Pitch hardness

[Guest]

I've got some pitch and I'm trying to judge how hard it is. The book I have describes the "Everest" hardness test which involves seeing how long it takes to make a mark 1/4 inch long with your thumbnail with a force of 1 pound but it is not an easy test to carry out. The pitch seems pretty hard to me but I'm reluctant to soften it at the moment.

Does anyone have better method of hardness testing without having to build elaborate test equipment.

I've had the pitch for a couple of years but it has been in a sealed container so I don't think it should have hardened too much in that time.

Cheers

Steve

 

[mapstar]

Hi Steve,

When I tested my pitch it was with the thumb nail and I could just mark it.

You can soften the hardness with boiled linseed oil or bees wax. You only need a drop at a time.

If the pitch is too hard it will just seem to skate over the surface. 

Make sure it is pressed out properly though over night. You can check the contact by warming the lap and then putting the lap face up with a little cerium mixture on. Turn the mirror face down and put it ontop of the upturned lap.

You may find wetting the back allows you to see through the mirror to judge how the facets contact. 

To add I used Gugolz 55 pitch and 64 

 

[Guest]

Thanks Damian,

I'm not sure what my pitch is and I can't remember where I bought it. With the thumbnail test my nail did make a mark but it was barely a mark if you know what I mean. I suppose I was expecting it to sink in a bit more than it did. In fact I found a design somewhere on this forum for a hardness tester so I thought I'd make that just for the hell of it.

By the way I've ordered a Ronchi screen from the place you suggested so thanks for that.

Steve

 

[mapstar]

No worries Steve

The hardness of your pitch sounds to be ok are you sure it is optical polishing pitch though?

Did you find the Ronchi generation program by John Upton that I mentioned.

[Guest]

Yes I did find it thanks.

I'm pretty sure it is optical polishing pitch. I may have bought it from Lapmaster UK. It wasn't Galvoptics anyway.

Did you ever start a thread about the actual telescope construction?

 

[mapstar]

4 hours ago, woodblock said:

Yes I did find it thanks.

I'm pretty sure it is optical polishing pitch. I may have bought it from Lapmaster UK. It wasn't Galvoptics anyway.

Did you ever start a thread about the actual telescope construction?

 

I didn't Steve but it is still on the cards and nearing a time of publish. I'm still tweaking it and it's been a long journey.

Should be very soon though and glad you've jogged my thoughts to get it done

[Guest]

Looking forward to seeing it.

Steve

 

[Guest]

So I've made a hardness tester according to guidelines. It's basically a blunt point pressing on the pitch with a 1kg weight behind it. Then you measure how far it penetrates over a five minute period. The range is quoted as 0.6mm (very hard) to 6mm ( very soft) with an optimum of about 2mm. I've just tested my pitch and it comes out at 0.2mm which is way hard. The pitch is still in the tray it was delivered in. A bit like the foil trays you get from the Chinese takeaway. It's been in there for at least a couple of years and I tested the surface. I'm hoping that it's just the surface that's gone hard so I will melt it all and test a sample again.