Chapter II: Preface (2)

The uses of the copper alloys may also be arranged in two classes—(_a_) engineering uses, and (_b_) general uses. Of the brasses, those containing upwards of 70 per cent. of copper may be rolled cold, whilst the alloys with less than 70 per cent. are hot-rolled.

In the engineering industry large quantities of 70/30 brass are utilised in the form of condenser tubes, whilst for the multifarious requirements of general engineering work, very considerable amounts of brass of lower tenor are employed in the forms of taps, pipes, fittings, etc.

Muntz metal, the 60/40 brass, finds extended application for the sheathing of ships, whilst the employment of brass and of the other alloys for all manner of articles of general utility is a matter of common knowledge.

The close connection between properties, constitution, and the equilibrium diagram of these various classes of alloys has become manifest to a marked degree within recent years, and the effects of thermal treatment partly in modifying their constitution, and thereby the properties, and also in controlling the condition and distribution of the constituents, are at the present time having an important bearing on the manipulation of these alloys in the industries manufacturing them and adapting them for their various uses. The study and application of these equilibrium diagrams are highly important to those who have to deal with these alloys on an industrial scale.

=The Properties of Copper.=—The properties of the metal which render it of such service in the arts and industries are mainly its high electrical conductivity, its great ductility, malleability, and toughness, which enable it to be readily worked up into the different forms in which it is employed, its high thermal conductivity, and its resistance to the various agencies which lead to corrosion. These are consequently the properties to which close study is directed. Of perhaps still greater importance is a knowledge of the influence exerted upon these properties by the circumstances which usually attend working practice; such as, for example, the various common impurities, and the variations of temperature, as well as the previous mechanical and thermal treatment. These can only be indicated in general terms here, references to authorities on the different branches being given later.

TABLE III.—INFLUENCE OF IMPURITIES ON THE ELECTRICAL CONDUCTIVITY.

┌─────────────┬───────────────────────┬───────────────────────┐
│ │ Addicks. │ Johnson. │
│ ├─────────┬─────────────┼─────────┬─────────────┤
│ │Impurity.│Conductivity.│Impurity.│Conductivity.│
│ │Per cent.│ │Per cent.│ │
├─────────────┼─────────┼─────────────┼─────────┼─────────────┤
│Pure copper, │ .. │ 101 │ .. │ 101 │
│Copper with— │ │ │ │ │
│Aluminium, │ 0·006 │ 98·6 │ 0·01 │ 99·7 │
│ │ 0·109 │ 66·8 │ 0·02 │ 98·8 │
│ │ 0·739 │ 43·5 │ .. │ .. │
│Antimony, │ 0·007 │ 99·6 │ .. │ .. │
│ │ 0·022 │ 97·2 │ .. │ .. │
│ │ 0·047 │ 95·4 │ 0·05 │ 96·9 │
│Arsenic, │ 0·004 │ 99·6 │ .. │ .. │
│ │ 0·007 │ 96·8 │ .. │ .. │
│ │ 0·013 │ 93·2 │ 0·04 │ 92·4 │
│ │ 0·140 │ 62·3 │ 0·06 │ 82·0 │
│Bismuth, │ 0·028 │ 99·6 │ 0·01 │ 95·7 │
│ │ 0·045 │ 99·3 │ .. │ .. │
│Cadmium, │ 0·062 │ 99·5 │ .. │ .. │
│ │ 0·113 │ 99·1 │ .. │ .. │
│ │ 0·427 │ 96·1 │ .. │ .. │
│Cobalt, │ .. │ .. │ 0·05 │ 92·0 │
│Gold, │ 0·089 │ 98·9 │ 0·05 │ 99·7 │
│ │ 0·149 │ 98·4 │ .. │ .. │
│ │ 0·317 │ 96·4 │ .. │ .. │
│Iron, │ 0·042 │ 96·8 │ .. │ .. │
│ │ 0·046 │ 92·9 │ .. │ .. │
│ │ 0·068 │ 89·6 │ 0·09 │ 98·8 │
│Lead, │ 0·083 │ 99·1 │ .. │ .. │
│ │ 0·052 │ 98·7 │ 0·06 │ 100·6 │
│ │ 0·347 │ 98·3 │ .. │ .. │
│Manganese, │ .. │ .. │ 0·02 │ 98·8 │
│Nickel, │ .. │ .. │ 0·05 │ 91·4 │
│Oxygen, │ 0·020 │ 100·7 │ .. │ .. │
│ │ 0·050 │ 101·4 │ .. │ .. │
│ │ 0·100 │ 100·5 │ 0·10 │ 99·8 │
│Phosphorus, │ 0·08 │ 52·3 │ 0·004 │ 98·5 │
│Platinum, │ .. │ .. │ 0·02 │ 93·6 │
│Silicon, │ 0·007 │ 99·4 │ 0·004 │ 99·7 │
│ │ 0·042 │ 99·0 │ 0·01 │ 98·4 │
│Silver, │ 0·003 │ 100·5 │ .. │ .. │
│ │ 0·137 │ 100·0 │ 0·05 │ 99·8 │
│ │ 0·340 │ 98·3 │ .. │ .. │
│Sulphur, │ 0·053 │ 100·0 │ 0·01 │ 98·5 │
│ │ 0·135 │ 99·0 │ .. │ .. │
│ │ 0·236 │ 98·9 │ .. │ .. │
│Tellurium, │ 0·065 │ 100·4 │ .. │ .. │
│ │ 0·181 │ 100·2 │ .. │ .. │
│ │ 0·405 │ 98·7 │ .. │ .. │
│Tin, │ 0·052 │ 97·6 │ 0·05 │ 100·5 │
│ │ 0·097 │ 92·7 │ .. │ .. │
│ │ 0·295 │ 79·8 │ .. │ .. │
│Zinc, │ 0·048 │ 98·3 │ 0·02 │ 98·5 │
│ │ 0·095 │ 96·3 │ .. │ .. │
└─────────────┴─────────┴─────────────┴─────────┴─────────────┘

PHYSICAL PROPERTIES.—The _colour_ of copper is familiar, being a fine salmon pink. The _appearance of the fractured surface_ is a useful guide in several respects as to the condition of the metal, and in the process of manufacture the refiner relies upon this appearance as an important criterion of the progress of the refining operation. Copper containing an excess of oxygen, for example, has a purplish-red colour and a coarse brick-like fracture; this is known as “_dry copper_,” and the metal is brittle and commercially useless when in that form. The ingot of dry copper is also characterised by a depression running along the surface. _Tough copper_ (“tough-pitch”) the mechanically useful variety resulting from the furnace-refining operation, possesses a bright salmon-coloured fracture, finely granular to silky in appearance, whilst “_overpoled copper_,” also brittle and industrially valueless whilst in that condition, has a very light salmon-coloured fracture, and is more coarsely fibrous.

