Electro-plating, with numerous engravings and diagrams

Front matter

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Edited

Chapter I

introduction: tanks, vats, and other apparatus.

Electro-plating is the process by which a metal or alloy held in solution by a liquid is deposited electrically on a prepared surface. From the solution, the metal (nickel, silver, brass, copper, etc.) is thrown on to the object receiving the deposit, and simultaneously an equal quantity of a similar metal to that held in solution is fretted off from metal plates (anodes) suspended in the electro-plating bath. Thus, although the solution is being constantly robbed of metal, it is yet being fed, and so its strength is maintained. Of course, the anodes must be plates of similar rhetal to that contained in solution ; thus, nickel anodes in a nickel solution, silver anodes in a silver solution, and so on.

Anode is the name given by Dr. Faraday to the positive plate or wire in a solution undergoing electrolysis. It is derived from two Greek words : ana, meaning "upwards"; and odos, "a way'' — the way in whicli the sun rises — and is an indication that the current rises from the battery to enter the electro-plating solution, this way being from the negative element of the battery to the positive element in the solution to be electrolysed (see the arrows in Fig. 1). The negative element of a battery receives the electric current immediately it is generated, and transmits it to the anode

T o Electro-pla riNG.

placed in the electro-plating bath ; the negative element is, therefore, the positive pole, and its connection to the anode by a wire makes the anode the positive pole or element in the solution. By way of illustration : the carbon element in a Bunsen battery (see pp. 28 to 30) is the negative element, because it receives the electric current generated by the positive, or zinc, element of the battery ; but it is also the positive pole, because, through it and its metal connections, the current is transmitted to work outside the battery. Fig. 1 shows at a glance what is meant.

Another scientific term has now to be referred to. Anion is a term invented by Dr. Faraday to indicate the radical of an acid, or the portion of a salt set free at the anode during electrolysis. It has been defined as " the electro-negative, or chlorous radical of the acid or salt decomposed." Assume that a solution of the double cyanide of silver and potassium is being used in the work of electro-plating. The salt of silver in this solution is combined with a salt of potassium, and three distinct substances are present, apart from the water which holds them all in solution ; these are —silver, potassium, and cyanogen. When the electric current is passing through the solution in the process of plating, silver and potassium are set free at the goods being plated, and cyanogen is set free at the silver anode. The salt is thus broken up or decomposed, and cyanogen is the anion of this salt.

Anodes may be soluble or insoluble in an electrolyte, as may be required to suit the nature of the work in hand. Insoluble anodes are used when it is wished to decompose an electrolyte, and break it up into its several component parts without adding another element to it, as when acidulated water is decomposed by the electric current to form oxygen and hydrogen — in which case a platinum anode is used, because platinum

Tanks, Vats, and other Apparatus. ii

is not soluble in the acidulated solution. Insoluble anodes are also used when it is wished to extract all the metal from its solution, and deposit it in a pure condition on the cathode (the article plated). Anodes are said to be insoluble when they are made of elements which are neither soluble in the solution to be electrolysed, nor can be made soluble therein under the influence of the electric current. Some solutions of the acids and alkalis will act very feebly, or not at all, on an element, even when heated to boiling point, but

ANOC

CATHODE

1. — Diag-ram showino- Positions of Anode and Cathode in Electro-plating-, will dissolve it freely when a current of electricity is passed from it through the intractable solutions. For instance, gold is only feebly soluble in a strong solution of potassium cyanide when exposed to air, even when the solution is heated ; but it is freely soluble in the same solution when only a feeble current of electricity is passed from it through the solution. Insoluble anodes are generally made of platinum or carbon.

Soluble anodes are used when it is wished to maintain an electrolyte (the plating solution) at its original strength— that is, to contain the same quantity of metal in the solution after it has been worked as it had when first made up. To do this, the anode must be only soluble in the solution whilst the current is passing, and then only to an extent equal to the rate of metal deposited. Unless an equivalent of metal is dissolved from the anode for each equivalent of metal deposited at the cathode, the original composition of the solution cannot be maintained during the process of electrolysis or plating.

The cathode is the opposite of the anode, the term being derived from the Greek fcafa, meaning " down,'^ and odos, "a way'." Thus the word signifies that the current passes down to it from the anode ; the current returns to the negative pole of the battery attached to the positive plate, thus completing the electrical circuit. The object being plated forms the cathode ; and just as the anode is connected electrically to the battery's positive pole, BO is the cathode connected to the negative pole.

