[ATM] Acculap vs Gugolz

[eric.shrader@comcast.net]

Hi Mike,

>Nice work Eric, and thanks Mark for posting it.  

Thanks, and thanks to Mark for formatting it.

>I have a similar graph, plotted by hand on a piece of notebook paper, 
>for various laps I have made and used with success on different size 
>and focal ratio mirrors.  I don't have data for the pitch straight out 
>of the "can", or whatever it comes in; I have data measured directly 
>on the lap that I was using.

>Some thoughts:

>The Gugolz 55 line seems VERY high based on what I've experienced. 
>I've never seen fall rates that high from my own Texereau-style 
>penetrometer (also with 1kg mass, fall measured after 5 minutes).  My 
>numbers are about half of those values, in the 0.100" to 0.200" range.

>It's possible that just my (gentle) melting and pouring of the lap 
>were enough to cut the numbers in half for my Gugolz 55, or I simply 
>got a harder batch.  My Gugolz 55 laps definitely did NOT get twice as 
>hard during use.

I was surprised how soft it was too, The data was taken on pitch squares that overlapped the edge of the lap (I usually use a mold) so mine was once melted too.  I have more of this batch in "the can" and some still left over in a plastic bag.  I will re-measure it to make sure of the measurement and to see if it has changed over time.  I also need to check my notes as I used to put about 0.5% beeswax in to make the pitch less sticky, but I don't think I did it to this batch.

>So, the differences we see may highlight inconsistencies in batches of 
>pitch, but they definitely illustrate the need for serious mirror 
>makers to quantify the hardness of their pitch if they want to get 
>more predictable results when polishing/figuring.

Agree completely, I was uncomfortable with the thumbnail test...

	Mike Lockwood

>PS:  Texereau's lower limit is quite hard - I use that hardness when I 
>am working on flats.  That's the range of Gugolz 73, in my experience. 
> It's hard to figure a smooth parabola with that type of pitch, 
>unless you pre-warm the lap before figuring sessions.

Agree here too, although I have found prewarming to be a very bad technique as everything is constantly changing temperature and large scale roughness results.

Unfortunately I am on vacation for two weeks, so I won't get to this until mid July, but I will follow up with a re-measurement of the G55 and maybe we could trade a bit and do a cross-test to see if it is a batch to batch variation, if you still  have the tester.

Cheers, Eric
_______________________________________________
ATM mailing list http://www.atmlist.net/