making your own pitch

[mura_gadi]

Hello,

I have contacted A.E.C. and got a quote for rosin at $110 per kilo and another $18 in postage. I think Gotgrit gugloz came in slightly cheaper than that, even after postage from USA. Still a little pricey imo, so I thought I do more searching and found this on iceinspace.

Quote from "Satchmo"
"My old copy of the AOS publication Building a Low Cost telescope lists the mixture for an 8" mirror at 20 dgerees C as :

1lb Gum Rosin
3 oz of wood flour
55ml Castor Oil
1 oz pitch

Adjust Ratios of Rosin to oil for hardening and softening ."

Now I can get colophonium rosin gum for $36 a kilo(link below), but not sure if this is what I need. It may need to be strained, but that's not a big issue.

Castor oil is easy.

The wood flour I can get at 50micron, for free (half kilo amount anyway) and uses sawdust from the mills, the miller said they replace the blades about ever 6 years, so I am not expecting many containments from them. I thought I could just float the flour on some water to take out any steel left over from the saws anyway.

The above post seemed to suggest that the pitch wasn't needed.

Question is, will this approach get me a useable lap at the end of the day? By useable I mean for a F9 mirror, so its fairly slow, thumbnail tests applied of course.



Thanks
Steve
Rosin Gum Link:
"https://www.ebay.com.au/itm/1KG-Bulk-Colophonium-Rosin-Gum-Prevent-Slippery-Pine-Use-Resin-Colophony-Blocks/273846650003?hash=item3fc2884493:g: 5ocAAOSwUjFc27v6"

[astro744]

I used coal tar pitch (came with my kit) to make my first mirror which was 8” f9 back in 1986. The pitch you listed components for is called tropical lap. I had enough coal tar pitch to complete my mirror so didn’t need an alternative.

Whatever pitch you get, buy extra in case you need to pour a new one whether it is too hard, too soft or simply set on you too quickly or you need to pour a new one if you’ve left the project for too long. If you get coal tar pitch use a pot you don’t want as it’s never going to be used for anything else again. The whole house will smell like roadworks pouring new bitumen. I recommend you get a portable gas or electric single element hot plate and heat it outside to avoid fumes in the house.

I found that Koppers is the only Australian manufacturer of coal tar pitch but where you can get some I’m not sure. Perhaps email them.

See https://koppers.com.au/carbon-materials-chemicals/#CMC_News

(Scroll down).

[dave brock]

I've made many mirrors using rosin and Stockholm Tar. Generally 7 to 1 ratio. Stockholm Tar can be purchased from equestrian supply stores.

[hamishbarker]

Hi dave, where does one buy rosin in nz?


this stuff?
https://www.nzsoapandcandle.co.nz/product/1865118

[dave brock]

Oops, sorry for the delay in replying Hamish. I forgot I had posted.
I've managed to gather Rosin over the years so have never had to buy from a supplier. If I did have to buy some the supplier you linked to looks good and I would try them. I don't do as much mirror making as I used to so if you need Rosin I probably have some spare.