For the present, this is all the theory of the process of electrolysis that it is desirable to explain, further information being given as the practice of electro-plating is dwelt upon. It must not be inferred from the theoretic explanation just given that electro-plating consists merely in taking any metal article, making the necessary connections, immersing the article in the solution, and then taking it out bright with its new coat of deposited metal. There is much to be understood before successful plating can be accomplished ; the appliances necessary for work on a big scale are many and expensive ; and the manipulation of the various metals calls for a special knowledge of the characteristics of each one. This handbook will deal with many matters of importance to the electro-plater, and will discuss, in the order in which the items are here mentioned, the electro-

Tanks, Vats, and other Apparatus. 13

plater's shop, tanks, vats, and other general requisites ; batteries, dynamos, and electrical accessories ; the appliances for preparing the rough metal surfaces to receive the electro-deposited coat, and for polishing the plated ware ; the proper materials, appliances, and methods employed in electro-plating with silver, copper, gold, nickel, iron, lead, tin, platinum, and alloys, and the methods of finishing plated goods.

Persons desiring a small plating plant for the purpose of treating trinkets, miscellaneous small articles of the nature of spoons, forks, etc., need not be deterred from carrying out their wishes by the magnitude of the operations or by the expense of the apparatus described in this and succeeding chapters. Metal can be polished by hand to look almost as ))right as machine-polished metal, using the same polishing materials and suitable brushes. Small quantities of the same solutions can also be used in smaller vessels, and gold, silver, etc., can be deposited in good condition with current obtained from small cells. It must be understood, however, that the same strict attention to absolute cleanliness, and care in preparation, must be observed in small operations, and the same rules must guide the worker when plating a spoon or a screw as when plating the handle-bar of a bicycle. Readers will find that special attention is paid to the requirements of the amateur and small professional plater in later chapters, their limitations having been taken into careful consideration.

The plating shop must be well lighted and ventilated, and kept at a temperature of about 60° F. Good light, preferably from high windows or from a skylight, is necessary for the examination of the work ; but receptacles containing silver solutions should be protected from the action of direct sunlight. Good light is required over the scouring trough and vats, to enable the workmen to note changes of colour in the metals ; but the light over the vats should be a northern light if possible, because sunlight decomposes all plating solutions. The plating shop should be roomy and clean, for the most perfect cleanliness in the process is necessary to success. The plating vats should be covered with canvas when not in use, to exclude dust. Ventilation is necessary, because all the exhalations of gas from the solutions are extremely poisonous and soon debilitate the strongest workmen when fresh air is absent. Not only is an even temperature conducive to regular working of the solutions, but the employes do better and more work when the shop is well ventilated and comfortably warmed.

If the solution vats are placed in a corner of an ordinary workshop, they will soon get contaminated with dust and dirt. If the dynamo is near the polishing lathe, or where metal dust is flying about, it will soon be ruined.

Metal castings, forgings, etc., are surfaced for plating on emery bobs, etc., as will be described in later chapters ; they are then immersed in a hot solution of caustic potash, to loosen and remove grease and oil contracted at the polishing lathe. The solution for this purpose is contained in a wrought-iron tank, the usual dimensions for which are 4 ft. by 2 ft. by 1 ft. 9 in., to hold 50 gallons of liquid ; or one for larger work, measuring 5 ft. by 2 ft., to hold 100 gallons. The size of the tank will be determined by that of the work, and the vessel must be large enough to receive the longest articles likely to be plated. It is made of wrought iron, which is not attacked by caustic potash. Tanks of galvanised iron, lead-lined tanks, zinc tanks, and tanks with soldered joints are unsuitable, because all these metals and solder are soluble in caustic potash, which will hold the dissolved metal in solution and deposit it on any articles cleaned in the tank.

Tanks, Vats, a.yd other Apparatus. 15

As the potash sohition will have to be worked hot, the tank must be placed in a position where ifc can be conveniently heated, either by steam passing through a coil of iron pipe in the solution, hy an ordinary furnace in which coal or coke is burnt, or by gas jets. Steam and gas are preferable to coal, because the heat can be regulated to a nicety, and there is no danger from fire after the day's work is done. Even when heated by steam, it is advisable to support the tank above the floor on brickwork to a height suitable for easy working. The solution is composed of American potash dissolved in water, at the strength of 1 lb. to each gallon of water.

The articles are rinsed in hot water after they are taken from the potash tank, so a second tank, of equal dimensions and of the same material, must be fitted up in a similar manner next to the potash tank. This is kept supplied with clean hot water.