The _melting point_ of copper is 1,083° C., and is slightly lowered by the small quantities of impurity usually present in commercial metal. Molten copper is of a pale apple-green colour. The _boiling point_ under ordinary conditions is about 2,300° C. (1,700° C. _in vacuo_). The _electrical conductivity_ is of much importance. Copper ranks second only to silver as a conductor, the relative conductivity of the best copper being about 98 compared with silver as 100. The resistance of 12 inches of pure copper wire, 0·001 inch in diameter, is 9·612 ohms. The conductivity of the metal is decreased by mechanical working, and it follows the general straight-line law connecting conductivity and temperature.

The effect of even small quantities of impurity on this property is very marked, so much so that only the purest varieties are suitable for electrical work, and for this reason electrolytic refining is often a necessary operation in the manufacture of copper intended for this purpose.

Table III. on preceding page, summarises the results of the work of Addicks and Johnson, and indicates the effects of small amounts of different impurities on the conductivity of the metal.

The notoriously destructive effect of arsenic on the conductivity is very apparent.

The influence of most of the common impurities is of a similar nature, and detailed investigations indicate that the effect is more or less progressive as the quantity increases—within the limits usually present in commercial metal. The results of Hiorns and Lamb’s experiments with reference to arsenic and antimony are indicated in Fig. 4.

The _specific gravity_ of copper naturally varies according to its condition and composition. When pure and in the worked state, its density is 8·95; cast metal, more open and inclined to porosity, has a density of about 8·2 to 8·6, depending on the purity, rate of cooling, etc. Impurities lower the specific gravity.

The _conductivity for heat_ of the metal is high, being 898 compared with gold as 1,000, and as a conductor it is two and a-half times more efficient than iron. It is this property, combined with its toughness and resistance to corrosion, etc., which largely determines its employment for heaters, steam-coils, and the like.

_Power of Dissolving Gases._—When molten, especially under reducing conditions, the metal possesses the property, common to many others, of absorbing gases such as carbon monoxide, hydrogen, hydrocarbons, sulphur dioxide, etc., which are moreover, to a large extent insoluble in the solid material, and are, therefore, often liberated at or about the moment of solidification; though some may remain dissolved. This action is one of the causes of the difficulty which is experienced in making sound castings of the metal, particularly since the gases mentioned are present in quantity during the poling and refining operations. The presence of certain materials in the copper, as in the case of steel, appears to reduce the dissolving power of the liquid metal for these gases, or possibly to increase their solubility when the copper is solidifying, and in this way tends to minimise their injurious effects. It would seem that one of the functions of the cuprous oxide, which is purposely introduced into the metal when “bringing it up to pitch,” is to exert this action. The ridge in the ingot of overpoled copper is, to some extent, accounted for as being due to the effects of the evolved gases, and this appearance indicates the absence of the requisite quantity of cuprous oxide necessary to counteract the effect.

Copper is also supposed to be capable of holding certain quantities of gas in solution after it has become solid, and the resulting metal is more brittle and often commercially useless. Several of the characteristics of overpoled copper probably arise from this cause also.

=Impurities[2] in Copper.=—In view of the marked influence of impurities on the properties of metallic copper, it may be advisable in this place briefly to review the results of recent scientific work as to the condition in which they exist in the metal, thus offering some clearer indication of the manner in which they affect the mechanical and other properties. The common impurities in ordinary commercial metal may be oxygen, arsenic, antimony, bismuth, lead, and to smaller extents, iron, sulphur, tellurium, and selenium.

A factor of much importance is that the effect of two or more of the common constituents when present together, may be of even greater moment than that of each one separately, and in this connection Hampe’s classical work should be consulted. The investigation of the joint effects of impurities becomes so complex that systematic study progresses but slowly. Metallographic work is, however, revealing much evidence, and the researches in progress at present at several laboratories will, when published, afford greatly increased knowledge on the subject. Recent papers by F. Johnson give valuable detailed information (see References, p. 34). The importance of oxygen in this connection is particularly marked: its effects are profound, since in addition to its own specific influence as oxide, it also brings about chemical changes in some of the other constituents, thus leading to the formation of entirely new compounds possessing quite different properties. The beneficial influence of certain definite proportions of oxygen in addition to the other constituents of commercial copper is well known in practice, and has been systematically studied by Hampe, and later by several other workers with more delicate means of investigation at their disposal.

_Oxygen in Copper._—Molten copper has the power of dissolving its oxide, Cu_{2}O. When the melted metal is exposed to oxygen, this oxide is produced and passes into solution in the liquid, yielding a series of binary alloys, of which the oxide acts as the second constituent. The equilibrium diagram of the series, as worked out by Heyn[3] (see Fig. 5), affords a good indication of these relationships, and throws light on several features connected with the presence of oxygen in copper.

It will be observed that when molten oxygenated metal containing less than about 0·38 per cent. of oxygen solidifies, copper crystallises out first, whilst later, in between the copper crystals, there solidifies a eutectic of copper and cuprous oxide. This eutectic contains about 3·45 per cent. of cuprous oxide, equivalent to 0·38 per cent. of oxygen; it melts at a temperature about 18° C. below that of the pure metal. The presence of this material, which is of a blue colour when viewed under the microscope, constituting slightly more fusible, tough, non-conducting areas between the copper crystals, accounts for many of the well-known effects of oxygen in metallic copper.

When oxygen is present in quantities above the eutectic proportion, the first constituent to solidify from the molten over-oxygenated copper is brittle copper oxide, and the presence of such brittle material disseminated through the metal explains why “dry copper” cannot be worked.

