This section is from the book "Soldering For Workshop, Farm And Home", by John Bonert. Also available from Amazon: Soldering For Workshop, Farm And Home - Information On Soft And Hard Soldering - Projects For The Workshop Explained And Illustrated.
Flux is applied to the surface of metals that are to be soldered. It helps to keep oxide from forming and makes the solder flow freely. A demonstration of this can be seen when tinning the soldering copper. Clean the point of the copper and apply enough heat to just discolor the copper. This discoloring is a coat of oxide and runs to a brown color. It is caused by exposure to the air while the copper is heating. If tallow candle or rosin is applied the bright metal will reappear. But if the copper is heated beyond this point, the oxide will be too heavy for the flux to remove it. In this case the copper will have to be allowed to cool somewhat and be filed over again. It would be impossible to do a soldering job without flux. The following information on fluxes will be of benefit to the beginner and as he progresses he should try to memorize it.
Rosin-used on, Brass, Bronze, Copper, Tinned Iron,
Iron, Iron, Steel, Zinc.
Stearine } used on, Copper, Lead.
Soldering Paste (non-corrode)-used on, Copper,
Brass, Bronze, Tin. Sal-Ammoniac-used for tinning soldering copper.
Rosin can be used in solid form or it can be powdered by hammering. Tallow candle such as is used by plumbers will be used for much of the work in the lessons. Do not use ordinary paraffin candles as they will not do. Sal-ammoniac comes granulated and in solid form. The solid is recommended for tinning the copper. Rosin, tallow candles and sal-ammoniac can be purchased at plumbing supply houses. Non-corrode soldering paste comes in small tins and can be bought at the hardware store.
The beginner must be warned not to use soldering paste or zinc chloride for radio work on account of their corrosive effect on insulation and apparatus. Rosin and tallow will solidify as soon as the soldered surface has cooled, but zinc chloride or soldering paste will remain wet and creep beyond the point soldered. Where an insulated wire is soldered to a lug the zinc chloride or soldering paste would soon get to the insulation where corrosion would occur. For this kind of work rosin-core solder is best. Where zinc chloride is used always wash the work with water after the soldering is finished to remove any that may remain on the surface. In addition to the fluxes mentioned there are a number of formulas for making your own. As the book is meant to help the beginner the fluxes given will be sufficient. In figure (1) directions for making a small brush to be used for applying zinc chloride are given. Figure (17) shows a small scoop for applying borax when hard soldering is being done. This can also be used for applying powdered rosin.