It is of the utmost importance that plumbers should know something about the principal metal they use - namely, lead. The Latin name for lead is "plumbum," hence we get the title of "plumber " - a worker of lead. This title is only given to men who use lead when in a manufactured state, and not to those who extract it from the ores.

Lead Ores

There are several ores, but only two which contain lead in sufficient quantity to extract the metal from on a large scale: Galena - sulphuret of lead, lead, and sulphur; cerussite - carbonate of lead, lead, oxygen, and carbonic acid. The sulphur, oxygen, and •carbonic acid are the chief impurities which have to be removed to get the pure metallic lead. Antimony, copper, and iron are also found in some specimens of galena, and nearly all contain silver.

Lead ores are found in several places in the British Isles, in the United States, Spain, Saxony, Australia, and other parts of the world.

Smelting Of The Ores

The ores are first prepared by being sorted by hand, and then broken or crushed to small pieces. They are then washed to remove earth or any soluble matter. The sulphur has to be expelled by means of heat, and this process is called roasting. This operation is conducted in different places in different ways, which depend to a •certain extent on the impurities which the ores contain. The principle is nearly the same in all cases, and consists in roasting the prepared ores in a kind of large oven called a reverberatory furnace.* This is shown in section at Figure 1, where A is the hearth, B the fire, and C the chimney. The ores being spread over the hearth and the fire lighted, the flames rebound from the arch on to the ores, the sulphur in which enters into combination with the unconsumed oxygen brought in with the flames, and passes away up the chimney as sulphurous-acid gas. Part of the lead also combines with oxygen, forming oxide of lead. This being again roasted with fresh sulphide of lead, the sulphur and oxygen combine and pass away as sulphurous-acid gas, leaving the lead in a metallic state. Sometimes lime is thrown into the furnaces to act as a flux to form any earthy matter into a slag.

* Reverberate - to bound back.

Separation Of Other Metals

Antimony, copper, and iron have the property of making lead very hard, and should always be extracted when soft lead is required. This extraction is done by means of the calcining or roasting furnace, which is a reverberatory furnace with a large, shallow cast-iron pan instead of a hearth. The melted metal is exposed to the action of the flames, when part of the lead, the copper, iron, and antimony are converted into oxides and float on the surface of the metal, and are skimmed off at intervals until the lead acquires the requisite degree of softness. Type-metal is an alloy of lead and antimony in proportion of about four of lead and one of antimony. Sometimes a small proportion of tin is added to improve it.

Figure i.

Extraction Of Silver

Silver is separated from lead by cupellation and by the Pattinson process. Mr. Pattinson's process is to melt the lead containing silver and to allow it to cool slowly, when part of the lead crystallizes. These crystals are fished out with a perforated ladle, the portion remaining being rich in silver. This is then placed in a cupel, when the remainder of the fluid lead is converted into an oxide of lead, sometimes called litharge, the air-blast used for this purpose driving the litharge over the further end of the cupel. The silver, which is not affected by oxygen, remains behind, cools, and sets in a large cake. Before the introduction of Mr. Pattinson's process the whole of the lead had to be converted into an oxide, but since this discovery it is only the liquid portion left after the lead crystals are fished out that has to be so converted. This saves a great deal of time, as the oxygen has to be expelled from a much less quantity of lead to reconvert it into its metallic condition. Some lead ores are richer in silver than others, but the value of silver is such that two parts in a thousand will pay for extraction.

Physical Properties

The physical properties of lead make it one of the most useful of metals.

Its tenacity is so low that it is useless for any purposes where strength is required; neither can lead be rolled into very thin sheets, because of its want of tenacity. Lead, on account of its softness, is very malleable, and easily worked without the aid of heat, but because of its low tenacity care must be taken not to reduce its thickness too much, or it will break into holes. The application of heat, which should not exceed about 3500 Fahr., makes lead softer and more malleable, but a higher degree of heat makes it brittle, while at about 6200 Fahr. it is melted or converted into a fluid state.

The ductility of lead is so very low that it is impossible to draw it into a thin wire, and even if made into wire its want of tenacity prevents its application to any useful purpose where strength is required.

Lead is very heavy, the specific gravity being 11.36, or about 11 1/3 times heavier than an equal volume of distilled water at a temperature of 60° Fahr.

Lead is not a good conductor of either heat or electricity. Compared with silver or copper, its conducting power is only about one-twelfth.

Fusibility

The low temperature at which lead melts renders it of great utility for certain purposes, such as for making into pipes; for casting into useful forms without the aid of a blast furnace; for fastening iron into stone-work, although this is not so much practised now as formerly, on account of a galvanic action setting up between the metals when exposed to moisture, which results in the iron being eaten away.