Composition of Flux

by Marta

Why do you need to wear a fume mask when soldering? What is the chemical flux compositon? Why does solder bond with copper foil when using flux?


The composition of each type of flux is a bit different, but the basic ingredients are a mixture of sarcosine and tartronic or tartaric acid dissolved in water and an aliphatic alcohol. The flux is employed for tin/lead soldering of a range of metals and alloys including copper, nickel and nickel-iron. Break down of the flux is stopped by the addition of lead carbonate, phenol or salicylic acid in the composition.

That should tell you why you need to wear a fume mask or use a fume extractor. You don't want to be breathing acidic fumes, as well as lead fumes (from the solder).

The reason we use flux for soldering is: The metals we work with, which include copper, oxidize when they are exposed to air. We can't always see the oxidation, but it's there. Solder will not adhere to oxidized metal. Flux cleans the metal so that solder will bond to it.

There is one natural flux that some people use, especially in the UK. It is called tallow. Tallow is a rendered form of beef or mutton fat, processed from suet. You would still need to wear a mask or have a fume extractor running near where you are soldering.

I hope this answers has been helpful.

Comments for Composition of Flux

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Jul 28, 2010
How do you Clean Solder Flux
by: Dexter

Are all solvents applicable? Hope you could advice.

In my opinion, if you're pregnant, stay away from all solvents. Don't take a chance in harming your baby.

Nov 20, 2008
Soldering While Pregnant
by: Tracey

I am a beginner. I took a class a while back and decided that I would make some small stained glass christmas ornaments this year to get back into stained glass. I am also 3 months pregnant and didn't even think about soldering as being something I should avoid while pregnant. Do you think, in your opinion, that I should not solder while pregnant? Or do you think that I would be ok (it would not harm my baby) if I used a fume mask? I know you aren't a Doctor...but I just thought I would ask your opinion.
Thanks, Tracey

Awesome site, by the way!


Hi Tracey,

This is such an important topic and I'm glad you brought it up.

Do not solder while you're pregnant. This is not just my opinion, it's common knowledge that all stained glass instructors should pass on to their students.

Don't do anything that creates fumes or involves using chemicals. Don't take a chance doing something that could harm that precious baby.
Spend your time finding patterns for future projects. You can cut and foil glass (wearing a mask when grinding),then put the foiled pieces in a zip lock bag to prevent them from oxidizing. They'll be ready to solder after you've had the baby.

Have a safe and worry free pregnancy.

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