In the preparatioii of articles to be plated, the polisher aims at getting the finest polish obtainable on the surface of the article ; whilst the plater tries to free the surface from every trace of grease, oil, sweat, and other foreign matter. The polished work may be beautifully clean from a polisher's point of view, and yet be very dirty when viewed from the plater's point of view. The surface musi be chemically clean before the article is placed in the plating vat, to ensure the deposited coat oE metal adhering firmly to the surface of the metal on which it is deposited. The vessel in which the articles are thus further cleansed is named the scouring trough or tray, and is show^n by Fig. 2. It consists, as shown, of a shallow trough of thick wood divided into two parts, lined with lead, and furnished at one end with a plug and overflow pipe. In some scouring trays a narrow shelf is fixed a little below the upper edge, and the work rests on this whilst being cleaned. Some workmen hold the work on a piece of board placed across the tray. The tray must be placed on strong wooden trestles, at a height to suit the workman, and kept half full of clean water, to rinse the articles after scouring.

The plating vat is the name given to the vessel intended to hold the plating solutions. This vessel must be roomy enough to take the largest article likely to be plated in it, and then leave an abundance of space all round. It is usual to plan plating vats to hold several such articles at

Fig. 2. — Scouring Trough.

once, so that time may be saved. Those in general use for nickel, copper, brass, and silver-plating, are made of thick pine-wood, well jointed, and strengthened with iron bolts across the ends, lined with sheet-lead put together with burnt joints, then match-lined to prevent metallic contact with the lead lining. The appearance of these vessels when finished is shown by Fig. 3. They are made in a great number of sizes, the capacities ranging from 19 gal. to several hundred gallons.

A vat to hold the solution may be of vitreous stoneware ; but this material is liable to fracture.

Tanks, Fats, and other Apparatus.

A7 and is not easily made into large rectangular vessels. The next best is enamelled iron, which, being smooth, retards the creeping action of the cyanide salts, so that the sides of the vat are kept clean, and enamelled iron vats are suitable for small work ; their use is almost compulsory when the solution must be used hot. In large plating establishments the vats are wood, lined with lead, as already mentioned.

Plating vats constructed of thick slate slabs, grooved and well bolted together, are also sold for the purpose, but apparently they cost 50 per

Fiof. 3.— Platino- Yat.

cent, more than the lead-lined wood vats. Vats made of iron only, unless they are lined with thick enamel, are altogether unsuitable as plating vats ; copper, brass, iron, and zinc are readily corroded by the action of the acid or the alkali present in the plating solution.

A light vat should be supported on short stout trestles, at a height suitable to the workmen — that is, with the upper rim about the same height as an ordinary table.

B

Two, three, or four such

I 8 El EC TR O-PLA TING,

supports will be required, according to the length of the vat. If, however, the vat is large and heavy, it should be supported by a strong foundation of masonry, and this should be so built as to ensure free access of air to the under side, thus preventing rot in the wood. If the floor round the vat can be coated with cement or asphalt it can be easily kept clean, and it is also advisable to have w^ooden grids on the floor by the sides of the vat, on which the workmen may stand.

Special iron stands for vats can be obtained (see Fig. 4) ; these are provided with means of heating the vat by gas.

It is possible to make a small electro-plating bath with glass plates cemented together and enclosed in a wooden box. The cement may be either marine glue, which can be bought, or 2 parts of pitch and 1 part of gutta percha can be melted together. Pour some of the melted material into the wooden box and spread in a thin layer over the bottom with a warm iron ; while the cement i3 warm (not hot) press the bottom plate firmly into it. The sides may be treated in the same way, but before putting in the glass plates warm them gently and coat all the edges with a thin layer of the cement, so that when they are pressed together the surfaces will adhere thoroughly ; the joints will then be waterproof. Make the wood box a trifle large, allowing room for the layer of cement, or if the glass plates be forced in and nothing allowed for contraction and expansion the glass box will probably crack at the weakest part. Another suitable cement is prepared as follows : Make some good flour paste, and allow^ to get cold. Prepare a quantity of thin glue, add 10 gr. of powdered bichromate of potash to each gill of glue, bring it to a boiling temperature, and then pour it into the cold paste w^iilst stirring. Use this cement warm, and expose to strong sunlight afterwards. It will resist acids, but not hot

Tanks, Vats, axd other Apparatus.

cyanide of potassium solutions. Still another cement is made by dissolving best isinglass in acetic acid by the aid of heat ; it is used hot.

The electrical fittings of the plating vat are described in the next chapter.