The effects of comparatively small quantities of oxygen are greatly increased on account of the fact that one part of oxygen, when present as cuprous oxide, yields a constituent in almost nine times as great a proportion by weight alone, since Cu_{2}O : O :: 142 : 16 or 9 : 1; whilst oxygen existing as oxide-eutectic is represented in the ratio of nearly 30 : 1. The presence of excess of copper oxide in the metal is particularly dangerous when copper is to undergo annealing in a reducing atmosphere, since the reducing gases acting upon the oxides at the crystal boundaries destroy them, thus tending to produce that rottenness in the material which is so often encountered under such circumstances.

The great value and importance of oxygen in copper lies in its property of bringing the metal up to pitch as indicated above.

The effect of carbon on oxygenated copper was the subject of much enquiry in early years. It was thought at one time that the influence of carbon _per se_ in the copper was responsible for the beneficial effects resulting from the melting of brittle “dry” copper with carbon, but the work of Percy, since confirmed, showed that its sole action is in the reduction of the injurious excess of oxide.

In addition to the specific influences of oxygen as just recorded, and to its important physical effects with regard to the solubility of gases, etc., oxygen in copper performs other valuable functions, by forming with reduced impurities which are exceedingly dangerous, oxygenated compounds more infusible and more insoluble; and this has the effect of segregating or distributing such injurious impurities into forms and positions much less harmful.

Fig. 6.—Microstructure of Copper containing Oxygen (_Heyn_).

_a._ Hypo-eutectic. _b._ Hyper-eutectic.

Oxygen 0·13 per cent. = 1·16 per cent. Cu_{2}O.

Oxygen 0·53 per cent. = 4·7 per cent. Cu_{2}O.]

_Arsenic in Copper._—When arsenic and copper are melted together chemical combination occurs, and a series of arsenides is produced; the system, which has been investigated by Friedrich (from whose work the following diagram has been constructed), Hiorns, Bengough & Hill, and others, being one of considerable complexity. With proportions of arsenic such as are usually present in commercial coppers, the compound produced is probably Cu_{3}As (28·3 per cent. of arsenic), which passes into solution in the excess of metal, and on solidification the copper retains this arsenide in solid solution. As in the case of all such solid solutions, the solidification takes place over a range of temperature represented between the liquidus and solidus curves; the purer metal crystallising out first, followed gradually by crystals of copper which become progressively richer and richer in arsenic (still in solid solution). In the case in question, diffusion of the arsenic throughout the crystalline mass proceeds but slowly, and as a result, the metal, as usually obtained in the cast state, shows fringes of such arsenic-rich copper. By annealing, diffusion is greatly assisted, and the material gradually becomes homogeneous, as is seen on microscopic examination. There appears further to be some decrease of this solubility with fall of temperature when the arsenic is high, leading sometimes to a separation of the arsenide itself at the crystal boundaries.

_Antimony_ appears to form an analogous compound, Cu_{3}Sb, also capable of passing into solid solution in the copper, but to a rather smaller extent than the corresponding arsenide. The fringes are therefore more pronounced, and the decrease of the solubility on further cooling is also more marked.

_Bismuth._—The influence of even minute quantities of bismuth on copper is notorious. Bismuth appears to be soluble in liquid copper, but not in the solid metal. In consequence, when copper containing bismuth solidifies, the copper crystals separate first, whilst the liquid bismuth still remains between them, until the metal reaches a temperature of about 268° C.—the melting point of bismuth—when it too solidifies _in situ_. The presence of such envelopes of very brittle, fusible, and limpid bismuth material explains much of the harmful effect of this impurity. These envelopes are found to consist almost entirely of practically pure bismuth. Oxygen converts the bismuth into a more compactly crystalline oxide, much less fusible and harmful. Arsenical copper tends to the scattering of the bismuth globules among the fringes which are formed during the gradual process of solidification over the range of temperature already indicated, and thus renders this impurity to some extent less dangerous.

_Lead_ behaves in apparently much the same way as bismuth, and the effects produced upon it by the presence of oxygen and arsenic are probably similar.

_Selenium and Tellurium_ probably exist in the form of selenides and tellurides, which are characterised by marked brittleness and fusibility.

=Mechanical Properties of Copper.=—The mechanical properties of commercial copper are influenced to a vital degree by the conditions associated with working practice, such as composition, previous mechanical and thermal treatment, temperature of working, etc. As has been already indicated, it is the possession by the copper of certain mechanical qualifications which leads to its employment by engineers, and it is, therefore, necessary to consider the influence of the above conditions, when reviewing the mechanical properties of the metal.

Much of the copper employed for general engineering work (apart from electrical and alloying purposes) is of the quality designated as “tough-pitch” copper. This tough copper generally contains certain impurities which render the metal exceedingly useful for mechanical service, and their presence is, indeed, almost essential in copper intended for such purposes. At the same time, such elements would render it absolutely unfit for the other uses just specified, where purity is practically the first necessity.

The standard works and the papers indicated in the appended list of references should be consulted for details concerning the effect of each circumstance on the several mechanical properties; certain general considerations must, however, be noted here.

Not only should the composition of the metal be carefully considered, but attention must be directed to the actual condition and distribution of each constituent. Owing largely to the difficulties of determining the oxygen contents in copper, and to a want of definite knowledge as to the condition, amount, and effects of the dissolved gases in the metal, the information at present available is not sufficiently concise to allow of a systematised statement being made as to the direct influence of the constituents on the mechanical properties. This is more especially the case since the other attendant circumstances of working practice may react through these to a considerable extent.

Many of the more general results have, however, long been known to engineers from practical working, and these have been placed on record from time to time.

The _malleability_ and _ductility_ of copper are considerable. Cold rolling and hammering causes a reduction in this respect, and the metal is hardened, but the properties are restored by annealing. The annealing effect commences at about 300° C., but proceeds more effectively at higher temperatures, the factors of annealing temperature and duration necessary for annealing being inversely connected. The impurities which influence these properties most adversely are bismuth and tellurium. The effect of other constituents, oxygen _per se_, sulphur, and iron, in the quantities usually present in commercial copper, is very small. Arsenic and antimony up to 0·4 or 0·5 per cent. have no deleterious effect on the malleability and ductility of copper _of the correct pitch_, and may even improve the metal when tested in the cold; the hot malleability is, however, somewhat decreased.

