Physical Properties. — Copper is distinguished by its red color from all other metals.

Malleability and Ductility. — It can be rolled into very thin sheets beaten out into leaves and drawn into fine wire, and thus possesses the properties of malleability and ductility in a high degree. By cold rolling or other mechanical treatment it becomes hard, but its mallea­bility is regained by annealing. It is immaterial whether the copper be quenched in water or cooled slowly after the annealing process.

Tenacity. — Its tenacity when cast is 8 to 10 tons per s.i.; when rolled or drawn 16 to 23 tons or even more, according to the amount of mechanical treatment it has received.

Specific Gravity.— The specific gravity of pure Cu, rolled, forged, or drawn, and afterwards annealed, may be taken as 8.89 at 20°C, but that of ordinary commercial copper usually ranges from 8.2 to 8.6.

Action of Heat. — The melting point of Cu lies somewhat about 1,083°C. When molten it is rapidly oxidized with the formation of cuprous oxide (Cu₂O), which dissolves in the metal. It also absorbs hydrogen, carbon, monoxide, and sulphur dioxide, which are given off during solidification, part, however, remains in the metal producing more or less porosity. Hence arises the difficulty of obtaining sound casting of copper *.

The metal is not volatilized at the temperature of ordinary furnaces, but readily in the electric furnace, its boiling point being about 2,100°C. When heated to near its melting point it becomes so brittle that it may be easily powdered.

Electrical Conductivity. — As a conductor of electricity copper is only surpassed by silver, and hence is largely used for electric wires and cables. Its electrical conductivity is 976 if silver be taken as 1000. This is greatly reduced by the presence of impurities, especially by cuprous oxide, phosphorus, arsenic, antimony, and silicon.

Conductivity for Heat. — Copper is an excellent conductor of heat,/ and in this property is about two and a half times more efficient than iron.

Atomic weight is 63.57.

Chemical Properties. — Copper undergoes no change in dry air at ordinary temperatures, but in moist air a green coating of basic car­bonate is formed.

When heated to redness with access of air, as in annealing sheets, etc., a dark colored scale is formed, which consists almost wholly of cuprous oxide. It may be removed by plunging the copper when red-hot into cold water; in practice the water used contains sulphuric acid. Copper is slowly dissolved by weak acids in the presence of air. It is soluble in nitric acid, "aqua regia", and in hot concentrated sulphuric acid.

Copper Oxides. — There are two oxides of copper — viz. red or cuprous oxide (Cu₂O) and black or cupric oxide (CuO).

Cuprous oxide is readily fusible at furnace temperature, and at a red heat is easily reduced by hydrogen, carbon monoxide, charcoal or other carbonaceous matters, and by iron, zinc, and metals having a strong affinity for oxygen. When heated with iron sulphide or cuprous sulphide it is reduced to metallic copper, according to the following equations —

3Cu₂O + FeS = 6Cu + FeO + SO₂

2Cu₂O + Cu₂S = 6Cu + SO₂

Cuprk Oxide. — This oxide is as easily reduced as cuprous oxide and by the same reducing agents, but it is not fusible.

Alloys of Copper. — In addition to the extensive application of copper itself, it is largely used in the manufacture of alloys. Its alloys, especially those with zinc (the brasses) and with tin (the bronzes) are of vast importance for engineering and other purposes, while its alloys with other metals have numerous uses in many industries. When copper is cast in what is termed a closed mould — i. e., a mould with a small aperture or "ingate"— unsound vesicular castings only can be produced. To obtain a sound casting the mould must be an open one, hence none but articles of simple forms can be cast of the metal. This is why alloys of copper have come into such extensive use.

Brass is an alloy of copper and zinc in various proportions. The color of brasses varies widely with the composition. When increasing amounts of zinc are added to copper, the resulting brasses show a range of color all the way from copper red, which persists for about 5 pct of zinc through a bronze color at about 10 pct, a golden color at about 15 pct and increasing dilution of color to the typical brass yellow at around 30 pct of zinc. With more than 38 pct of zinc (alloys obtainable only in rod form) the alloy again takes on a buff red cast. This question of color is tremendously important in some applications, particularly costume jewelry, fasteners, fuse boxes, architectural trim, and so forth. In the range from about 80 to 90 pct of copper, the color changes very rapidly, and for such uses a given nominal alloy must be held within rather narrow limits in order that different parts of an assembly may not show variations in color.

Red brass, containing 85 pct of copper, is, next to cartridge brass, the most important of the nonleaded brasses, being produced in very large quantities in all the common forms but particularly favored for pipe for use in plumbing because of its high resistance to aqueous corrosion, and for a host of manufactured parts for which a combination of ductility, good fabricating properties, golden color, and ease of polishing render it especially well suited.

