The Alloys of Silver.
Fine silver enters freely into combination with nearly all the useful
metals, but its most important alloys are those prepared from copper,
the latter substance being more suitable for the production of
silversmith's work than any other; whilst it produces a more pleasing
effect, if not over-alloyed, in regard to finish. Silver articles,
especially of the _filigree_ kinds, if the designs are good, possess
a very tasteful appearance. In treating of the alloys of silver, it
is our intention, first, to give a cursory glance at the chemical
and physical properties of the metals which form these alloys. Such
a description, although brief, will, we believe, prove of essential
service, not only to working silversmiths and metalsmiths, but also to
goldsmiths and jewellers, who are constantly manipulating with these
inferior metals in precisely the same way as the silversmith. Besides,
such information cannot, we apprehend, fail to be useful, whether to
the student, the theorist, or the practical worker.
An alloy is the union of two or more metals by fusion, so as to form
a metallic compound. It may consist of any number of the metallic
elements, and in any proportion, provided they will chemically combine,
always excepting mercury as one of the ingredients. In this latter case
the mixture is called an _amalgam_. Chemistry has made us acquainted
with about forty-nine metals; of that number, however, not more than
fourteen are employed to any considerable extent for industrial art
purposes. They are as follow: Gold, silver, copper, zinc, platinum,
aluminum, nickel, iron, mercury, lead, tin, arsenic, antimony, and
bismuth. Some of these are occasionally employed for _special_
purposes in the arts in their pure state; but where hardness is to be
a distinguishing characteristic, combined with certain variations in
shades of colour, a union is effected of two or more of these metals
in different proportions, by fusion and stirring, so as to form the
requisite alloy. Metals used in the pure state, that is, without any
mixture of alloy, have very few applications in regard to industrial
pursuits and the arts. The precious metals--gold, silver, &c.--would
be much too soft, while, on the other hand, arsenic, bismuth,
and antimony would be far too brittle to be employed alone for
manufacturing purposes. It is quite possible to effect some thousands
of alloys, but there do not appear to have been more than about three
hundred practised successfully for commercial purposes.
The principal alloy of silver, as we have already remarked, is copper;
but, occasionally, nickel, and even zinc are employed in the case of
the commoner qualities of silver. Tin is also used in the preparation
of solder for these qualities, in order to render it the more easy of
fusion when used for soldering the work. Of the distinctive features of
these elements of silver-alloy we shall now speak with some amount of
detail.
Silver will unite with copper in various proportions by melting the
two ingredients together, and stirring them whilst in a fused state. A
product will thus be formed differing physically in character from fine
silver, caused by the loss of some little of the latter's ductility
and malleability; but, on the other hand, a compound will be produced
harder and more elastic, which is in every sense better adapted to the
manufacture and also to the durability of the articles made by the
silversmith.
_Copper_, like the precious metals, appears to have been known from
a very early age, being one of the six metals spoken of in the Old
Testament; and described by the historian as being also one of the
seven made use of by the ancient philosopher. It is of a reddish
colour, malleable, ductile, and tenacious. It is largely employed in
alloying both gold and silver for the manufacture of jewellery and
other articles. With regard to malleability, it stands next to gold and
silver in the list of useful metals; in ductility it occupies the fifth
position; and in tenacity one only is superior, viz. iron. It is not
very fixed in the fire, for if subjected to a long-continued heat it
loses a part of its substance; for this reason the alloys of silver and
copper should be carefully watched in the crucible to prevent this loss
when under the action of the fire.
When struck copper gives only a feeble sound, and is easily abraded by
the file. It fuses at a good white heat, or about 1994° Fahr., although
some authors have given it as 1996° Fahr. Its specific gravity varies
between 8·88 for cast copper, and 8·96 when rolled and hammered. It
loses between one-eighth and one-ninth, or 4/35ths, of its weight in
water. When exposed to a damp atmosphere a greenish oxide, called
verdigris, is produced on its surface, and this is one of the reasons
why silver articles containing a percentage of copper become so
readily discoloured if left exposed to atmospheric influences; copper
also, if heated in contact with the air, quickly becomes oxidized,
and, on being touched, scales fall off: these form the _protoxide
of copper_. If this process is frequently repeated under a great
heat, each time the metal is operated upon it loses a part of its
malleability and ductility, which are both eminent characteristics of
the pure metal. Most of the ordinary acids act on copper but slowly in
the cold, but nitric acid very readily dissolves it, even if largely
diluted. Copper amalgamates with most of the metals, and its subsidiary
alloys are very largely employed in the arts and manufactures of every
kind.