The presence of impurities raises the temperature required to bring about the full effects of annealing after the metal has been hardened by mechanical work. This action is probably explained by the interference of the impurities upon the molecular freedom of the metal, which controls the mechanism of annealing. The conditions, whether reducing or oxidising, during annealing, may exert an important influence on the results.

_Hardness._—Pure copper is a comparatively soft metal. It is hardened by mechanical work—the hardness of rolled copper, determined by the Brinell Test, being 74 compared with mild steel as 100—and by the presence of even small quantities of impurities, tin possessing a particularly marked effect in this connection. The worked metal is softened on annealing.

_Tensile Strength and Elongation._—The strength of copper, being a property of such practical importance, has been the subject of much extended investigation. The work has, however, been conducted under such a great variety of conditions, many of which have been left unrecorded, that co-ordination of the results is barely possible, and does not allow of establishing on a definite basis the effect of different influences on this property of the metal. Later work, some already published, some still in progress, should eventually allow of more general standardisation than is at present possible. The tensile strength of pure cast copper is 8 to 9 tons per square inch. Mechanical work causes an increase in the value up to 14, or even 16 tons, cold work exerting a still more marked influence; whilst 33 tons and more per square inch has been recorded with cold-drawn fine wire. The elongation varies according to the mechanical work which the metal has undergone; the amount ranges from 35 to 40 per cent. and upwards, measured on a 3-inch length.

Tensile strength is reduced on annealing, but never to so low a degree as that of the cast material, the usual figure being 12 to 14 tons per square inch. The effect of temperature in reducing tensile strength, especially when impurities are present, is important from the industrial point of view. The reduction in strength caused by annealing appears to be considerably smaller in the presence of arsenic and antimony.

Arsenic increases the tensile strength of copper when the metal is of the correct pitch, generally to well over 15 or 16 tons, in the presence of the proportions usually found. Antimony has a similar effect. Some workers state that, within certain limits, the strengthening effect of this element is even more pronounced. Excess of antimony exerts, however, a much more adverse influence than does excess of arsenic. The elongation is increased by the presence of moderate quantities of arsenic.

Oxygen _per se_, when present in moderate quantity in copper, has but little effect on the tenacity. Bismuth, tellurium, sulphur, and lead are the impurities which lower the strength, even when present in minute quantities, and especially on heating. Bismuth in the proportion of 0·005 per cent. lowers the malleability and ductility considerably, and recent reports state that 0·02 per cent. bismuth renders copper cold short, that 0·05 per cent. makes it red short, and that 0·005 per cent. is the limit for electrolytic copper which is to be rolled. The deleterious effects of bismuth are, as already explained, to some extent masked by the presence of arsenic and by oxygen.

The strength is increased by the presence of nickel, tin, and zinc in the proportions usually present in the commercial metal; these are, however, generally small.

From the foregoing review, indications will be afforded of the reasons for the choice by engineers of “tough-pitch” copper for much of their work, and the explanation for the 0·3 to 0·5 per cent. arsenic often particularly specified for. The frequent use of arsenical coppers for such purposes as fire-box plates will also be understood, since the arsenic not only improves the mechanical properties of the metal, but ensures the retention of rigidity and strength at the high working temperatures required, to a greater degree than would have been the case had pure copper been employed.

The effect of the above factors on the elastic limit of copper, is also very marked and of much importance, the influence being closely analogous to that produced on the other mechanical properties.

=Chemical Properties.=—The atomic weight of copper is 63·57. The metal is unchanged in dry air at ordinary temperatures; in the presence of moisture and of carbon dioxide a green coating of basic carbonate is produced. When heated in air, a black scale, consisting of cuprous oxide, Cu_{2}O, is obtained, which is readily detached by quenching and hammering. Water at ordinary temperatures is without effect upon copper; concentrated sulphuric and nitric acids have little action upon it in the cold, but attack it on heating. The best solvent for the metal is dilute nitric acid, which dissolves it very readily. Copper is liable to corrosion when subjected, whilst hot, to the action of chlorine or hydrochloric acid gas; this action has provided an explanation of the corrosion of copper boiler tubes where the coal employed had been exposed to sea water.

Copper is deposited from solution as a dull red, spongy mass, by iron, zinc, or aluminium, but it is more electro-positive than gold or silver, and readily precipitates these metals from solutions of their salts, these effects being extensively made use of in practice. The metal possesses a powerful affinity for sulphur, and this property has very important applications in the smelting processes.

Copper readily alloys with gold, silver, tin, zinc, and nickel, but not with lead or iron.

References.

Composition and Properties of Metal for Railway and Locomotive Work
(p. 20).
_Proc. Inst. Mech. Eng._, 1893; Dean, p. 139; Blount, p. 164;
Watson, p. 168; Gowland, p. 176; Aspinall, p. 193;
Tomlinson, p. 182.
Webb, F. W., “Locomotive Fire-box Stays.” _Proc. Inst. C.E._, 1902.
Milton, J. T., “The Treatment of Copper for Steam Pipes.”
_Inst. Marine Eng._, 1908–9.
Hughes, G., “Non-ferrous Metals in Railway Work.”
_J. Inst. Metals_, Sept. 1911.
Law, E. F, “Alloys.”
Influence of Impurities on the Electrical Conductivity of Copper
(p. 24).

