Fig. 2. Fire-clay Crucible.
Fig. 2. Fire-clay Crucible.
The assaying of the genuine ores is performed in the following manner; that is, if they contain but little earthy matter. They may then be conveniently treated by fusing with carbonate of soda, on account of its cheapness, and borax, in a fire-clay crucible (Fig. 2). The dimensions of the crucible should be as follows: 4½ inches in height, and 2½ inchesin its greatest diameter, which should be at the top. A quantity of litharge (a semi-vitrious substance, oxide of lead), more than is actually necessary to take up the whole of the silver in the ore, should be added, so as to promote fusion, and collect the ingredients into one mass at the bottom of the crucible. In preparing the ore for the crucible, it must be well pounded, and intimately mixed with the undermentioned chemicals:—
Place two crucibles to warm during the time occupied in the preparation of the mixture, then put it into the warm crucible; take 100 grains more of litharge, and powder it over the contents in the vessel. Prepare in this manner a second mixture for the other crucible, place them both in the furnace, and put plenty of coke round them. The mixtures may be melted in an ordinary wind or melting furnace, such as is used by jewellers in the preparation of their material for art working. The fusion should take place very gradually at first, as silver in combination with lead is sensibly volatile at a high temperature: itmay then be continued at a low heat for twenty-five minutes, and finally the operation may be completed with a full red heat for five minutes longer.
During the process of fusing the contents of the crucible may be watched by removing one of the bricks from the top of the furnace, and when the whole mass has become quite liquid the crucible must be seized with a pair of suitable tongs, tapped once or twice very lightly against the side of the furnace to procure the settlement of the contents, and immediately poured into an iron mould, previously warmed and greased to prevent adhesion and spitting. Allow the mould to remain for some time, in order to partially cool, and then plunge it into a vessel of cold water. On cooling, the metallic elements will be found incorporated into a button, the slag can then easily be removed by tapping with a hammer on the edge, and the plunging into cold water greatly facilitates this separation. The whole mass has then to be cupelled, in order to separate the silver from the lead and other metals.
Fig. 3. Fire-clay Fusing Cup.
Fig. 3. Fire-clay Fusing Cup.
Silver ores, containing a large proportion of the sulphides (chemical combinations of sulphur with metallic substances) of other metals, may be easily assayed by the scorification process, which is, without exception, applicable to the assay of all kinds of argentiferous ores; and is one of the best,most simple, and most exact methods that can possibly be employed in the extraction of silver from its ores. This process, like that of fusion with litharge, already described, has the effect of producing an alloy, and subsequently requires cupellation. The ore is first well pounded, and then put into a small shallow vessel made of close-grained refractory fire-clay (Fig. 3), with an excess of finely granulated lead and some borax. The fusing cup or scorifier employed in this process should be about 1½ in. high and 2½ ins. in its greatest diameter; some assayers, however, use them deeper in proportion to their width, and representing in form the end of an egg. The object of this shape is to preserve the bath of molten metal at the bottom, and that it may always be well covered and protected by the slag on the top during the process of fusing. In the scorification method the principles are exactly the reverse of those of the crucible assay; for in the latter the object is to reduce the oxide of lead to a metallic state, whereas in the former the metallic lead added to the pounded ore in the scorifier is oxidized by being fused in contact with the air. The charge for this assay may be as follows:—
The cups or scorifiers should be charged in the following manner: well mix the silver ore with 300 grains of granulated lead; place this mixture in a scorifier, and add 300 grs. more of granulated lead, and over the top of the whole put the burnt borax. The vessel may then be placed in an ordinary assay furnace or muffle, as many being introduced at one time as there is room for in the furnace, and submitted to the strongest heat for about thirty minutes; during the greater portion of this time the door should be kept closed, especially for the first fifteen minutes. On opening the muffle-door a current of air passes through the furnace, converting a portion of the lead into litharge; this enters into combination with the earthy portions of the ore, the other metallic sulphides, and also the borax, producing a fusible slag on the surface of the metallic bath, extending over the whole surface of the scorifier. The excess of lead is thus protected by this film or flux from the oxidizing effects of the currents of air admitted into the furnace, and remains united with whatever silver there may be in the ore, in a metallic state.
