CHAPTER III

Fluxes Used in Soft-soldering

=Why a Flux is Required.=--The great essential to successful soldering is the chemical cleanliness of the surfaces to be united, and the proper use of a flux. Although work may be filed or scraped perfectly bright and clean, this is not the kind of cleanliness which is alone sufficient; there is always in course of formation a film of oxide present, and the duty of the flux is to dissolve this and keep any more from forming. Then, and not until then, will the molten solder “run” and spread over faces in the intimate contact necessary. If this vital precaution of cleaning and fluxing is always observed, the difficulties which many beginners experience in effective soldering will vanish.

=Variety of Fluxes.=--There are a good many fluxes employed, including tallow (largely used for lead and pewter), resin (used for lead, compo-pipe, and tinned metals), hydrochloric acid, diluted (for zinc and galvanised iron), and chloride of zinc (the well-known “killed spirit”). The last-named is the most generally used, being suitable for tinplate, tinned iron, new zinc, copper, and brass. Sal-ammoniac is also utilised, sometimes in conjunction with chloride of zinc. The small worker who does but a moderate amount of soldering will find it convenient to use a soldering paste such as “Fluxite,” which is sold in a tin, and can be kept handy and applied to the work with a sliver of wood. “Tinol” is a paste flux in combination with a solder.

=Preparing Zinc Chloride (“Killed Spirit”).=--Make this flux at home from finely snipped new sheet-zinc and _pure_ hydrochloric or muriatic acid. (This is sufficiently cheap at any working druggist’s stores, and infinitely preferable to the contaminated oil-shop quality known as “spirits of salt.”) Stand the acid outdoors in a stoneware crock, add the zinc cuttings a few at a time at first, and when the first violent ebullition moderates, put in the rest. Be sure to provide an excess of metallic zinc, observing that a quantity remains undissolved after all chemical action ceases. Leave the metal in the liquor for twelve hours (covering the crock with a pane of glass), then decant and filter into a wide-mouth glass jar of handy size. _Do not add water_ to the concentrated zinc chloride solution; dilution is sometimes recommended, but should never be done; the heavy, slightly syrupy, water-bright liquor should be used as it is. The alleged “cleaning” qualities of this chloride can scarcely be admitted to exist, and its principal function is to shield the surfaces of the work from oxidation; this it fulfils by the formation of a viscid glaze on the heated metal when the salt reaches its anhydrous (waterless) condition by evaporation. The addition of water to the flux, therefore, only uselessly prolongs the period occupied by evaporation, and wastes heat.

Always remove all trace of flux from finished work, first by soaking in water, and afterwards by washing with soda, soap, and water. Otherwise, there is the risk of the work being corroded.

Special “soldering solutions,” obtainable ready prepared, should not be used in preference to zinc chloride made as before explained or to the well-known paste fluxes.

=Applying the Flux.=--A short heavy bottle about 3 in. or 4 in. high is best for bench use as a flux container. It should be particularly noted that soldering and soldering tackle should be kept as far away from other work (and iron and steel goods and tools) as possible.

A pointed wooden stick is not a good tool for applying killed spirit, because the acid acts on the wood, which becomes unpleasant to handle, and the liquid does not leave the wood readily enough to place the right quantity on the exact spot to be soldered. A galvanised iron wire is better. Another good tool is a thin steel or iron “spit,” about 12 in. long, and a steel knitting-needle is also excellent. Should a brush be preferred, take a few hairs from a broom, place them in one end of a thin metal tube, and then flatten the end with a blow from a hammer.

A brush made by hammering the ends of a short length of cane until the fibres are like bristles is frequently used for the purpose, the handle end being soaked in molten wax before using the cane brush the first time.

A convenient method of applying liquid flux is to have a bottle with a screw cap sprinkling top such as is often used for perfumes, and to push a length of thick galvanised iron wire through the orifice in the stopper, leaving about 1-1/2 in. projecting above. The lower end should just reach the bottom of the bottle, and may be flattened and pointed. The lead nipple is squeezed round the wire to hold it firmly, and the projecting end bent into a ring to form a handle, as shown in Fig. 1. The cork part should be thinned a little to render it an easy fit in the bottle neck. The flux can be quickly applied with the wire exactly where it is wanted, and in very small quantity; for a long seam the wire can be run along with one dip in the solution. The fingers need not be brought into contact with the flux; the cork will not go soft and will not sink down owing to the lead flange supporting it.

=Flux Used on Food Containers.=--Killed spirits is objectionable as a flux for soldering vessels intended to contain food of any kind. Not only is this flux a poison, but it is liable to produce subsequent rusting wherever used unless all traces of it are thoroughly removed immediately after soldering. A good non-poisonous flux suitable for tin boxes may be made by dissolving resin in oil. Place a quantity of powdered resin in an iron vessel, add colza, olive, or any similar oil, and apply gentle heat, meanwhile stirring it until the resin is dissolved. Dissolve as much resin in the oil as possible without making the flux too thick (when cold) to apply with a brush. One or two small experiments will soon decide the required proportions. The resin is really the base of the flux; but the oil is added to facilitate its application and removal before and after the soldering process.