Lawrence Addicks,
_Trans. Amer. Inst. Elect. Eng._, 1903, vol. xxii., pp. 695–702;
_Electro-Chemical Industry_, 1902–3, pp. 580–583;
_Trans. Amer. Inst. Min. Eng._, 1906, vol. xxxvi., p. 18.
Walker, A. L., _Mineral Industry_, 1898, vol. vii., p. 248.
T. Johnson, “Some Features in the Metallurgy of Copper.”
_Proc. B’ham. Met. Soc._, 1906.
Hiorns and Lamb, “Influence of Arsenic and Antimony on Copper.”
_Journ. Soc. Chem. Ind._, May, 1909.
Condition and Influence of Impurities on the Mechanical Properties
of Copper (p. 32).
┌─
│ _Zeitschrift für Berg. Hutten and Sal. Wesen_,
Hampe ─┤ 1873, xxi., 218; 1876, xxiv., 26
│ _Chemiker Zeitung_, 1892, No. 42, p. 16.
└─
┌─
Heyn, E., │ _Reports, Royal Tech. Testing Institute._
“Copper and Oxygen.” ─┤ Charlottenburg, 1900, p. 315.
│ _Metallographist_, vol. vi., 1902, p. 48.
└─
Arnold, _Engineering_, vol. lxi., p. 176. Feb. 7, 1896.
Roberts-Austen, Second Report, Alloys, Research Committee.
_Proc. Inst. Mech. Eng._, April, 1893, p. 114.
Rudeloff, _Mittheil. König. Tech. Versuchs. Anstalt._, 1894, ii.
(_b_), pp. 292–330; 1898. 16_a_, pp. 171–219.
Lawrie, _Bull. Amer. Inst. Min. Eng._, 1909, pp. 857–66.
“Bismuth in Wire Bar Copper.”
Johnson, F., “Impurities in Tough Pitch Copper containing Arsenic.”
_Proc. Inst. of Metals_, 1910, vol. iv.; No. 2, p. 163, _et seq._
Johnson, F., “The Influence of Impurities on the Properties of
Copper.” _Metallurgical and Chemical Engineering_, Oct. 1910,
p. 570.
“Annealing of Copper and Diseases of Copper.” _Ibid._,
Feb. 1911, p. 87.
“Notes on the Metallurgy of Wrought Copper.” _Ibid._,
August, 1911, p. 396.

See also Standard Specifications for Copper Wire-Bars (recommendations by the Committee of the American Society for Testing Materials). _Eng. and Min. Journ._, Jan. 20, 1912, p. 181.

LECTURE III.

Compounds of Copper — Copper Mattes — The Varieties of
Commercial Copper — Ores of Copper — Preliminary
Treatment of Ores, Sampling.

=Compounds of Copper.=—From the smelting point of view, the three most important classes of copper compounds are the oxides, the sulphides, and the silicates.

_Copper Oxides._—Of the oxides, two are of importance—cuprous oxide, Cu_{2}O, and cupric oxide, CuO—the first-named particularly, having extensive connection with smelting practice.

Cuprous oxide is black when in the massive form, and has a red hematite colour when powdered. It is readily formed by the oxidation of copper, and melts at a red heat without decomposition; further heating in the presence of air produces the cupric oxide which is less fusible. As has been already indicated, cuprous oxide dissolves in the molten metal. It is easily reduced to metallic copper by heating with carbon, the metal being also obtained if the oxide be heated in the presence of reducing gases; it combines readily with silica when heated, yielding fusible silicates.

When cuprous oxide is heated with sulphide of iron, the copper, having a greater affinity for the sulphur than iron possesses, enters into combination with it, forming copper sulphide and iron oxide, and if sufficient silica be present, a silicate of iron slag is produced. When melted with copper sulphide, cuprous oxide yields metallic copper with liberation of sulphur dioxide according to the equation—

Cu_{2}S + 2Cu_{2}O ➡ 6Cu + SO_{2}.

This reaction is a quantitative one, and takes place in the Direct Process of Nicholl and James as operated at Swansea. The excess of either constituent remains unchanged. The reaction is of great importance in the processes of copper extraction, since upon it depends the liberation of metallic copper from the sulphide, both in the old roaster process and in the modern converter operation.

_Sulphides of Copper._—Of the sulphides Cu_{2}S and CuS, the former only is of metallurgical importance. It is grey black, brittle, and crystalline, its melting point is about 1,135° C., and its specific gravity, when cold, about 5·5. Owing to the great affinity of sulphur for copper, this element acts as practically the universal carrier of the metal in smelting work, detaching the copper from all other forms of combination, and collecting it as sulphide, mixed with the sulphides of other metals, particularly that of iron—copper sulphide and iron sulphide alloying in all proportions.

When copper sulphide is melted with an excess of sulphur, it remains unchanged; when melted with copper and subsequently cooled, the sulphide and metal separate as such, although it is believed that small amounts of copper are present in solid solution in the sulphide on solidification, but that they separate from it during a dimorphic change in the material, which occurs at about 103° C. The sulphide reacts with iron with liberation of some metallic copper, and the formation of some iron sulphide which associates itself with the rest of the copper sulphide, forming a matte. This matte is not further affected by iron, so that it is not possible to completely decompose copper sulphide by this means.

When heated in a powdered condition in excess of air, copper sulphide is oxidised, oxides of copper and sulphur being produced. There occur probably several intermediate reactions, and several intermediate products are formed, but the main effect is represented by the equations—

┌─
│ Cu_{2}S + 2O ➡ 2Cu + SO_{2}
─┤
│ 2Cu + O ➡ Cu_{2}O
└─

which take place simultaneously, the copper represented in the first equation being oxidised spontaneously according to the second, and the resultant is the reaction Cu_{2}S + 3O ➡ Cu_{2}O + SO_{2}.

In the furnace operations, some of the SO_{2} in the presence of air and oxidisable material, and in contact with the heated brickwork becomes oxidised to SO_{3}, which, interacting with the oxides and sulphides present, combines to form copper sulphate and cupric oxide. At a higher temperature the sulphate is again decomposed to CuO and SO_{3}, some of which passes off and is free to oxidise more sulphide; the rest is decomposed to SO_{2} and oxygen. These reactions occur during the roasting of charges containing copper sulphides.

=Copper Mattes.=—On smelting a furnace charge which contains both copper and sulphur, the sulphur appears to have a stronger attraction for the copper than for any of the other metals usually present, and only when this affinity has been satisfied is the excess sulphur free to combine with other constituents of the charge. The fusible copper sulphide which is thus produced, has the power of mixing completely with any more sulphides which may be present, especially with sulphide of iron.

The fused sulphides resulting from such furnace operations are termed _copper-mattes_. They may contain from a mere trace to upwards of 80 per cent. of copper, and in ordinary work, sulphide of iron is the other constituent present in the greatest proportion, but sulphides of nickel, silver, zinc, or lead, etc., may also be found, as well as arsenides and antimonides.

These facts relating to the collection of the copper as a constituent of a fused sulphide product, form the basis of modern copper-smelting work. In view of the practical importance of the mixed sulphides, the diagram representing their equilibrium requires notice. A number of workers have studied the question with widely differing results. Röntgen made an exhaustive investigation of the system FeS—Cu_{2}S, and published a very complete diagram of the series, working with FeS and Cu_{2}S in the pure state.

