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Need a good description of flux

Hi All,

Could you provide a good description of flux, it’s various components
and styles and what it does exactly?

Why is Postassium Biflouride in flux a bad thing, meaning, why is
flouride free flux a good thing.


Karen Christians

Flux is a substance that helps clean and helps the solder flow into
the join. Flourine is a very corrosive chemical and when combined
with hydrogen (water).makes hydrofluoric acid which is probably the
most corrosive and hazardous acid


Karen – In case you were not aware–; fluorides can be very
corrosive to flesh, lungs, etc. etc. Very dangerous! (A drop of
(concentrated) hydrofluoric acid on your hand would probably eat all
the way through to the other side.)

Margaret the chemist

Could you provide a good description of flux, it's various
components and styles and what it does exactly? 

Well, Karen, the easiest way is Wikipedia:

Almost TOO much, maybe, and a heavy tilt towards soft soldering, but
especially the first few paragraphs are pretty good.

I don’t know about potassium bifluoride except it’s fairly toxic and
corrosive in it’s pure form. That’s because I don’t know what the
fluoride in fluoride flux, like Battern’s, actually is. But I’ve
used that my entire career, I love it and have no problem whatever
with using it every day.

BTW, going backwards - The words “fluorine” and “fluoride” come from
fluorite, the mineral. The name fluorite comes from Latin roots
meaning “to flow”. The property of fluorides to aid in the flow of
molten metals was known long before anybody knew what fluorine was.

I googled flux and potassium fluoride and came up with the

Flux is explained in section 3:

According to this, potassium fluoride is poisonous if ingested and
breathing it can cause upper respiratory problems:


Most fluxes, like Borax, when heated become molten glasses - surface
oxides dissolve in the glass and surface particulates become
suspended in the glass and are both are carried away by the liquid
glass thereby cleaning the metal surfaces so the solder can wet and
bond to them.

Flux is a substance that helps clean and helps the solder flow
into the join. Flourine is a very corrosive chemical and when
combined with hydrogen (water). makes hydrofluoric acid which is
probably the most corrosive and hazardous acid 

Excellent. Thank so much. Sometimes its the last piece of
that is so important to round out the whole. I know what
flux is, what it does and when it has exhausted its protective
properties during heating. It was the missing flourine reaction that
I could never find. Of course. Makes perfect sense.


Thanks Margaret. The chemistry of the products we use is important to
our health. We brush our teeth with fluoride and we drink it in our
water. So why is bad when we use it in our flux. Now I understand.


Hi Karen,

I assume that your query is prompted by needing or wanting to solder
(precious) metals.

To solder successfully it helps to understand how it works. Soldering
refers to the process whereby pieces of metal are joined by melting
another metal (the solder) so that it wets the joint and holds it
securely when it freezes. That’s basically it, but there are a few

The important word is “wets”. It’s just like water. Put a nice clean
pipe in water and freeze it; it’s then difficult to get the pipe
out, but use an oily pipe and it’s much easier. The difference is
that the water wetted the clean pipe, but couldn’t wet the oily one.
If the pipe wasn’t oily but dirty, the effect would be the same.
Solder works in exactly the same way. So all you have to do is make
sure the joint is nice and clean and you’ll get a good joint, right?
Well, no, it’s a little more complicated than that, but not a lot.

To melt the solder you have to heat it up. The trouble is that the
very act of heating it also makes it dirty. The oxygen in the air is
only too eager to oxidise everything it touches, and, as far as
solder is concerned, metal oxide is dirt. Most metals oxidise rather
slowly at room temperature, but heat them up and the effect is very
rapid - so rapid as to make soldering impossible, unless you prevent
the oxygen from reaching the hot joint.

There are only three ways to do this: solder in a vacuum, solder in
an atmosphere devoid of oxygen, or coat the joint with an oxygen
barrier that can withstand the heat. The first two options are rather
impractical, but the third is fine; the barrier is called a flux.
The job of the flux is to cover the joint with a barrier to stop the
oxygen from oxidising it. Resin is a good flux for soft solders, and
borax for hard solders or brazing. If the joint and the solder are
nice and clean then these fluxes work rather well, but that’s all
they do; they are known as inactive fluxes. If the joint is a little
dirty these simple fluxes do nothing except act as a barrier, but
there are others that can do a small amount of cleaning too. Baker’s
Fluid is one such active flux for soft solder and EasyFlo is one for
hard solder. (I use a product called Auflux, which is a sort-of
luminous green liquid. It is used straight from the bottle and
doesn’t froth up as much as the powdered fluxes.) The active fluxes
are certainly better than the inactive ones, but they are not magic,
you shouldn’t rely on them to do the cleaning for you. Active fluxes
usually contain some rather toxic and dangerous additives (potassium
biflouride?) which do the cleaning bit.

