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Flux problems

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Bernard Greatrix26/03/2020 23:14:05
32 forum posts
6 photos


I'm using Silver-flo rods and Easy Flow flux to connect pieces of mild steel.

My heat source is a 1/2" Bullfinch torch fed from a 7kg Propane cylinder.

So far so good you might say, and very often it is. The flux forms a silvery appearance and the Silver-flo rod runs nicely into the joint.

But .... every now and then the flux goes black and the Silver-flow beads up and the joint doesn't make.

At this point I have to scrape off the black residue, clean up the metal again and hope for the best.

What am I doing wrong?

I've tried using a Borax flux with the same results.



fizzy26/03/2020 23:17:49
1705 forum posts
115 photos

easy to answer, your flux is burning out before the silver solder is applied. Use tenicity flux and problem should go away - you might be offering too much heat and 'burning' the metal, but its classic flux burn

Bernard Greatrix27/03/2020 16:35:19
32 forum posts
6 photos

Thanks Fizzy, I'll get some Tenicity.

I must admit that I do find it difficult to get quickly to temperature, so I do finish up cooking the metal for quite a while.

I am using Silver-flo 40 (which I bought in error) which probably doesn't help.


JasonB27/03/2020 16:37:45
17850 forum posts
1954 photos
1 articles

Your 1/2" burner may be a bit on the small side, 25 or 28mm would get the heat into the work far quicker, also make sure your regulator is turned right up you want at least 2bar for the larger burners.

Bernard Greatrix28/03/2020 23:16:31
32 forum posts
6 photos

A bigger burner will have to go on my birthday list.

As I usually only 'cook' small pieces of metal I've found the 1/2" nozzle to be adequate.

The one which prompted this post was 1/2" square x1/8" thick being attached to a 1/4" round rod - much like a key blank and I thought it would be a doddle - but it took ages to get to temperature.

Anyway thanks guys - at least I have some food for thought and future improvements



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