The sulphides as commonly met with, especially in smelting practice, do not however, occur as materials of the composition denoted by the formulæ Cu_{2}S and FeS. The ordinary commercial sulphide of iron corresponds more closely to the impure eutectic of the iron-FeS system, containing about 85 per cent. of FeS and 15 per cent. of iron, and melts at about 970° C., whereas the pure FeS has a melting point of upwards of 1,180° C. At the elevated temperatures of the copper-smelting furnace, pure FeS tends to lose sulphur and to assume the composition of the eutectic. There are, further, good reasons for believing that copper sulphide behaves in a somewhat similar manner, so that the series of sulphides constituting the mattes of practice are not represented by pure materials so well as by a series composed of mixtures of the respective eutectics.

The diagram of this series of industrial sulphides was worked out by Hofman, Caypless, and Harrington, and gives a fair summary of the melting points of the series of mattes. It is reproduced in fig. 8. The temperatures may be supplemented by Gibb’s determinations of 1,121° C. for the 71·7 per cent. copper matte, and 1,098° C. for the 80 per cent. matte.

The problem of the constitution of mattes is, however, a very complex one, and is not yet satisfactory settled. An interesting view was put forward by Gibb and Philp. Mattes corresponding to the formula 5Cu_{2}S . FeS (copper 71·7 per cent.), when examined microscopically, appeared to be homogeneous, and indicated some form of combination between the sulphides in these proportions. Lower-grade mattes were assumed to consist of this compound substance and excess FeS. Iron sulphide was held to be capable of carrying a certain quantity of copper in solution, and mattes might, therefore, carry this copper, according to the amount of excess FeS which they contained. Within certain limits the lower the grade of the matte—_i.e._, the more FeS present—the more copper was held in solution, and with a fall of temperature this solubility was lessened, and moss copper was set free in the solid matte.

Deposition of copper may also be accounted for by a variation in the solubility for copper, accompanying the well-marked dimorphic change occurring in FeS at 130° C. whilst another possible cause of the separation of moss copper is the partial decomposition of Cu_{2}S, being effected, as previously indicated, by the free iron of the iron-FeS eutectic which constitutes the iron sulphide component of copper mattes. The whole subject is thus of considerable complexity, and involves questions of thermal and chemical equilibrium.

The appearance, chemical constitution, and physical properties of mattes vary according to the rate of cooling, and are further influenced by the nature and amount of the impurities they contain, and the following statement must be understood to be more or less general:—Usually low-grade mattes (up to 20 per cent. or so of copper) are more or less stony in fracture, with a bluish-purple colour; as the copper contents increase, a reddening of the colour occurs, and also an increase in the crystalline character and brittleness. Considerable quantities of moss copper are present in these mattes. Beyond 30 per cent. of copper, increased softness and brittleness result, with a darkening towards blue-black in the colour, whilst with the 60 to 70 per cent. mattes the colour becomes in general of a steel-grey hue.

Increase in the copper contents leads to an increase in the density—a matter which has important applications in connection with the economical separation of matte from slag, and the slag-losses in smelting practice.

The specific gravity of the 13 per cent. copper matte is about 4·80.
" 43 " " " 5·18.
" 60 " " " 5·42.
" 80 " " " 5·55.
(_Gibb_ and _Philp_.)

The density in the fluid state, which is the important condition in smelting, is less than this, and may indeed be somewhat different, owing to changes in the constitution of the material.

_Copper Silicate_ is formed by the action of copper oxide and silica on heating. The silicate is decomposed when heated in the presence of sulphides, resulting in the formation of sulphide of copper and silicate of the second metal, in consequence of the great affinity of copper and sulphur. Upon this fact depends the extraction of copper from various silicate ores, as well as the cleaning of slags high in copper, which are often added to the sulphide charges in the furnace with this object. When heated with iron, the silicate is reduced to metallic copper with the production of silicate of iron; it is also reduced by carbon in the presence of metallic oxides capable of uniting with the silica which is liberated.

=The Varieties of Commercial Copper.=—The copper employed industrially comes into the market in widely differing forms. Different varieties are named according to the method of manufacture, the uses for which they are intended, the locality in which they are produced, or by special trade names. The most important variety is:—

_Electrolytically-refined High-conductivity Copper_, which is largely used for electrical work. The methods by which it is produced ensure that most of the impurities inimical to high conductivity have been removed, and the metal is specially free from arsenic, antimony, and bismuth, as well as from silver and gold. As ordinarily produced at the electrolytic refinery, it is in the form of cathode plates, often about 3 feet × 2 feet 6 inches by ¾ inch thick, weighing 150 to 170 lbs. It is then remelted in order to bring it “up to pitch,” and to give it the necessary mechanical properties, so that it may be transformed at once into the particular form suitable for the electrical purposes intended. Such metal often comes into the market in the form of wire-bar ingots, cakes, or billets, weighing from 70 to 500 lbs. when in bar form, and from 100 to 400 lbs. when in other shapes. Electrolytic copper is also suitable for the manufacture of alloys.

_Lake Copper._—The copper ores of the Lake Superior district are particularly pure, and on smelting and furnace-refining yield a metallic product of great purity which also possesses good mechanical properties. It is, therefore, particularly suitable for electrical work. By reason of its satisfactory properties, Lake copper realises prices which usually rule somewhat higher than those of ordinary electrolytic copper as quoted on the New York market.

_Best Select Copper._—For the production of copper alloys, such as best brass, etc., it is essential that the copper should be pure. The impurities which are present in ordinary tough copper, and which may be valuable for imparting strength to the material, have a very harmful effect when present in alloys. In the older Welsh process of manufacturing copper, a special method was employed for obtaining metal free from these impurities, especially arsenic and antimony. This was known as the “best selecting” process.

The principle underlying the method was to conduct the furnace operations to the stage at which a small quantity of copper, known as “copper bottoms,” was obtained. The metal so produced has the property of collecting from the rest of the matte-charge in the furnace, the gold, the silver, and the great bulk of the other impurities, owing to its greater solvent power for them. As a result, the greater part of the matte (“white metal”) was left pure, and from this material the copper was extracted by continuing the furnace operations in the usual manner, the resulting product being known as “best select” (B.S.) copper.

The process was later used principally for the extraction of the gold in the charge, rather than for obtaining specially pure copper. The product is essentially a British one, and was largely used for the manufacture of high quality alloys.

