook free
> what clogged the pits and apparently some of the polish.
>
> After the cleaning, I made a new lap.  Polish returned slowly, along with
> cerium in the all the openings.  Solvents and soap haven't cleaned the
> porcelain as well.
>
> As of today, it's still not polished uniformly.  And, the image is good.
It
> was apparent that a Sun spot was in fact a conjoined pair of Earth sized
> Sun spots.
>
> I don't plan to glaze or silver this porcelain.  Somewhere there's a
> slightly larger blank.  I'll try that when it surfaces.
>
> Anthony
>
>
> NEMO inquisitium Hispanum exspectat.
> NO ONE expects the Spanish inquisition.
>
>
>
>
>
>

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