The fusing should be continued longer than thethirty minutes—in fact until the slag or flux is reduced into a perfectly liquid state; stirring it well with a slender iron rod will facilitate the operation, as it will tend to mix with the mass any hard portions remaining undissolved and attached to the sides or other parts of the vessels. This condition of the flux is absolutely indispensable; when the slags are quite liquid, which with a strong fire will take place in from thirty to forty minutes, wrap up in a piece of paper the powdered anthracite, and drop it into the scorifier while still in the furnace or muffle. The object of adding the anthracite at the last moment is to reduce any minute portions of the metal that may exist in the slags, and remain separated from the bulk. When the anthracite has burnt off, which process usually takes about five minutes, this point is considered to have been attained, and the operation is then complete. The scorifier may be immediately withdrawn from the fire, and the contents poured into a suitable casting-mould, of the form represented inFig. 4, a button of silver lead being the result. When cold, the metallic mass is readily separated from the slag or flux by slightly tapping with a hammer; the former may then be passed on to the next operation, viz. to be purified of its lead by the process of cupellation, which will be presently described.
When there is not enough borax present the assayer will observe an infusible skin floating upon the surface; should this be the case more borax must at once be employed, in order to dissolve such impurity. When a chloride of silver ore is to be assayed, carbonate of soda must be added to the mixture to prevent sublimation.
The following method of assaying is adopted in several large Continental establishments, where the ores have, beside the usual earthy matter and the sulphides of lead, an admixture of zinc, iron, and copper. The process is precisely similar to the crucible assay, in the case of genuine silver ores, as already described—with this exception, that no more lead is added than the ores then contain—that is, if we are treatinggalenaorsilver lead; other ores require different treatment according to their known composition. In this process wrought-iron crucibles are employed having the form and shape as shown inFig. 5. They are made of thick iron plate, and are rendered secure by welding the edges firmly together. Their dimensions are as follows: a depth of 4½ ins., with a thickness of iron at the bottom of 1½ in., and a ¼ of an inch in the sides; the diameter at the top of the crucible should be about 2½ ins., and at the bottom between 2 and 2¼ ins. A mechanicalmixture or flux is prepared to use with the ores to which we have referred, consisting of the following chemicals, all of which should be finely powdered and well mixed with the ore to be assayed:—
The furnace used for this assay is the ordinary one, having rather a high chimney, to insure a perfect draught. In effecting the reduction of the silver, the crucible is first placed as before on the fire, and allowed to become hot; when this is accomplished, take
These ingredients should be thoroughly mixed together, and put into the red hot crucible. Fuse at a low heat for about twenty minutes, when the whole will be in a perfect state of fusion; then give about five minutes strong heat, and at the end of that time the crucible may be withdrawn, and its contents poured into an iron mould, as represented inFig. 4, having one or two conical holes for the reception of the fused mass. The silver and lead collect at the bottom of the mould by reason of its high specific gravity. It may be removed by reversing the position of the latter,when a gentle tap or two will deprive it of that slag or flux which is usually attached to it. A large quantity of silver can be readily collected from its ores by an alternate use of crucibles, in which case it is possible to make a regular number of fusions per hour. Wrought-iron crucibles, when strongly prepared and carefully made, will stand about thirty of these fusions, giving way in the end on account of the action of the sulphur contained in the ores.
Fig. 4. Iron Casting-moulds.
Fig. 4. Iron Casting-moulds.
Another kind of crucible, in addition to those already mentioned, is used by the trade, and is recommended by many assayers as superior to all others.Fig. 6represents the form of it. It is about 4½ ins. high, and 2 ins. in its greatest interior diameter, being in the form of a skittle. The charge consists of the following in this assay:
Put the powdered ore into the crucible, andplace upon it the iron, which should not be in the form of filings or dust, but in small pieces; upon the ore and iron should be put the black flux, and lastly the common salt must be placed above all these substances as a protection against the air. The crucibles, as many as convenient, may now be introduced into the furnace, and slowly raised to a strong red heat, at which temperature they should be kept for about half an hour; at the end of that period they should be removed from the fire, slightly tapped to settle the contents, and then placed aside to cool. When this has taken place, a few blows with a hammer near the base of the crucibles, each in turn, will soon expose the button of silver attached to the undecomposed iron; the latter substance may, however, be easily detached by a few well-directed blows with the hammer.
Fig. 5. Iron Crucible for Assay.Fig. 6. Fire-clay Crucible for Assay.
