┌─────────────┬───────────────────────┬───────────────────────┐
│ │ 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.
[Illustration: Fig. 4.—Influence of Arsenic and Antimony on the
Electrical Conductivity of Copper.]
_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.
[Illustration: Fig. 5.—Relations of Copper and Oxygen.]
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.
[Illustration: _a_ _b_
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.