The percentages given in Table VI are for convenience in comparison and for figuring large heats. The beginner, however, will generally melt but 1 or 2 pigs of copper at one time. These he will weigh first, and then figure the other portions of his mixture from this weight. In this case a formula given in pounds and ounces is much simpler.
From what has been said, it is understood that it is possible to vary these mixtures to meet special conditions. To harden or toughen an alloy, increase the tin; to soften it, reduce the tin. The same is true with zinc, but it will require larger proportion-ate changes in this metal to effect similar results in the alloy.
Phosphorus is not a metal, but is a very active chemical element manufactured from bone ash. It has such an affinity for the oxygen of the air, that in its pure state it must be kept under water, because the slightest scratch would cause it to burn fiercely. It forms the principal substance used in making the heads of matches.
Proportions or Allot
As a rule it is never used in the foundry in its pure state. For the production of phosphor-bronze castings there are several combined forms of phosphorus on the market. The most convenient of these is known as phosphor tin, which is metallic tin carrying various fixed percentages of phosphorus, of which 5 per cent is one very common proportion. Knowing the amount of phosphorus carried by the tin, the exact proportion for the entire alloy may be readily calculated. This element should not be used in alloys containing zinc or lead.
Phosphorus acts as a flux, combining with any oxidized or burned impurities in the bath of metal and driving them to the top. It tends to make the tin crystalline in form, in which condition it unites more firmly with the copper. It apparently unites chemically with copper, making that metal harder. The proportion of phosphorus should not exceed 0.75 per cent, while 0.25 to 0.40 per cent are safer proportions.
Two typical mixtures, one using 5 per cent phosphor tin; are given in Table VII.