From: Guy Brandenburg 
To: Rich Ball 
CC: ATM 
Reply-To: Guy Brandenburg 


80 degrees isn't very hot, frankly. I donno where you live, but around
here, that's room temperature or cooler for much of the year, unless you
have A/C. For a hot press, either use a serious heat lamp or hair dryer, or
put it into a bath of water that is a bit hotter than your hand can stand.
Try sticking your thumbnails (if you have them) into the pitch while the
heating is going on, to test how soft the pitch is becoming. When it is
really, really soft, sort of like a room temperature Mars Bar (!!), then
try pressing with the mesh for a BRIEF time with a good bit of hand
pressure, (up to your entire body weight! - experiment) with a cerium oxide
or rouge slurry on the mirror. You should immediately see the mesh making
an impression on your lap. If your lap has become so soft that it has
melted, you will also lose your channels. Thus, you probably don't want to
melt the pitch before doing this.

There are arguments on whether you should heat the mirror or not while
doing this. If you don't, then it acts as a huge heat sink, cooling down
the pitch almost immediately, once it has begun to make good contact with
your mirror and once the mesh has done its job; you can simply them do a
cold press for 20 minutes and start polishing, or else start polishing
immediately. If you do heat up the mirror, then everything will take much,
much longer to cool down, and you will need to NOT let the mirror and lap
stay in contact while they are cooling down, lest you lose all of the
channels that you made so carefully.

One thing you can do, before this, while the lap is cool or room
temperature, to bring 'virgin' pitch more to the surface and to provide
more room for the pitch to conform to the surface, is to take a single-edge
razor blade, or a sharp knife, and seriously score the lap in a cross-hatch
pattern with the tip of the knife or the corner of the razor blade. Then
try the hot press.

While polishing, and with everything at room temperature, the channels
should start to close somewhat in about 30 minutes to an hour. If they
close much faster than that, then the pitch is too soft. (Mind you, my
experience is that pitch takes roughly 24 hours to reach its terminal
hardness level after you've melted it, so Texereau's pitch tester is of
marginal utility unless you have that much time to wait around.) If you see
no change in the channels after an hour, then it's too hard. In either
case, it's easy to fix by chipping off all of the pitch and either boiliing
it for a while to harden it by driving off the more-volatile elements, or
by adding a little bit of turpentine to make it a bit softer.

Good luck.

Guy B




Rich Ball wrote:
> Mentors - I need help again.
>
> My 8" mirror project has been dormant for more than two years.  My tool
> (pitch lap) has been stored pitch up in a covered tray.  Today I set myself
> to figuring after finally making sense out of Mr. Foucault.
>
> I warmed the tool to about 80 degrees and pressed it with a mesh and cerium
> oxide for about 1 hour with a 30 pound weight.
>
> But even after so severe a pressing the mesh was not taken up in the pitch
> and I fear the pitch is too hard to take the pressing.  What should I do?
> More heat?  New pitch?
>
> --Rich Ball
>
>

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