Re: [ATM] Pitch hardness

[Mike Lockwood <melockwo@uiuc.edu>]

Paul,

Paul Bradley wrote:
> I have some gugolz 74, but have just measured the temperature in my
> garage where I'll be polishing and it's about low 50s fahrenheit.

That's quite cool for just about any pitch.  G73 (you probably meant 
73) is rock-like at even 70 degrees!

> I've heard people say they never soften their pitch, but is this
> likely to be OK at this temperature? If not, I presume I need to
> add turps or beeswax, roughly how much will I want to add?

I soften pitch often.  It works, but I don't know how it will work at 
that temperature.

I would highly recommend polishing in the house where it's a 
reasonable temperature, or heating up the garage to at least 60 or 65 
degrees.  It will be difficult to figure a smooth mirror unless you do 
that.

As for softening:

Turpentine will evaporate out quickly, leaving the pitch hard.
Beeswax may cause roughness and other problems.  Gugolz doesn't need 
it, in my experience.

I recommend unboiled linseed oil for softening Gugolz.  I got it at my 
local hardware store (the Do-It-Best variety).

I have softened G73 with it quite successfully, and other Gugolz 
pitches on countless occasions.  A few teaspoons did the trick (for 
~0.5-1 kg of pitch).  You probably don't want to add it all at once. 
Add a couple teaspoons, mix it in for a few minutes, and add a couple 
more.  Then follow with lots more mixing.

If you add too much oil you will end up with an oil residue oozing out 
of the pitch.  (If that happens, you need to strip the lap and heat it 
to evaporate some oil away.)

My standard test for pitch hardness is that a lap's channels should 
require trimming after ~2-4 hrs of hand polishing with good contact.

What size/focal ratio mirror are you polishing?

	Mike Lockwood

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