In order to ascertain the exact amount of theprecious metal—that is, the silver—contained in the buttons of lead obtained as the results of the foregoing operations, they are subjected to a purifying process by the metallurgist, called cupellation. By this means the lead and other impurities are driven off by heat in contact with a current of air, and the silver is left behind in a pure state. To perform this operation it is necessary to expose the buttons on some absorbing medium or porous support, and this support is commonly known as acupel. No doubt many porous substances could be made available for the formation of cupels, but bone-ash is the best for all practical purposes, such as are required by the assayer. The bone-ash, in the condition of a very fine powder, is mixed with a little water in which has been dissolved a small quantity of potash, and moulded into the desired shape. The cupels are tightly consolidated by pressure in an iron mould of the form shown inFig. 7, which is the best in use, being well adapted for the manufacture of cupels. It consists of a slightly conical steel ring, 2 ins. in depth, and about 1½ in. in diameter at the top internally; a steel die with a wooden handle (Fig. 8) is made to fit the mould. To make a cupel the space in the ring is nearly filled with the moistened bone-ash, and pressed down by the hand, and afterwards bythe die, the latter being driven into the ring by the application of a wooden mallet (Fig. 9) to the handle of the die. It will be seen from the illustration that the die forms a cavity in the cupel capable of receiving the charge of metal for assay. When the bone-ash has been sufficiently compressed, the die is withdrawn, and the cupel removed from the ring. This is a delicate operation, as sometimes the edges of the cupel are liable to be injured; to prevent which and facilitate the removal a loose plate of iron, exactly fitting the bottom of the mould, should be introduced previous to putting in the bone-ash. The iron plate of course being removed with the cupel, it must be replaced before another can be made. By introducing a cylindrical piece of wood to the lower aperture of the steel ring, the cupel can be removed without difficulty.
Fig. 7. Cupel Mould.Fig. 8. Die for Cupel.Fig. 9. Wooden Mallet.
The size of the cupel should always be regulated according to the quantity of foreign matter to beabsorbed, it being generally understood that the material of which it is formed takes up double its weight of lead. The process of cupelling is conducted in the furnace of the assayer, an apparatus of peculiar construction, the most important part of which, however, is the muffle (Fig. 10), consisting of a small arched oven of fire-clay closed at one end, and furnished with perpendicular slits in the sides, in order to allow of a free access of air to the cupels inside.
Fig. 10. Assayer’s Muffle for Cupels.
Fig. 11. Cupel Tongs.
Fig. 12. Cupels, section and perspective views.
The position of the muffle in the furnace is so arranged that it can be readily heated on every side; and when it has become red hot, six or eight cupels, previously well dried, are taken and placed on the floor of it, which should be covered with a thin layer of bone-ash. The form of tongs required for this purpose is shown inFig. 11. When the cupels have been raised to the temperature of the muffle itself, the assays are put in by a very slender pair of tongs, the door of the furnace is then closed for a few minutes, when the metal will have becomefused, and the litharge will begin to be taken up by the bone-ash of which the cupel is composed. The temperature of the furnace is now lowered as much as possible, although not to such an extent that it will retard the progress of oxidation and absorption. When nearly the whole of the lead has been thus absorbed, the bead remaining will have become very rich in silver, and, as the oxidation proceeds, will appear much agitated, assuming a rapid circular movement, and revolving with great rapidity. The silver gradually concentrates itself in the centre of the cupel, taking the form of a globule, and at this stage the fire should be made sharper, the operation being carefully watched. When the last particle of lead leaves the silver, the agitation will suddenly cease, and a beautiful phenomenon be witnessed, called by assayers thebrightening. The button of silver then becomes brilliant and immovable, and the operation, when this takes place, is complete. The cupel must be cooled with very great care, in order to prevent the silver fromsprouting; which if allowed to take place would result in considerable loss, besides destroying theaccuracy of the assay. To prevent this sprouting it is a good plan immediately to cover the cupel by another, which has been heated for that purpose; the two are withdrawn together, and allowed to remain at the mouth of the muffle until the silver has become solid; the metal is then in a state of almost chemical purity, and may be detached and weighed. Previous to the latter, however, it should be carefully cleansed from all foreign matter, and flattened on a smooth-faced anvil, this process greatly assisting in the removal of any oxide of lead, which not unfrequently attaches itself to the globule of silver. The weighing is conducted with a pair of scales having an extremely delicate balance; and where any commercial transaction depends upon the accuracy of the assay, it is always imperative to make several tests of the same sample, to avoid the consequences of any accident or mistake.