A 90-10 mixture, commonly known as commercial bronze because of its color, finds a wide field of application for such things as angles, channels, costume jewelry, etching bronze, grille work, hardware, projectile rotating bands, screen cloth, screws, rivets, and many others.

The brasses containing from 61 to 54 pct of copper are called "Muntz-metal". The essential features that distinguish the Muntz metal brasses from the plain brasses are extreme plasticity at red heat followed by conditions tending in the opposite direction when cooled to room temperature; viz., these alloys can be worked hot easily and drastically by rolling, extrusion, and other methods, but are not suitable for severe cold-working operations. The former condition makes for cheapness of production because heated billets may be almost instantly extruded through dies into rod or other shapes at a minimum expenditure of energy, and the long extruded rods may be finished by a cold-drawing operation to give them the exact dimensions and mechanical properties required by a given specification.

Moreover, the qualities of hardness, stiffness, and strength, not softness and pliability, are desired in countless objects such as screws and other shapes rapidly turned from rod on high-speed automatic screw machines. The necessary shortness of chip or general free-cutting quality is secured by adding amounts of lead up to approximately 3.5 pct. Some manufacturing operations are favored by combining hot-working in the breaking down or initial processing followed by a series of cold-working and annealing operations substantially as would be conducted with plain alpha brasses.

The alloy that is used in greater quantity than all others in this group combined is free-cutting brass, containing 61.5 pсt of copper, 3 of lead and 35.5 of zinc. It can be cut at high speeds with low tool pressure, causing minimum rate of tool wear and with very short chips that clear the tool well. However, since free-cutting brass does not lend itself particularly well to cold-working procedures often required in addition to machining operations, a medium-leaded or high-leaded brass, or perhaps leaded Muntz metal, might be used in preference; viz., a compromise is necessarily made between optimum machining properties obtainable only with the higher lead content and good cold-fabricating properties obtainable with lower lead content (and usually somewhat higher copper content).

Where hot pressing or hot forging is to be used as the principal shaping procedure, forging brass containing nominally 60 pсt of copper, 2 of lead and 38 of zinc is by all odds the favored material. This alloy, which is extruded readily, also can be forged into intricate shapes over a wide range of temperature, and the 2 pсt of lead facilitates extensive and ready machining thereafter.

In fabricating various articles from strips involving drawing or forming operations and where some machining operations are also necessary, various lead-bearing alloys are available, including low-leaded brass, medium-leaded brass, high-leaded brass and extra-high-leaded brass. The choice between cartridge brass containing no lead and one of these four lead-bearing brasses will depend on the relative importance of the cold-fabricating operations necessary, on the one hand, and the degree of machining required, on the other, it being impossible to attain the maximum of both these qualities in any one material.

Of the tin and aluminum brasses admiralty and aluminum brass are used almost exclusively for heat-exchanger tubes, primarily because of their high resistance to most conditions of aqueous corrosion encoun­tered in such service. The admiralty alloy, which has been a leader in this field for many years, is now usually modified by the addition of a few hundredths of one per cent of an element from the group including antimony, arsenic, phosphorus, and silver, for the purpose of inhibiting dezincification (preferential removal of zinc by corrosion, leaving spongy metal), which otherwise might occur under some con­ditions of operation. The aluminum brass is more specifically used where high water velocities are encountered, as for instance in marine condensers, it having been found highly resistant to so-called impingement attack frequently resulting from such conditions of use. It also is now currently made with an inhibitor, as is the admiralty mixture.

The plain and leaded naval brasses and manganese bronze are widely applied where a high-strength structural material resistant to salt-water corrosion is needed. In point of strength, the manganese bronze is distinctly superior to the naval brasses. A leaded variety of the latter is of course used in preference to the nonleaded one when extensive machining is necessary.

Phosphor bronzes containing 5, 8, or 10 pсt phosphorus are the pre­ferred alloys for springs or other applications requiring high strength, great resiliency, and corrosion resistance. The 1.25 pсt phosphor bronze finds its principal application in electrical contacts, flexible hose, and pole-line hardware.

Although cupronickels of varying nickel content up to some 40 pсt are often employed, by far the most important is the 30 pсt nickel variety* presently considered to be the outstanding alloy for heat-exchanger use, owing to its exceptionally high resistance to most of the conditions of corrosion there encountered.

The two nickel silvers listed are in demand mostly for their color, which is desired as a base for plated flat and hollow tableware, for zippers, costume jewelry, nameplates, radio dials, and other articles.

The silicon bronzes are general-purpose structural alloys of very wide application where a combination of high strength, great toughness, ease of forming, resistance to corrosion, and other properties are essential.