The articles to be plated are suspended in the plating solution from copper rods (see p. 51) by means of short lengths of wire, named slinging wires. These are made of bare copper wires, ranging in gauge from No. 8 to No. 18, the larger

Fig. 4. — Iron Stand for Vat. Fig-. 5. — Copper Hook.

size being employed for heavy, and the smaller sizes for light, articles. Two wires of No. 18 will hold up a cycle handle-bar ; but it is always advisable to have more wires or larger wires than required merely to support the articles, since these wires convey the electric current from the goods being plated ; and an insufficient wire conductor here w^ill prevent the nickel from going on as fast or as well as may be required. Small wires offer a greater resistance to the current, and should be avoided for this reason.

The connection between the slinging wires and the articles to be plated is by means of copper hooks, the usual pattern of which is shown by Fig. 5.

The plated articles are rinsed in hot water after leaving the plating vat, and are then transferred at once to hot boxwood sawdust, in which they are dried quickly. This treatment is needed to prevent the coat of deposited metal from being marked by stains. The sawdust is held in a shallow pan or tank of iron placed next the hot water tank. This pan is heated by hot water or steam. If hot water is used, it occupies the double bottom, this being heated by steam jets ; by the other method, the steam passes through a coiled pipe in the double bottom (see Fig. 6). Direct heat from a fire or from gas jets is inadmissible, because this would char the sawdust, and thus stain the work. If live high-pressure steam is employed, this should be used in a coil of pipes not directly in contact with the sawdust (see Fig. 6), as such very hot steam is liable to char the sawdust. Boxwood sawdust is employed, because this does not contain resinous matter, likely to be extracted by heat, and etain the metal ; but the sawdust of sycamore could be used as a substitute if more readily obtainable. The sawdust tank and the hot-water tank may be of galvanised iron, if this is of any advantage.

The Bunsen burner will frequently be useful. It is composed of a short burner inside a piece of gas barrel some five or six inches in length, to which air is admitted at the lower end. The air mixes with the incoming gas in the barrel, and the mixture burns together at the top with an intensely hot, smokeless, and nonluminous flame. When the tube of the burner is surmounted with a cap, the flame spurts out of the holes, and forms a " rose burner," the flame from which can be used for general heating purposes.

- The balances or scales required by an electro-

Tanks^ Fats, and other Apparatus. 21

plater will vary with the class of work on which he is engaged. For the ordinary work of weighing the goods before and after plating, to determine how much metal has been deposited, a pair of scales with a stout steel or brass beam will be required. For weighing gold and gilded articles a lighter pair, indicating a turn on one grain at least, should be provided ; these will also serve for weighing out the ingredients used in making up solutions. For rough assays and estimations a small cheap balance indicating a turn

Fig-. G. —Steam-heated iSawdust Pan.

of i\) grain will serve the purpose ; the cost of this, together with weights from 500 grs. down to y^Q grain, will be about £l 10s. For assays, analysis, and calibrations, a still more elaborate and delicate balance will be required, such as an Oertling assay balance fitted with agate bearings and indicating a turn with at least y^oth of a grain. Such a balance, with a full set of weights, will cost from £5 to £lO.

In some plating establishments the weight of deposited metal is determined during the operation of plating by means of a plating balance. This is merely a pair of large scales furnished with a scale pan at one end of the beam and a metal frame suspended over the bath at the other end. The goods to be plated are slung to the metal frame, and the whole is balanced by weights placed in the scale pan. The pillar of the beam is connected to the negative pole of the machine or battery. As the metal goes on th-^ goods to

7. — Roseleur's Plating- Balance.

be plated, the beam is thrown out of balance, and the exact weight deposited can be ascertained at any time by additional weights placed in the scale pan.

Roseleur's plating balance is shown by Fig. 7. Tlie beam carries at one end a metal ring e, from which the objects to be plated are suspended in

Tanks, Vats, and other Apparatus. 23

tlie bath B. The anode a is shown as a rod connected through terminals, etc., to the positive pole of the battery. From the opposite end of the balance the pan s is hung. A metal pin m dips into the mercury cup n, and this cup is connected to the negative pole of the battery. The path of the current is therefore from the battery by way of the anode a through the bath, and thence, by way of the articles to be plated, through the beam and mercury contact back to the battery. In the horizontal position of the beam, m does not make contact with /(. But the weight of the articles is balanced by adding weights to the pan s, and further weights are then added equal to the weight of silver to be deposited. This brings m down and completes the circuit. When the required w^eight of metal has been deposited, the left arm of the beam (as seen in Fig. 15) rises and breaks the circuit. It will be seen that the balance is automatic, cutting off the current when the articles are plated to the extent intended.