The bean-shot copper of commerce, costing about a shilling per pound
avoirdupois weight, is quite good enough for all the practical purposes
of the silversmith.
[Illustration: Fig. 14. _Venus._ Egyptian Mark for Copper.]
The name given to this metal by the alchemists was _Venus_ (Fig. 14),
which is one of the principal planets, whose orbit is situated between
the Earth and Mercury. The scientific name of _cuprum_ for copper
is derived from the Isle of Cyprus, where, it is said by Pliny, the
Greeks discovered the method of mining and working it. Copper is found
distributed all over the world; a considerable portion, however, is
found in the United Kingdom.
_Nickel._--This metal is found chiefly in the Hartz Mountains. It
was formerly called by the Germans "Kupfer nickel," or false copper,
"nickel" being a term of detraction. It was first discovered about a
century and a half ago by Cronstedt. It has a greyish-white colour, and
is slightly magnetic, _i.e._ it is attracted by the magnet in the same
way as iron and steel, but it loses this property if heated to about
600° Fahr. Its specific gravity varies between 8·40 and 8·50, according
to the amount of compression it has received, and it is rather brittle;
it may, however, be drawn into wire, and rolled flat, or into sheets.
It is considerably harder and less ductile than any of the other metals
employed in jeweller's and silversmith's work. In hardness it nearly
approaches iron, and on this account, when polished, a characteristic
brightness is produced. The malleability of nickel is less than that of
iron, standing tenth in the list of useful metals; and in ductility it
also occupies the tenth position. Nickel is very infusible, and does
not so easily oxidize or tarnish at ordinary temperatures as copper
does. Several countries have tried to employ it in the manufacture of
small coin for the currency, but its use has now been almost abandoned.
Nickel alloys are much used in the arts for manufacturing purposes,
under the name of "German silver," there being large demand for
this metal, as it forms the hard white alloy much used in making
"electro-plate," and on which silver is afterwards deposited. It also
is used in common silver alloys, in order to keep up the whiteness of
the latter element, the addition of too large a proportion of copper
maintaining the tint of the latter metal, in too strong a degree to
be altogether employed by the silver-worker. Nickel is sometimes
_specially_ employed, in combination with other metals, to replace
or imitate silver in the manufacture of commercial wares, while with
copper, zinc, tin, &c., it forms very useful alloys, producing great
hardness.
_Zinc._--This metal in its pure state is sometimes called _spelter_. At
the present day it is not much used for alloying silver; but, as it is
commonly employed in the preparation of silver-solder, it is necessary
that the amateur and the student should know, as well as the practical
mechanic, the distinctive characteristics of it, together with the
qualities it imparts to others when in combination with them. As a
metallic substance it was unknown until a long time subsequent to the
discovery of the principal metals; and only since the commencement of
the present century has its uses been thoroughly known and appreciated
in the industrial world. In its pure state, zinc is a bluish-white
metal, hard and highly crystalline; but, when raised to a heat of
between 250° and 300° Fahr., it is malleable, and may safely be rolled
and hammered: it is in this way that the zinc of commerce is produced.
Zinc may be annealed by placing it for a time in boiling water.
Its specific gravity varies between 6·8 and 7·2, according to the
previous kind of mechanical treatment it has received. At 773° Fahr.
it melts, and is quickly oxidized by exposure to a current of air,
emitting white vapours, which rise into the air, and are not unlike
cotton-flakes; oxide of zinc is thus formed by the burning away of
the zinc. Spelter or zinc is employed by jewellers in the manufacture
of bright gold alloys, as it gives liveliness of colour to their
wares not to be equalled by any other metal. (For the proportions
and treatment of this composition see "The Goldsmith's Handbook.")
It may be alloyed with most of the metals we have named; its uses in
roofing, gutters, spouting, and chimney-pots being all well known.
All the acids very readily attack it in the gold, and even when
largely diluted; it speedily tarnishes, and becomes covered with a
white oxide which protects the metal from atmospheric influences. In
point of malleability zinc stands eighth among the metals, seventh in
ductility, and as regards tenacity about seventh also. In chemistry it
is represented by the symbol _Zn_. Its value when in a state of purity,
commercially speaking, is about 4_d._ per. lb.
