Solder is a mechanical mixture of tin and lead, a fact which is susceptible of very simple demonstration. A bar of solder of a grade even as low as 30 per cent, tin and 70 per cent, lead, passed through a buffing machine, will show a surface practically identical with that of a bar of second-quality or reclaimed tin. The buffings, on chemical analysis, will prove to be almost pure lead.

According to the best practice, solder is made in the following manner. Virgin pig lead is first melted, and when it is thoroughly liquefied, virgin pig tin is added, together with a small amount of flux ; the latter is for the purpose of bringing to the surface the so-called "liver" consisting of impurities that may have remained in either the lead or the tin as a result of incomplete refining. The combined material, when completely liquid is thoroughly stirred for some hours, and is then cast into small pigs. Just before casting, and continuously during this operation, the molten metal yields dross, consisting largely of the oxides of lead and tin; this should be carefully skimmed off.

After the pigs have cooled, they are taken to a smaller kettle, re-melted, and cast into the desired shape for use ; or if wires, ribbons, etc., are to be made, the solder is cast into slugs suitable for extrusion and rolling. During this second operation, the skimming of dross should be even more carefully done than at first.

Hand mixing has proved to be the only reliable method for the production of the best quality of solder, irrespective of its percentages of lead and tin. The best quality of solder is not necessarily that which contains the highest percentage of tin, but rather is that composition which performs best on the required piece of work. In order to produce a thorough mechanical mixture, it is necessary to stir for a long period ; experience has shown that to perform this operation satisfactorily takes from 5 hours to 6 hours, irrespective of the quantity of material being mixed, and also irrespective of the proportion of tin in the mixture, whether 60 per cent., or as low as 30 per cent.

Throughout the casting process what occurs is that the lead solidifies in skeleton crystals until the remaining liquid has the eutectic composition, when it freezes at a constant temperature as a mechanical mixture of tin and lead containing some tin in solid solution. It is remarkable how many shapes these skeletons take. The seeming explanation of this variation is the presence of other metals than tin and lead, in very small proportion, or even traces.

An analysis made, in 1901, of borings taken from a section of a pig of solder at the different points showed the tin content to be as follows : No. 1, 59 06 per cent. ; No. 2, 52-99 per cent. : No. 3, 38-43 per cent. ; No. 4, 39 07 per cent. ; No. 5, 45-62 per cent. ; No. 6, 39-33 per cent.; No. 7, 38 82 per cent. The breaking stress of wire solder of No. 9 Birmingham gauge, 46% tin and 54% lead, was 91 lb; that of 50% was 95 lb. Tin wire of this gauge breaks when subjected to a stress of 120 lb. and lead wire when subjected to a stress of 45 lb.

With the idea of conserving tin, solder should be separated into two classes :-

(1) ;That which is used strictly for soldering, that is, joining and holding together two pieces of metal.

(2) ;That which is used primarily for the filling of an interlocked joint, so as to prevent the escape of the contents of a container. It is these filling metals that offer the greatest opportunity for the conservation of tin. It is only necessary that the metal shall flow into the seam, and solidify into an impenetrable mass.

The greatest abuse of solder occurs in the use of high-tin mixtures for filling metals. A mixture of 25 per cent, tin and 75 per cent, lead, worked at the right temperature and with proper fluxing, is high enough in tin for any filling purpose, as has been demonstrated in the practice of the oil canners, notably the Standard Oil Company.

The filling operation is usually conducted by machinery, but the users have frequently not realised that the baths are considerably richer in tin at the top, through which layer the container is being dragged, than the solder that is put into the baths. When the 40 : 60 solder, most commonly used on automatic can-making machines has not worked entirelv satisfac-torily, it has often been found that the addition of lin. or 2in. to the depth of the bath has made the solder work very much better. Hence, one of the best means of conserving tin in canmaking solder is to deepen all baths, whether on line machinery or for hand dipping, thus permitting the use of a lower-grade solder.

The fact that solder dross contains a higher percentage of tin than the original solder has usuallv been explained on the assumption that tin oxidizes more rapidly than lead. The probable explanation is than in solder baths the lead is gradually worked toward the bottom and the tin to the top, where it is exposed to the oxygen of the air ; thus the oxide of solder is richer in tin than the original solder.

The overheating of solder is not only detrimental to the work, but also causes some, though not a very great, waste of tin through the excessive production of exide. While this oxidation may be a source of considerable expense to the package manufacturer, it is not actually a very serious loss of tin because the reclaiming of these drosses, or oxides, has been so perfected that very little of the original metallic contents is lost. In these days, however, when every ounce of tin should be conserved, both to insure a sufficient supply for the most essential work, and to save the useless transportation of a material which comes such long distances by boat, overheating should be avoided, and all baths should be covered with a protecting material such as sal ammoniac, oil, charcoal, or ash.

In this connection, it should be emphasised that every particle of solder oxide should be preserved, and sent to the reclaimer. A teaspoonful of solder dross contains enough solder to make a 5-gallon can or 100 No. 1 cans. In many plants, even those of some of our largest consumers of solder, this dross is not collected and saved with sufficient care. It is seldom that a thorough cleaning and gathering together of the oxides takes place more than once a week.

Fire will purify these reclaimed materials when properly refined, and in purity they will compare favourably with the virgin materials. However, too little attention has been paid to the proper refining of these scrap metals. Usually they have simply been put into a kettle, melted down, and then brought up or down to the required composition. This is not sufficient. Reclaimed metals are never equal to virgin metals, no matter how much refining they undergo ; nevertheless, for certain classes of work they are economical and efficient. The repeated use of metal affects its physical permanency ; yet the margin of safetv in the use of solder is so large, and the length of time that solder is required to remain on the container is so comparatively short, that any lack of permanency can usually be safely disregarded.