Re: [ATM] Acculpa vs. Gugolz: Hardness Comparison

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Nice work Eric, and thanks Mark for posting it.  This graph is nice to 
see - someone has taken the time to measure many different types of 
pitch BEFORE using them.

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.

Equilibrating the pitch for 24 hours was a good move.  I have measured 
laps just after pouring and then days later - in my experience (based 
on measurement), my laps reach a stable hardness only after about 8-12 
hours of cooling.  (No doubt that is a function of indoor temperature, 
lap size and thickness, etc.)

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.

	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.

Mark Holm wrote:
> A couple of weeks ago, Eric Shrader presented a graph showing hardness
> over a range of temperatures, measured with a Texerau style
> penetrometer, for different grades of Gugolz pitch and Acculap synthetic
> pitch replacement.  Eric has graciously allowed me to make a web page
> using his graph and write-up and post it at http://www.atmsite.org
> 
> Specifically, it is at
> http://www.atmsite.org/contrib/Shrader/Acculap/acculap.html


_______________________________________________
ATM mailing list http://www.atmlist.net/