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Gugolz: #64 or #73

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#1 kcoles

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Posted 13 July 2020 - 06:41 PM

I'm resurrecting an ancient mirror project (my first). I'm polished out, have a tool and a tester, and will find a barrel or make one. I need to order pitch. [Thanks to Vern (N7GTB) on another thread who reported the status of First Discovery on the web - I was wondering too.] I'll be working in my basement, which at least has a consistent temperature. It currently ranges from 69 F on a cooler summer day to 72 F (or about 20-22 C) on the hot days (over 90 F/32 C) we had in Pennsylvania last week.

 

Seems like I fall right on the line between these two grades of pitch. I'm inclined to go to the harder one (73), as working it may warm it enough. And I'm told you don't want a lap that is too soft. Your thoughts and opinions are welcome. Regards,

 

Ken

western Pennsylvania


Edited by kcoles, 13 July 2020 - 06:42 PM.


#2 Steve Dodds

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Posted 13 July 2020 - 07:01 PM

I use 64 for almost everything.  64 is medium 73 is pretty hard.  In winter I mix 55 with 64.



#3 jelloptic

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Posted 13 July 2020 - 07:11 PM

For figuring, I suggest the softer (64) pitch.  You have the option of mixing in some 73 if you think it too soft.

 

regards,  kbl



#4 dogbiscuit

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Posted 13 July 2020 - 07:38 PM

What size and f/# ?

 

64 should work pretty good in that temperature range.

For me Gugolz 64 works particularly well somewhere between 72° and 76°.


Edited by dogbiscuit, 13 July 2020 - 07:38 PM.


#5 kcoles

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Posted 14 July 2020 - 09:59 AM

This is a basic first project, a 6-inch f/8 (measures at about f/8.3) that I ground and polished out at the Delmarva 2006 workshop but didn't finish figuring. Full-size tool (though of course it needs new pitch). I suspect I should go back to spherical with it and re-parabolize, but that's another thread. I've made time over the rest of this year to get this and some other projects done.

 

That's three votes for 64, but I'll order some 73 also in case I need something harder now or later. Thanks.

 

Ken



#6 Mike Lockwood

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Posted 14 July 2020 - 11:40 AM

I recommend 64.  You can harden it by cooking it, but I doubt you will want to.  73 is indeed pretty hard.


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#7 SteveV

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Posted 14 July 2020 - 11:56 AM

May I ask how to make a hard pitch soft?  73 to 64.


Edited by SteveV, 14 July 2020 - 12:23 PM.


#8 Steve Dodds

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Posted 14 July 2020 - 12:36 PM

Add a teaspoon of linseed oil, a little goes a long way.


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#9 Mark Harry

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Posted 15 July 2020 - 05:17 AM

I use #73 down to low-mid 60's. I haven't bought any pitch for almost 20 years. I soften it with 30wt motor oil because it takes a long time for it to polymerize and let the pitch get hard. A -VERY- little goes a long way.
   #73 will work down to about F/4.5 pretty easily on smallish mirrors in the 6-8" range, provided the right technique is used.
I tried linseed oil and turps, but the pitch gets hard to quick for my taste.
Winter, I just don't do much polishing then- wait for spring.


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