Now my pitch is miles too soft

[mura_gadi (Steve)]

Ok, while its warmer now than when I tried, my pitch is way way too soft.

They say you can boil off the pitch for a while to make it harder, what is a boil? Do I just have the pitch runny and going for a few hours or should there be a slight simmer bubble in the pot?

Can I just add more sawdust instead of a long cook?

Any tips for getting the pitch firmer? I have no resin or tar to add....


Thanks
Stuff for brains

[astro744]

Quote from “A Manual for Amateur Telescope Makers”, Karine, Jean-Marc, Lecleire, Willmann-Bell publisher.

Page 101, para 3 half way through. ’To harden pitch, warm it over medium heat for an hour or two, without quite letting it boil. Just a hint of smoke should escape from the pitch, which is evidence that it is losing some of its solvents. Occasionally take a sample with a spoon, cool it under running water for 10 minutes, and test its hardness with your thumbnail or the stainless steel ball’.

Para 4. ‘To soften pitch that is too hard, add a little turpentine to the melted pitch in the saucepan. ...’

It’s an excellent book worth getting if it becomes available again through Shopatsky. (Not yet listed).

[mura_gadi (Steve)]

ty, I have some books that mentioned heating for a prolonged period to boil off volatiles and testing every 15minutes, but nothing about a slight smoke...

[astro744]

See post #7 & 8 at https://www.cloudynights.com/topic/2...-lap-too-soft/

They mention to gently heat to remove volatiles but also mention adding filtered rosin as an option. I would heat to remove volatiles and not add anything. It does take a while and it’s also been a while since I’ve done any of this. I’m sure there are many methods that work.

Edit:

Also found this: https://stellafane.org/tm/atm/mirror...rit.html#Pitch

Perhaps you need a different pitch to begin with if working in summer, ie one with harder inherent properties. Eg Gugolz #84 or 91.

Just found the only book I ever used pre internet era and that is “Building a Low Cost Telescope”, by Eric Whitcombe, printed by ACME OFFICE SERVICES, Crows Nest NSW.

Bottom of page 11 and beginning of page 12 says, “A lap that is satisfactory for the winter months would be too soft in the summer. Swedish or Burgundy wood pitch is preferred but local coal tar pitch is satisfactory. To harden pitch add resin or cook longer - to soften add turps or linseed oil.”

My Astro-Optical kit had a black pitch substance that I used to make an 8” f9 mirror and later 10.1” f6.3 what seems like an eternaty ago now. I got the 10.1” f6.3 refigured professionally by Suchting and it cameback 10.1” f6.4 and I have been very happy with it. The 8” f9 I sold to help fund the 1991 Eclipse trip. Kind of wish I still have it sometimes as it was my first hand made mirror.

[mura_gadi (Steve)]

I got the pitch to a workable stage and managed a quick polish last night.

I went down to 14000 grit on the smoothing and I can't believe how fast the blank took a shine.

Another two steps forward one step backwards day but something learnt at least... now for something remembered...

[By.Jove (Jove)]

Ah. Another trick is to paint the surface of the pitch with melted beeswax before pressing with a string mesh, and the next polishing session, something I did long ago when making some mirrors.

You might find some of the posts in this thread interesting https://www.cloudynights.com/topic/7...-vs-polishing/
In particular with soft pitch I'd be worried at the possibility of turned-down edge, but one of the posts indicates polishing underwater (ie the mirror and the tool are both submerged in a waterbath) is beneficial in preventing TDE. Aside from the quality of the polish.

[OzEclipse (Joe Cali)]

I don't want to say I told you so but, I told you so!

https://www.iceinspace.com.au/forum/...94&postcount=9

If you use one of those methods to firm up the pitch, it will then become too hard during the cooler weather to get a good press. So astro744's suggestion of two different pitches makes more sense or just wait and work in cooler weather. Be very careful adding sawdust. If it has any hard contaminants, you'l get sleeks and scratches. And yes, I agree that a soft pitch may result in a TDE.

In the old ATM club I was involved with in Brisbane, we did most of the optical polishing in late Autumn, Winter and early Spring and did aluminium casting and metal machining during the hot weather stupidly hot as that was.

Joe