The chief element in combination with silver on the large scale is lead. Formerly the plan adopted in the separation of this metal was cupellation alone. This process on the large scale is somewhat different from that just described; and as it may appear to the reader interesting and instructive, a brief explanation of it may not be considered out of place.
CHAPTER IV.
The Cupellation of Silver Ores.
This interesting process is performed in a reverberatory furnace of a very peculiar construction, the cupel employed on the large scale differing somewhat from the ordinary one, being considerably larger and varying also in form. It consists of a strong oval wrought-iron ring, with a part of the full shape omitted, as shown in accompanying sketch, in order to allow of the overflow of lead during the process, in the form of litharge. This iron ring, known as thetest ring, contains the cupel, and in order to prepare the latter, the frame, which measures about 60 ins. in its longest diameter, 40 ins. in breadth, and 6 ins. in depth, is strengthened by having a number of broad strips of iron seamed across the bottom by riveting to the sides of it. The cupel itself is prepared for use by taking finely ground bone-ash, together with a little carbonate of potash, and working themup with just sufficient water to make the mass cohere properly; the carbonate of potash may be advantageously dissolved in the water; the latter is then applied in small quantities at a time to the bone-ash until the proper coherency has been obtained; of the total quantity of bone-ash employed in the operation, 2 per cent. of potash will bequantum sufficitto mix with it. The iron frame, or test, is then filled with the mixture, and it is pressed down into a solid compact mass, the centre part being hollowed out with a small trowel, the sides sloping towards the concavity in the middle; the hollow should not however be extended more than within 1 to 1½ in. of the bottom of the frame, and above the iron bars. The cupel forms the hearth of the furnace we have spoken of, and of whichFig. 13is a sectional view; it is removable, and not a fixture in the furnace. It must be left for several days to dry, after having been constructed as described, when it is ready for use, and only requires firmlywedging in its place beneath the arch of the furnace.
Fig. 13. Cupels, section and perspective views.
The fire should be only very moderate at the commencement of the operation, and the furnace slowly raised in temperature, lest the cupel should crack by being too quickly heated. As the temperature increases, if without any apparent defects in the bone-ash cupel, or hearth, which it may now be termed, the wind or blast, generally driven by a fan, is thrown in through a nozzle, or an aperture in the furnace, which, for facilitating the immediate removal of the bone-ash hearth, is placed upon an iron car, and runs beneath the vault of the furnace on rails, so that it may thus be very readily withdrawn when found necessary. The admission of a current of air into the furnace oxidizes the excess of lead, in combination with the silver, producing litharge on the surface of the molten mass; the formation of the litharge takes place rapidly, and it is continually blown forward by the strength of the blast as fast as it is produced, running through a gap or channel specially made for the purpose in the mouth of the cupel into a movable iron pot which is placed for its reception. The continual oxidization and flow-off of the lead alters the respective proportions of the metals in the cupel. For this reason it isalways kept full of lead ore, which is effected by taking it in its fused state from a kettle in which it is ready melted by means of a long-handled ladle; and thus about 500 or 600 lbs. of metal are constantly kept in this bone-ash cupel or hearth.
As the silver necessarily increases in the hearth, it will require to be occasionally withdrawn, in order to make room for a further supply of lead ore. This process is adopted when it reaches about from 8 to 10 per cent. of silver to the ton (between 2,000 and 3,000 ozs.), and may be effectually performed by drilling a hole underneath the cupel, and letting the silver flow through it into a receptacle placed to receive it. Of course the operations of the furnace are arrested while these manipulations are being carried on. After the withdrawal of the silver, the hole is closed up again with a plug of moistened bone-ash prepared as before; when the process may be continued a second time by giving 500 or 600 lbs. of fresh lead ore to the cupel. Thus a single cupel will often last 48 hours, and 6 or 7 tons of lead may be oxidized upon it.
We have already observed that the prolongation of the cupelling process increases the richness of the remaining alloy, and this very rich silver-lead alloy is again subjected to a second operation in cupelling. This process of assaying or refiningis similar in every respect to the former, and is often performed in the same furnace, the cupel being first of all brought to almost a bright red heat, when about 600 lbs. of the silver-lead alloy are added, and a strong current of air given in order to oxidize the remaining lead in combination with the silver. In this operation the material under treatment, previous to its introduction to the cupel, should be melted in a kettle easy of access, and added in its fused state. The current of air in connection with the heat of the furnace immediately begins to purify the silver by oxidizing the lead, and forms litharge, which passes off through the channel provided in the mouth of the cupel; as this proceeds, fresh silver-lead alloy is added, to keep the level of the metal always at the same height. This is continued until some three tons of the alloy from the first cupellation have been put in, and when about 600 or 700 lbs. of silver are collected in the cupel.