“_Tough Pitch Copper._”—The operation of “bringing copper up to pitch” has for its object the imparting to the metal of the toughness and mechanical strength required for industrial service. The process resolves itself into the adjustment of the correct proportion of oxygen, the function of which is largely to eliminate the gases from the copper, or to overcome their deleterious effects, as well as to convert the otherwise more injurious metalloid impurities into a less harmful form.

In modern practice, practically all copper is brought up to pitch, but it is useful to distinguish between tough-pitch furnace-refined copper and tough-pitch electrolytic copper.

The former is the brand to which the general term “tough pitch copper” is best applied, this name having been given to the product from the refining furnaces of the old Welsh and similar processes. Before the converter method was introduced into copper practice, the furnace processes for extracting copper from the ores resulted in the production of a crude “blister” copper, into which several injurious constituents, if originally present in the ore, found their way. The principal impurity was usually arsenic. Although this was also removable by special refining methods, and with some difficulty, it was known, as has been indicated, that when arsenic is present under suitable conditions and in proper proportions, it is capable of imparting considerable strength and rigidity to the metal. Such copper being particularly suited for various engineering and mechanical uses, the arsenic being sometimes even specified for and purposely added—as in fire-box plates and stay bolts, though it is never employed for conductivity work or for the manufacture of alloys if any considerable proportion be present—the metal found a ready market when brought to pitch.

Tough pitch copper may thus vary largely in composition, especially in arsenical contents, up to about the 0·5 per cent. already indicated as being mechanically very useful. The actual process, as used for bringing all classes of metal to pitch, will be described in detail later, it being practically the same whether conducted on furnace-refined metal, converter metal, or on electrolytic copper, as a necessary preliminary to casting into the various forms of ingot in which it is to be marketed.

In preparing the tough metal from crude copper, the more oxidisable impurities (iron, sulphur, etc.) are first removed by a thorough oxidation during or after melting down, this being known as “airing.” The operation oxidises some of the copper, and it is probable that the copper oxide thus formed plays an important part in getting rid of impurities. By the time they have been thoroughly expelled, the metal is considerably over-oxidised. Samples taken at this stage exhibit the following characteristics:—The ingot has a depression down the centre line, the material is very brittle, the fracture is brick-like in texture and purple-red in colour, whilst much copper oxide and oxidule-eutectic are seen on examination under the microscope. This material is known as _Dry Copper_; it is merely an intermediate product, and is commercially useless. The excess of oxygen is removed by “poling”—that is, reduction, effected largely by charcoal, as well as by reducing gases—successive samples showing less and less of the characteristics of dry copper. The surface becomes level, the metal exceedingly tough, the fracture fine-grained to silky in texture, and a fine salmon-pink in colour. With satisfactory mechanical properties, the metal has now become =tough pitch copper=.

If the poling—that is, the reduction of the oxidised constituents of the tough pitch copper—be carried too far, the metal becomes brittle again, being known as _over-poled copper_. The fracture then tends to become coarse and fibrous, the colour lighter, and the upper surface of the ingot exhibits a ridge. The reasons for these effects have not yet been quite fully explained, but there is no doubt that they arise from the removal of oxygen from the oxygenated constituents, and the withdrawal from the metal of the protecting influence of the cuprous oxide. Such influences are to some extent physical, since they prevent the retention of the reducing gases; partly mechanical, in their effects on the properties of the metal _per se_, and partly chemical, as the oxide had probably entered into chemical combination with some of the objectionable impurities, producing compounds, in which form they were much less harmful. The removal of this oxygen from the metal breaks down such combinations, leaving the reduced impurity again to exercise its destructive effects on the properties. Over-poled copper, like dry copper, being brittle, is commercially useless as such, and is really an intermediate product, the metal being brought to pitch again by further aëration to make it “dry,” after which it may be poled back to correct pitch. As already stated, the over-poling effects are not due to any intrinsic action of carbon directly on the copper itself.

Summarising, it may be stated that the most important commercial varieties of copper are:—

=Electrolytically-refined metal=, employed for electrical work (also for alloy-making).

=Tough Pitch Copper= for engineering uses.

=Best Select Copper= for alloy manufacture.

And, in addition, Lake Copper and some Converter Bars.

A number of unrefined metallic products met with in practice include:—

_Converter Bars._—The product from the Bessemer operation on copper mattes. Most converter metal is subsequently electrolytically refined, but several varieties of Australian and American copper are put on the market direct in this form. Being produced from fairly pure ores, which carry but little silver and gold, the converter metal may be sufficiently pure to render electrolytic refining unnecessary, and too low in gold and silver values to make such an operation profitable.

_Cathode Copper_ is the product from the electrolytic refinery, and is usually remelted, brought up to pitch, and cast into ingots previous to use.

_Black Copper_ is produced by the smelting of oxide ores, and is subsequently refined.

_Cement Copper_ is produced by wet processes, usually by precipitation from copper-bearing solutions by means of iron, the product being a rather impure reddish-brown spongy mass. Many varieties contain arsenic. It requires melting and subsequent refining to adapt it for service.

_Blister Copper_ was the name given to the crude metal from the older type of furnace operations. Such copper contained large quantities of gas, particularly SO_{2}, which, tending to escape at the moment of solidification in the mould, gave a blistered appearance to the surface. It contained 96 to 98 per cent. of metallic copper, and was subsequently refined. The term is generally applied still to all crude copper exhibiting similar features.

_Chili Bar_ is an impure copper imported from Chili for refining. The composition varies, the metal usually containing 96 to 98 per cent. of copper, with indefinite quantities, sometimes small, of undesirable impurities.

Appended is a series of representative analyses of various copper products, compiled from different sources. The composition of such material as tough pitch copper and the various cruder varieties is, however, subject to very great variation.

=The Sources of Copper.=—Copper ores usually consist of various minerals of copper mixed with those of many other metals, and accompanied by very varied gangue, according to the locality in which they are found.

They are best classified under three groups:—

(1) _Native Ores._
(2) _Sulphide Ores._
(3) _Oxide Ores._

The most important points to be noted with regard to the distribution of these different classes are that—

(1) Native ores are localised in their occurrence, being chiefly confined to the Lake Superior district.

TABLE IV.—ANALYSES OF VARIOUS COMMERCIAL COPPERS.