So, now you have a nice clean joint and a flux, so it’s plain
sailing, yes? Well almost. Most problems are caused by heating the
solder rather than the joint. All that happens is that the solder
melts, goes into a ball, and refuses to flow into the joint because
it freezes before it can wet it. The secret is to heat the joint, not
the solder. When the joint gets hot enough it will melt the solder
which will then flow nicely into the clean, fluxed joint. The final
thing that can go wrong is to burn the flux. If you heat the flux for
too long, longer than a minute or so, it will lose its properties and
allow the oxygen to pass. This is normally the result of insufficient
heat, so if it happens, remove the heat, clean the joint, and start
over, perhaps with a better source of heat, or better insulation to
prevent the heat from leaking away.

So, the four main points for a good soldered joint are:

1. Make sure the joint and solder are both clean.
2. Use a good appropriate flux.
3. Heat the joint not the solder.
4. Complete the joint quickly.

I hope this helps.
Regards, Gary Wooding

The chemistry of the products we use is important to our health. We
brush our teeth with fluoride and we drink it in our water. So why
is bad when we use it in our flux. Now I understand. 

I am not trying to down play the fact that potassium bifluoride is a
toxic chemical. But equating it to hydrofluoric acid is like saying
table salt (sodium chloride) is dangerous as hydrochloric acid.
Potassium bifluoride is a potassium salt of hydrofluoric acid and
table salt is a sodium salt of hydrochloric acid. They are both
composed of corrosive elements, chlorine and fluorine are quite
similar in many ways like their corrosive abilities and toxicity.

That said potassium bifluoride is not something to be handled
carelessly. There are many flux formulations but the main ingredient
is borax often with boric acid as well these are the fluxing agents
they combine with metal oxides and strip the oxygen out of the metal
oxide when they are at working temperature and also provide a molten
glassy covering on the area to inhibit oxygen contact with the
metal. The other fairly common additives to fluxes we use are
potassium bifluoride ammonium chloride, sodium hydroxide, possibly a
wetting agent like trisodium phosphate. If possible many of the
chemicals used in flux for torch use are formulated from the
potassium salts rather than the sodium salts to reduce the intense
yellow sodium flare that you get when heating sodium compounds that
makes visibility difficult when the torch is on them.

The chemicals besides the borax boric acid fluxes are there to
further clean the metal, lower the melting point of the flux, make
the flux more fluid, make the flux residue more soluble in water
after fusing etc.

You don’t absolutely need to have those other chemicals in the flux
to solder but they make the flux much better at what it does.

James Binnion
James Binnion Metal Arts

I’ve been listening in to these posts as I have no confidence when it
comes to chemistry. I was using lab grade alum as a pickle, and the
flux paste we used at Uni (in Aus it is simply called tenacity 4A
etc) When the lab grade alum, which resembled sugar in consistency
became very hard to find and extremely expensive, I bought a bag of
alum (farm grade for use in dams as a water flocculent) which worked,
had a very chalky gritty consistency and was much more smelly.
However after buying an ultrasonic and reading that the pickle could
damage my machine, I moved across to sparex as a pickle, but a local
jeweller I am friends with told me I could just use the liquid flux
most jewellers use, as it is far easier to see what you’re doing.

I am trying very hard NOT to use overly harmful chemicals in the
studio; even in the Ultrasonic I am using a “green” powder that’s
been put out to prevent the issues of disposal etc.

I would love someone such as yourself James, or any of the more
experienced and confident practitioners to make up a list of safest
to least safest studio use chemicals in regard to soldering,
cleaning etc. I have looked through every book I own, and often there
is the assumption you know this or you have a chemistry
degree and you understand the technical info. Something in between,
put together by people who understand the various compositions would
be brilliant for someone like me. I will remember your phone number
for years, I can tell you exactly what you ate and what you were
wearing last time we met, but I can’t remember complex mathematical
equations and I have no idea how often I have looked up chemicals,
only to forget the whole thing by next day. Cheers, Thankyou.

Kathleen. Southern Vic, Aus.