Composition of Flux
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.