──────────────────────────────┬───────┬──────┬───────┬───────
│Copper │ Gold │Silver │ Lead
──────────────────────────────┼───────┼──────┼───────┼───────
1. Electrolytic conductivity │ 99·89 │ nil │ nil │ nil
copper, │ │ │ │
2. Lake copper, │ 99·77 │ nil │0·029 │ nil
3. Best select copper, │ 99·75 │ .. │ .. │0·024
4. Tough pitch copper, │ 99·41 │ .. │ .. │0·070
│ 99·25 │ .. │0·36 │0·0103
5. Copper fire-box plate (ran │ │ │ │
500,000 miles, Met. Ry.),│ 98·70 │0·0001│0·0346 │0·4085
│ │ │ │
INTERMEDIATE PRODUCTS— │ │ │ │
Refined converter copper, │ 99·25 │ .. │0·36 │0·0103
│ 99·08 │ .. │0·30 │0·0085
Cathode copper, │ .. │ .. │0·001 │0·00054
Black copper, │ 94·39 │ .. │0·11 │0·19
│ 97·70 │ .. │0·2133 │0·78
Cement copper (Spanish), │ 51·90 │ .. │2·35 │1·45
│ 76·93 │0·10 │ .. │ trace
Blister copper, │ .. │0·0009│0·04 │0·042
Chili bar, │ 98·60 │ .. │ .. │ trace
──────────────────────────────┴───────┴──────┴───────┴───────

──────────────────────────────┬─────────┬────────┬────────┬───────
│ Arsenic │Antimony│ Bismuth│ Iron
──────────────────────────────┼─────────┼────────┼────────┼───────
1. Electrolytic conductivity │ 0·016 │ trace │ nil │0·042
copper, │ │ │ │
2. Lake copper, │ nil │ trace │ nil │0·0077
3. Best select copper, │ 0·025 │ trace │ 0·011 │0·10
4. Tough pitch copper, │ 0·320 │ trace │ 0·010 │0·010
│ 0·0211 │ 0·630 │ 0·0044 │ ..
5. Copper fire-box plate (ran │ │ │ │
500,000 miles, Met. Ry.),│ 0·3726 │ 0·0346 │ 0·0360 │0·0069
│ │ │ │
INTERMEDIATE PRODUCTS— │ │ │ │
Refined converter copper, │ 0·0211 │ 0·0630 │ 0·0044 │ ..
│ 0·0290 │ 0·0254 │ 0·0035 │ trace
Cathode copper, │ 0·00034 │ 0·0008 │ 0·0003 │ ..
Black copper, │ trace │ .. │ .. │ ..
│ 0·052 │ 0·2380 │ 0·0035 │0·17
Cement copper (Spanish), │ 2·95 │ 0·50 │ 0·95 │7·00
│ 1·32 │ 0·02 │ .. │7·6
Blister copper, │ 0·108 │ 0·157 │ 0·055 │0·4
Chili bar, │ 0·100 │ trace │ nil │0·009
──────────────────────────────┴─────────┴────────┴────────┴───────

──────────────────────────────┬────────┬────────┬────────┬───────
│ Nickel │ Tin │ Oxygen │ Sulphur
──────────────────────────────┼────────┼────────┼────────┼───────
1. Electrolytic conductivity │ 0·006 │ .. │ 0·008 │ nil
copper, │ │ │ │
2. Lake copper, │ 0·0146 │ nil │ 0·070 │ ..
3. Best select copper, │ 0·061 │ .. │ 0·143 │ ..
4. Tough pitch copper, │ 0·060 │ .. │ 0·120 │ ..
│ .. │ .. │ 0·284 │ ..
5. Copper fire-box plate (ran │ │ │ │
500,000 miles, Met. Ry.),│ 0·3039 │ .. │ 0·0181 │ 0·0064
│ │ │ │
INTERMEDIATE PRODUCTS— │ │ │ │
Refined converter copper, │ .. │ .. │ 0·284 │ ..
│ .. │ .. │ 0·12 │ 0·01
Cathode copper, │ .. │ .. │ 0·005 │ ..
Black copper, │ 2·04 │ 0·07 │ .. │ 0·80
│ .. │ .. │ .. │ 0·796
Cement copper (Spanish), │ .. │ .. │16·00 │ 5·10
│ .. │ .. │ .. │ 0·48
Blister copper, │ 0·0–0·2│ 0·0–0·5│ .. │ 0·112
Chili bar, │ .. │ .. │ .. │ 0·909
──────────────────────────────┴────────┴────────┴────────┴───────

(2) Sulphide ores supply the bulk of the world’s copper, constituting upwards of 80 per cent. of the total.

(3) The oxidised ores are found in most copper districts, though usually to only a limited extent. They are often gossan deposits produced by weathering or by decomposition of sulphides, hence are generally found nearer the surface, changing to sulphide with depth. The supply of copper from oxidised ores, which was at one time very large, is decreasing rapidly, and the greater proportion of the copper now obtained from them comes from the more recently developed deposits, of which those at Tanganyika afford an example.

More than 200 minerals which contain copper are known, but most of them are unimportant from the smelting point of view. The characteristics of the more noteworthy may be fully studied from text-books of economic mineralogy.

=Copper Ores—Native Copper.=—Occurs extensively in the Lake Superior district of Michigan, in Precambrian rocks, sparingly in New Mexico and China, but seldom anywhere else in workable quantities by itself. Copper barilla or copper sand, an impure native metal from Chili, was formerly of importance. Native copper constitutes about 20 per cent. of the North American supply. It yields metal of exceptional purity, and the brands of Lake copper reach a very high standard, both as regards electrical and mechanical properties. A still purer variety is the native metal from Yunnan, China.

The Lake Superior copper occurs in three formations:—

(_a_) Vein deposits, from which the enormous masses of
copper are taken out.
(_b_) Copper-bearing ash beds, of amygdaloidal diabase.
Chief mine, Quincy.
(_c_) Beds of conglomerate in which the cementing material
consists partly of copper. This last class of deposits
yields three-quarters of the Lake copper supply. Their
average copper content is 2·9 per cent. The chief mines
are the Calumet and Hecla, the Tamarack and the Atlantic,
all situated on one ore chute measuring 3 miles in
length, and worked to a depth of 4,000 feet.