Re: [ATM] pitch lap question

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

John Lynch wrote:
> To the ATM listserv:
> I've talked with Randy at Salem Distributing for quite
> some time, and he's a great person to work with and
> very friendly and knowledgeable.  I have heard of
> Acculap but haven't tried it.  I'm sticking (pardon 
> the pun) with Gugolz because of past performance.  I
> like it much better than burgundy pitch because it
> doesn't chip as much.  It seems to me that adding
> turpentine to Gugolz or cooking it longer to achieve
> the correct temper is much easier than what was
> described earlier with Acculap and mixing different
> grades.  I suppose Acculap cannot be tempered the same
> way regular pitch is (with turpentine).  By the way,
> the Tywann viscosity tester described in Texereau
> (responding to John Graves comment in a previous email
> message) is a great tool and makes the pitch tempering

I agree wholeheartedly.  I have been tempering the same Gugolz for a
couple years over 10 mirrors, and it should last much longer.

I temper Gugolz with natural, unboiled linseed oil.  It doesn't
evaporate as fast as turpentine, meaning your lap will stay softer longer.

I highly recommend building a pitch tester so that you can quantify
the hardness of the pitch at various temperatures.

I want to point out that a pitch tester doesn't need to be an exact
duplicate of Texereau's or the Stellafane design.  Mine is my own
design, and it has been calibrated through experience.  After testing
a few laps (and recording the temperature at which the test was done)
and knowing how well those laps worked for their intended task, you
will rapidly get an idea of what hardness of pitch is acceptable.  I
now know what hardness is good for slow mirrors and what is good for
fast mirrors, and I can work F/4 or F/8 mirrors in a temperature range
from 65 degrees (winter) to 80 degrees (summer) and keep my results
consistent and my edges under control.

When the pitch is right, making mirrors is a lot more fun.

	Mike Lockwood




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