Silver solder pickles

Hi all, Ive not done much silver soldering, so Ive been reading up a bit. The way to remove the flux afterwards seems to be to 'pickle' the piece in
acid. Why acid? Does it matter which acid? I have some 40% weight Formic acid, wiki http://en.wikipedia.org/wiki/Formic_acid would this be a suitable pickle?
any insights appreciated.
Dave
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For steel, I use phosphoric acid (has to be heated a bit above 20C). For brass, citric acid.
Nick
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I believe, it creates tiny (microscopic?) indentations on the surface where some areas are dissolved and others aren't
Michael
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The acid just helps to dissolve the flux and clean the metal surface a bit.
Formic acid would be OK but 40% is too strong - if you splash that on skin or get it in your eyes it can do damage. It's also pretty smelly. I would dilute it down to 5 - 10%. Wear eye protection and avoid it splashing.
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Norman Billingham wrote:

I thought 40% would be a bit on the strong side. Thats the conc it comes in. Add acid to water ISTR, ie put 4 parts water in jug and add 1 part acid for about 8%?
Ive picked up some citric acid crystals from the homebrew shop, what sort of concentration of that would I use?
thanks
Dave
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20g per litre of water -that's what it says on my citric acid pickle. It seems to work OK.
Regards Kevin
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That seems about right to me too. I'd certainly use it in preference to formic acid - much nicer stuff. Formic acid can be very irritating stuff. If you do use the formic, 8% is probaly about right.
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Norman Billingham wrote:

8% formic acid seems a bit strong to me. The given figure for citric acid is only 2% after all, and formic acid is a stronger acid than citric acid. I'd try 1% formic acid to start, and add more if needed. If you do try formic acid, please let us know how you get on.
BTW formic acid can kill by inhalation - don't heat it. It's also poisonous as well as corrosive, and the ldlo-oral-human is only 2.4 mg/kg, so very small amounts can cause harm, though it will take a lot more than that to kill. I'm a little surprised that you have any to hand, it's a bit hard to get these days.
(the ldlo-oral-human is the smallest oral dose at which effects on humans have been noted in the literature)
http://ptcl.chem.ox.ac.uk/~hmc/hsci/chemicals/formic_acid.html
I agree that citric acid is much nicer - you may get some odd looks when you buy it though, as addicts use it to prepare raw heroin for injection. But you can eat small amounts of dilute citric acid without harm, unlike formic acid. And it won't eat holes in your skin unless you get very silly with it, again unlike formic acid, though rubber gloves are a good idea.
I usually just boil the formed joint in water for 20 minutes or so, brushing with an old toothbrush if necessary, occasionally adding a little citric acid if I want it to look prettier (esp. on stainless) or if the used flux has "gone over" or gone too glassy - it will dissolve in hot water even then, but it can take a loooongg time.
Note also that used pickle of any kind can contain dangerous levels of fluoride ions, fluorine-containing compounds and nasty metallic compounds - nothing immediately lethal, but to be handled and disposed of with care. It's also toxic to plants in any significant quantity.
--
Peter Fairbrother


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Peter Fairbrother wrote:

Given the following I think Ill give it a miss for the moment, esp as we have a small baby....

If you need some its on the shelf in boots (the chemist), as ataka bath stain remover and kettle descaler. it says 40% formic acid w/w on the orange bottle, with corrosive labels. ~1.75 for 250 ml.

Ive used citric acid to ceate 'batteries' when I was about 5, but thats a long time ago. Its also used in homebrewing, so homebrew shops, and most wilkinsons stock it.

My flux always seems to be glassy, or 'burnt' I think Im heating for to long? probably need more practice. Ive only silver soldered 4 things so far...

Hadnt though of that, thanks for the warning.
Dave
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snipped-for-privacy@bem.fki-et.com wrote:

Forget all the corrosive stuff We silver solder all the time and just leave the part in warm soapy water overnight. The flux (even glassy) will disolve. Regards Dave
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djkeates wrote:

:)
tomorrow morning can sometimes seem a long time away ...
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On 3 Sep 2006 03:27:03 -0700, " snipped-for-privacy@bem.fki-et.com"

Information from Johnson Matthey catalogue.
Flux Residue Removal
Easi-flo flux    standard`        550 to 800 deg C         Stainles steel grade 550 to 780 deg C         Aluminium bronze grade 550 to 800 deg C Tenacity No 6 grade 550 to 800 deg C
Residues are slowly soluble in hot water. However immersion in a warm 10% caustic soda solution followed by brushing in a stream of warm water is preferable.
Tenacity 4A        800 to 1000 deg C Tenacity 8        550 to 800 deg C Tenacity 5A        600 to 1000 deg C
Residues are virtually insoluble. Immerse in a 10% sulphuric acid solution followed by brushing in a stream of warm water. Mechanical removal may be necessary.
Tenacity 10        1000 to 1300 deg C Tenacity 12        1000 to 1300 deg C
Residues are insoluble. Sandblasting or mechanical removal is necesary.
Pretty well all the commonly used fluxes are flouride types listed in the first category. Acid pickle can be useful for removing metal discolouration but hot water or 10% caustic soda is the best route for flux removal.
The middle category is speciality fluxes, not often encountered. Tenacity 4A is an intermediate type useful for two step brazing with high followed by low temperature silver solders.
The last category is high temperature brazing fluxes pretty similar to borax.
Jim
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For those who silver solder and eat at the same time, brown sauce is a recommended pickle.
Feel sick? Being physically sick works as well.
Just thought that I would have a quick burst on my banjo!
Cheers
Norm
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As a professional chemist, dropping red hot metal into 10% sulphuric acid, 10% caustic soda or any % formic acid has me worried. None of these things is at all nice to get on your skin or in the air. We use formic acid in the labs but only in a fume hood.
Dilute citric acid on the other hand is pretty much harmless
Personally I'd feel much more comfortable with quenching into plain water then transferring to whatever acid/alkali bath takes your fancy once the risk of spurting is over.
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Norman Billingham wrote:

I might be wrong, after all I odnt know much about this area, but I wasnt planning on 'quenching' in acid. I thought the pickling was done after you have finished the job, and it has cooled down. I think gentle warming of the pickle is recommended, but I certainly wasnt planning on boiling acid... Far to scary! I await a more knowledgable confirmation of this.
Dave
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On Tue, 5 Sep 2006 23:46:47 +0100, "Norman Billingham"
SNIP

OOPS - the temperatures that I listed were just additional information indicating the working range of temperatures over which the flux is active. . They are NOT the dunking temperature of the soldered assembly which should be quenched or allowed to cool naturally before immersion in warm or cold solution.
Sorry about the confusion
Jim
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