When the cupel has received the above proportion of metal, the addition of the alloy ceases, and the silver is allowed to purify. The litharge which passes off towards the close of this process will be richer in silver than in the former one; consequently it is found best in practical metallurgical operations to treat in a special manner thelast part of silver cupelling on the large scale, for it needs very careful management indeed to secure all the silver, especially to do so in a fine state. Towards the completion of the process the fire should be increased considerably, in order to keep the silver thoroughly melted, and also to oxidize and completely remove every trace of lead that is possible. As it begins to purify itself from the remaining lead a characteristic brightness will be perceived. When this takes place the fire must be lowered, the wind or blast stopped, and the metal left to cool gradually. This latter proceeding is of some importance, as a too sudden cooling of the surface causes the interior of the metal to expand and shoot, by which means little globules of silver may be lost; therefore it should be allowed to cool very slowly.
The iron ring encircling the cupel with its contents may now be drawn from beneath the arch of the furnace, and the cake of silver taken from its bed in the bone-ash which formed the vessel, and cleaned of any impurity; when it may be re-melted in a plumbago crucible, and cast into ingot moulds. These moulds should be made of iron, and should always, when used for this purpose, be warmed and greased a little, previous to the introduction of the melted material, to prevent themetal from spitting and adhering to it. If skilfully treated during the process of cupellation, the desilvered lead seldom contains more than ·002 to ·003 per cent. of silver to the test assay of 200 grs., or between six and ten pennyweights to the ton, beyond which point it is unprofitable to carry on the operation.
The litharge which is formed and passes off during the process gradually grows richer in silver towards the end of the cupellation. It probably contains after concentration about thirty to forty ounces of silver to the ton of litharge. This is again subjected to the several operations of the same kind for the recovery of the silver.
It is somewhat remarkable that the present method of recovering and purifying this metal bears a strong resemblance to that employed in ancient times, and which is spoken of in the Holy Scriptures by the prophet Ezekiel (xxii. 18 and 20): “Son of man, the house of Israel is to me become dross: all they are brass, and tin, and iron, and lead, in the midst of the furnace; they are even the dross of silver.” And also, “As they gather silver, and brass, and iron, and lead, and tin, into the midst of the furnace, to blow the fire upon it, to melt it; so will I gather you in mine anger and in my fury, and I will leave you there, and melt you.” Thecelebrated metallurgist Dr. Lamborn says, “Only those who have seen, beneath the glowing arch at the smelting works, flames surging wave after wave across the surface of the liquid metal, carrying all the substances, here called dross, from the pure silver; and only those who have heard the roar of the fiery blast, that ceases neither day nor night, until its task of purification is accomplished,—can appreciate the terrible force of the figure made use of by the prophet.” According to the above scriptural passage it is evident that the ancients were in possession of the first rudiments of assaying, and understood to some extent the purification of metals; but scriptural testimony does not point out with what amount of skill and success these operations were performed. Judging from the appliances which have been handed down from generation to generation, we are inclined to think they must have been practised somewhat rudely; for it has been left to the present school of scientific and practical metallurgists to found and develop the art in the direction of that commercial success to which it has at the present day attained.
This plan of cupellation which we have just described is still adopted in many continental works in the assaying of silver-lead ores. InEngland the system has been almost entirely superseded by one invented by the late Mr. Pattinson of Newcastle, and which is confidently stated to be far more convenient in practice.
CHAPTER V.
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 thefiligreekinds, 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 anamalgam. 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 forspecialpurposes 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, andantimony 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 havebeen 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 soreadily 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 theprotoxide 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.
Fig. 14.Venus.Egyptian Mark for Copper.
The name given to this metal by the alchemists wasVenus(Fig. 14), which is one of the principal planets, whose orbit is situated between the Earth and Mercury. The scientific name ofcuprumfor 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 overthe 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 sometimesspeciallyemployed, 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 calledspelter. 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 thecommencement 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 oxidewhich 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 symbolZn. Its value when in a state of purity, commercially speaking, is about 4d.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 Phœnicians; 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.
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 inFig. 15, and related to the planet of that name, one remarkable for its brightness.In mythologyit 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.
Melting-points of the Principal Metals.
Physical Properties of the Principal Metals.