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From: "Barry L. Ornitz" <>
Newsgroups: rec.crafts.metalworking
Subject: Soldering Flux
Date: Sat, 30 Oct 1999 18:19:20 -0400

In an offline conversation with Alan Gouldburn, we discussed
solder fluxes.  I thought the group might be interested in some of
this too.

The common "acid" soldering fluxes usually contain zinc chloride
and/or ammonium chloride.  They may also contain some free
hydrochloric acid.  "Killed Acid" is just muriatic acid to which
zinc metal has been dissolved in to produce a solution of zinc
chloride.  Zinc dissolving in hydrochloric acid releases hydrogen
so keep all sparks and flames well away.

Ammonium chloride when heated decomposes into ammonia and
hydrogen chloride gas (vapor form of hydrochloric acid).  If metal
oxides are present, the released hydrochloric acid usually
dissolves them and promotes adherence and wetting of the solder.
You can produce a "cold smoke" by opening a bottle of household
ammonia near an open bottle of hydrochloric or muriatic acid.  The
fumes from both bottles will react to produce ammonium chloride
which is white and looks like smoke.

The action of zinc chloride is a little more complex.  Normally it
is present in water solution or in its hydrated form.  When
heated, the moisture and zinc chloride react to produce free
hydrochloric acid and zinc oxide.  They hydrochloric acid again
reacts with the metal oxides to dissolve them.  The zinc oxide
usually mixes with the metal oxide and chloride "crud" to produce
dross around the edge of the solder joint.

In neither case is free chlorine gas released.  The amount of
hydrogen chloride gas (hydrochloric acid) produced is also small,
and it generally immediately reacts with the metal.  So unless you
are in an enclosed space and having to breathe the vapors all day,
they generally never reach dangerous concentrations.  Still, it is
best to avoid these vapors when possible.

In electronic soldering, a different non-corrosive flux is
generally used.  This flux is commonly called rosin flux because
purified pine rosin is often the main ingredient.  Other natural
rosins and gums may be used too.  All contain abetic acid which is
capable of dissolving copper oxides.  Different grades of
electronic flux are available.  When soldering freshly tinned
surfaces, almost pure rosin is used.  When soldering tinned
surfaces that have oxidized a little, a more "activated" flux may
be used.  The "activation" means some zinc or ammonium chloride
has been added.  Very little is actually added as residual zinc
chloride will corrode the copper in the future.  When soldering
circuit boards, use the mildest flux that will work.  It is best
if the flux is washed from the boards after soldering if activated
flux has been used.  Commercial flux removers are available for
this; most contain mixtures of alcohols and chlorinated solvents.

As I mentioned, pine rosin is often the main ingredient of the
flux.  In a pinch, I have used raw rosin scraped from a pine tree,
but the smell is pretty bad.  [Still, I prefer it to the perfume
several manufacturers added to solder in the 1960's that smelled
of cheap incense.  But hey, it WAS the 1960's.]  It is best to
avoid breathing the fumes of any of the electronic grade fluxes
too.  For electronic work, I prefer the solders with a flux core.
For precision work, I like the Alpha Metals 63/37 eutectic solder
with a completely non-corrosive flux.  For most work, however, I
prefer the British 60/40 Ersin solders.  In my opinion, Kester is
my last choice in solder.

If you need some extra rosin flux, go to a music store and buy a
block of rosin like those used by violin players.  Ask for the
oldest, dried-out block which should make the store owner happy
(the musicians prefer the soft, sticky ones).  Dissolve as much of
this as you can in a small glass bottle containing acetone.
Remember acetone is VERY flammable.  But it evaporates quickly
leaving the rosin behind, ready for soldering.  One block will
normally last a lifetime.

If you ever have occasion to remove solder from an existing
soldered joint, you can buy a commercial product called "Solder
Wick" or something similar.  [I have a vacuum de-soldering iron, so
I rarely use the stuff.]  This is merely copper braid dipped in
flux.  You can easily make your own from old braid-shielded
cable.  Old Ethernet networking cable is often available for
dumpster-diving and it works well for this.  Remove the outer
insulation from a short section.  Then slide the braid off and
stretch it out.  Dip the end in your flux and place it on the
joint.  Touch the braid with a hot soldering iron and the braid
will soak up the solder like a sponge.  Cut off the end, dip in
flux, and you are ready to do it again.  The commercial wick may
work slightly better, but this is about a cheap as it gets.

Flux for soldering aluminum and brazing fluxes have far more
complicated chemistries.  These best selected for the particular

        Barry L. Ornitz

As always, remove the NOSPAM to reply.

From: "Barry L. Ornitz" <>
Newsgroups: rec.crafts.metalworking
Subject: Re: Soldering Flux
Date: Sun, 31 Oct 1999 16:37:32 -0500

Doug Goncz wrote in message
>>purified pine rosin is often the main ingredient.  Other natural
>>rosins and gums may be used too.  All contain abetic acid which is
>>capable of dissolving copper oxides.
>Acetic acid?

No, abeitic acid like I said, C20H30O2.  An older name is sylvic acid, the
name coming from the Latin reference to trees.

>>Flux for soldering aluminum and brazing fluxes have far more
>>complicated chemistries.
>Both aluminum and copper can be brazed without flux. For aluminum,
>rubbing with a stick of filler dislodges the oxides, and for copper, they
>sort of disappear at higher brazing temps, allowing a copper phosphorous
>alloy to wet without flux.

Actually aluminum brazing requires flux according to the book I have from
Alcoa.  The aluminum solders do not, but these melt at quite low
temperatures, around 400 F.  At the temperatures of aluminum brazing, which
is only 20 to 100 degrees F below the melting point of the aluminum, the
oxides form much too quickly unless flux is present.

The phosphorus in silver solders acts as a flux when brazing copper.  As
far as I know, the bronze rods for brazing iron and steel all require flux.

>Maybe gold also, that very noble metal. Never tried it.

Gold does not oxidize enough to require a flux.

        Barry L. Ornitz

From: "J. Kimberlin" <>
Newsgroups: rec.crafts.metalworking
Subject: Re: silver soldering again .. failure
Date: Mon, 20 Dec 1999 07:09:44 -0800

Grant Erwin wrote:

> I've tried again, this time just trying to tin one end of a 1/2" x 1" x 2"
> piece of steel, more precisely one end of the largest flat side. I think
> the flux is failing.

After reading all the previous very good observations and advice,
it seems to me that you have answered your own question.  Flux
failure during heat soaking is quite common.  In the first place
steel is a bear to silver solder.  I generally use a 15%
silver/phosphorous flux silver solder stick as it seems to flow a
bit easier for me.  On steel, I usually use one of those fluxes
that is meant for oxy-acet brazing using a brass brazing rod.
They are higher temperature.  You won't need the higher
temperature but steel doesn't conduct heat very fast compared to
copper, and suddenly, you are overheated and the lower temp flux
has gone useless.  If possible, try to get a flux that is proper
for a silver solder that contains much less than 45% silver.
Those fluxes work at higher temperatures than the ordinary SS
flux, but will still get liquid at lower temperatures proper for
the 45%.


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