From: George Anderson 
To: Anthony Stillman 
CC: atm{at}shore.net
Reply-To: George Anderson 


Hi Anthony

I had been wondering about porcelain or ceramic mirrors for a while, if you
were to glaze the substrate would there be any point in grinding beyond 220
prior to applying the glaze?

George Anderson
Montreal Canada

Clear skies and good health

Anthony Stillman wrote:
>
> 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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