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
be a suitable pickle?
any insights appreciated.
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.
I thought 40% would be a bit on the strong side. Thats the conc it
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
concentration of that would I use?
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)
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.
Given the following I think Ill give it a miss for the moment, esp as
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
~=A31.75 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
My flux always seems to be glassy, or 'burnt' I think Im heating for to
probably need more practice. Ive only silver soldered 4 things so
Hadnt though of that, thanks for the warning.
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
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
The last category is high temperature brazing fluxes
pretty similar to borax.
For those who silver solder and eat at the same time, brown sauce is a
Feel sick? Being physically sick works as well.
Just thought that I would have a quick burst on my banjo!
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.
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.
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