_Tin._--This appears to have been one of the oldest known metals, and
was employed in the Egyptian arts by the ancients, in combination
with copper. Its colour is white, with a shining lustre almost as
brilliant as that of silver, but it tarnishes much more quickly than
alloys of the latter metal. With the exception of aluminum and zinc,
it is the lightest of all the metals, its density being between 7·0
and 7·3, whether cast, hammered, or rolled. It is found in abundance
in Cornwall, where it was also obtained at a very early period by the
Ph[oe]nicians; and it is reported in Soame's "Latin Church," p. 30,
that it was through the medium of the trade in tin that Christianity
was first introduced into this country. Tin is not of a fixed nature
like gold or silver, but melts in a moderate fire long before it
becomes red hot, or about 442° Fahr. It is rapidly oxidized when kept
for a long time in a fire having a free access to the air; and it is
dissolved by hydrochloric, sulphuric, and nitric acids, the latter
acting on it most powerfully. Tin should not be alloyed with gold or
silver, as with either of these it easily enters into combination by
fusion, rendering them extremely brittle, especially in the case of
silver, which becomes by the least mixture of it so brittle that it is
totally unfit for the work of the silversmith. However, for solder,
for filing into dust, it may be advantageously employed to promote a
quicker fusion; but even for this it should be avoided where it is
possible to do so. The vapours of tin are also permanently injurious
in the melting of gold, silver, and their alloys, as they render them
very unworkable, and the operator being often at a loss to understand
the cause of his misfortune; therefore, in melting silver alloys, it is
advisable to avoid as much as possible the introduction of little bits
of scrap tin into the furnace. If such a thing should happen, however,
make the fire once or twice stronger in order that the tin may all be
destroyed before the crucible containing the silver alloy is put in.
[Illustration: Fig. 15. _Jupiter._ Egyptian mark for Tin.]
Tin is very malleable, moderately ductile, and tenacious, being fifth
on the list for malleability, eighth for ductility, and eighth for
tenacity. The Egyptian mark or symbol for tin (sign of "Jupiter") was
the same as is represented in Fig. 15, and related to the planet of
that name, one remarkable for its brightness. _In mythology_ it is
understood as representing the supreme deity of the Greeks and Romans.
The modern scientific name for tin is Sn. Tin loses over one-seventh,
or 4/29ths, of its weight in water from its absolute weight in air. In
the next chapter we shall treat of the mixing of silver alloys, &c.,
and in order to make our information regarding the various metals so
employed as complete as possible, we shall conclude this one with the
following tables, each of which will no doubt be found useful:--
Table of Metallic Elements.
+-----------------------+----------+---------------------+
| Names of Elements. | Symbols. | Specific Gravities. |
+-----------------------+----------+---------------------|
| Platinum | Pt | 21·40 to 21·50 |
| Gold | Au | 19·25 " 19·50 |
| Mercury | Hg | 13·56 " 13·59 |
| Lead | Pb | 11·40 " 11·45 |
| Silver | Ag | 10·47 " 10·50 |
| Bismuth | Bi | 9·82 " 9·90 |
| Copper | Cu | 8·89 " 8·96 |
| Nickel | Ni | 8·50 " 8·60 |
| Iron | Fe | 7·77 " 7·80 |
| Tin | Sn | 7·25 " 7·30 |
| Zinc | Zn | 6·80 " 7·20 |
| Antimony | Sb | 6·75 " 6·80 |
| Arsenic | As | 5·70 " 5·90 |
| Aluminum | Al | 2·56 " 2·60 |
+-----------------------+----------+---------------------+
Melting-points of the Principal Metals.
+--------------------+--------------+-----------------+
| Names of Elements. | Fahrenheit. | Centigrade. |
+--------------------+--------------+-----------------+
| Platinum | { Infusible, except by the |
| | { oxyhydrogen blow-pipe. |
| Cast Iron | 2786° | 1530° |
| Nickel | 2700° | 1482° |
| Gold | 2016° | 1102° |
| Copper | 1994° | 1090° |
| Silver | 1873° | 1023° |
| Aluminum | 1300° | 705° |
| Zinc | 773° | 412° |
| Lead | 612° | 322° |
| Bismuth | 497° | 258° |
| Tin | 442° | 228° |
| Antimony | Fuses a little below red heat. |
| Arsenic | Volatilises before it fuses. |
+--------------------+--------------------------------+
Physical Properties of the Principal Metals.
+---------------+------------+-----------------+
| Malleability. | Ductility. | Tenacity. |
+---------------+------------+-----------------+
| Gold | Gold | Iron 549 |
| Silver | Silver | Copper 302 |
| Copper | Platinum | Aluminum 300 |
| Aluminum | Iron | Platinum 274 |
| Tin | Copper | Silver 187 |
| Platinum | Aluminum | Gold 150½ |
| Lead | Zinc | Zinc 109½ |
| Zinc | Tin | Tin 34½ |
| Iron | Lead | Lead 27½ |
| Nickel | Nickel | [B] |
+---------------+------------+-----------------+
[B] The above weights were lbs. sustained by 0·787 of a line in
diameter, in wires of the various metals.