Re: [ATM] Pitch hardness

["Paul Bradley" <paul@paul-bradley.com>]

Mike,

Thanks for the reply. I'm polishing 2 8" F6 mirrors for a binocular 
newtonian.
I'll give the linseed oil a try, and I'll heat the garage to the mid 60s.

Paul

----- Original Message ----- 
From: "Mike Lockwood" <melockwo@uiuc.edu>
To: "ATM Mailing List" <atm@atmlist.net>
Cc: "Paul Bradley" <paul@paul-bradley.com>
Sent: Tuesday, October 18, 2005 4:02 PM
Subject: Re: [ATM] Pitch hardness


> 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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