The work of the plating balance is done in a different way by a special ammeter (p. 54) which records the amount of silver deposited per hour.

Beakers are tumblers made of very thin Bohemian glass for special use in chemical manipulations where small quantities of acids and other liquids are employed at high temperatures. Tumblers made of ordinary glass would soon break in pieces, but these thin glass beakers will bear boiling water being poured into them, or will hold acid whilst it is being boiled in them over a gas stove. They are also useful in analysis of solutions, as the clear glass enables the operation of precipitation to be observed whilst the operator holds the beaker away from his face, and thus avoids breathing the deleterious fumes.

Chapter II. Batteries, Dynamos, And Electrical Accessories

The electric current used to deposit metal from plating solutions must be obtained from a convenient source ; this may be a battery for occasional jobs, and for the general use of the amateur or small professional ; and it may be a dynamo when large quantities of work have to be treated. The advantages of the dynamo are many. The current is more constant, labour is lessened, and the cost of maintenance is much reduced. The current can also be regulated, and thus any class of work can be done from one machine. A plating dynamo gives a large current at a low pressure ; the armature may be of the drum or ring pattern, and the machine is shunt wound. With a large machine, capable of giving, say, 200 amperes, large surfaces of work may be acted on in several vats simultaneously, and nickel-plating may proceed at the same time as silver-plating.

The electro-motive force required to be supplied by the battery or dynamo varies according to the bath. Thus, f<>r gold and silver, it may be from ^ to 4 volts ; for copper, with an acid bath, it may be from ^ to 1^ volts, or, with a cyanide bath, from 3 to 6 volts. The amperage will vary with the area of the anode and cathode surfaces.

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French electricians speak and write of such generators under the name of "Piles/' doubtless in deference to the form of the first voltaic generator of electricity made — the pile of metal discs invented by Yolta. English electricians apply the word battery to all apparatus in which electricity is generated by chemical decomposition, and also to tAvo forms of storage cells known respectively as accumulators and Leyden jars.

A tabulated list of the batteries in use by electro-platers is given on p. 25, and the table will show at a glance the battery most suitable for any particular work.

Other batteries, such as the Fuller, Wollaston, Gassner, and very occasionally the Leclanche, are also used by some platers ; these will be referred to later.

The Daniell is best for depositing copper from its sulphate solution and silver from the usual plating solution, but the Bunsen may be used for this purpose if the exciting liquid is sufficiently weakened. One or two large cells in series will be enough for all ordinary purposes. The Bunsen is best for depositing copper and brass from their alkaline solutions, and also for the deposition of nickel, because its electro-motive force is high, enabling the current to push through high resistances. It is not suitable for the work of silver-plating, gilding, and electrotyping, because its high E.M.F. causes the metal to go on too fast and in a granular condition. In all these operations the Daniell will be found to be the best because its E.M.F. is lower than that of the Bunsen, and its current equally constant in volume. The Smee, and also Walker, are eminently useful cells for giving a current suitable to the w^ork of electro-gilding small articles of jewellery. Batteries with a high E.M.F. cause gold to go on too fast, and give the deposit a brown colour.

Some information as to the form and construction of the various batteries used by electro-platers may now be given.

The Daniell cell is known in several modifications, Fig. 8 showing the internal arrangement of the porous pot form. The glass or glazed vitrified stoneware jar J contains the cylindrical plate c (made of sheet copper), the porous pot p (made of uri glazed earthenware), and the zinc

Bunsen Battery.

rod z. Inside the porous pot dilute sulphuric acid is poured, while the copper plate c stands in a saturated solution of copper sulphate. The connections are made to the two wires shown in the illustration. The maximum electromotive force of the cell is about 1-14 volts, with an internal resistance of 3 ohms in the 3-pint size, and 1'6 ohms in the 3-quart size.

A Daniell battery is re-charged in the following way: Thoroughly clean all the parts and reamalgamate the zincs ; charge the porous pot, containing the zinc, with a solution of 1 part of sulphuric acid in 12 or 13 parts of water, and the outer jar, containing the copper plate, with a saturated solution of sulphate of copper. The small sieve or tray near the top of the containing jar, but below the level of the copper solution, should contain crystals of sulphate of copper to keep the solution saturated. If copper is allowed to deposit on the porous pot, the current will fail, not only on account of a higher internal resistance, but also because this short-circuits the cells. To prevent this, the zinc and also the zinc sediment must be kept from touching the porous partitions.