Modern electro plating. The principles involved in depositing gold, silver, nickel, copper, brass and other metals, by means of batteries or dynamos; the preparation and management of plating baths, chemicals used, etc

Introduction

This little work has been prepared chiefly to help out of his difficulties the plater in country towns who has purchased a small outfit and begun operations without previous training in a plating establishment where modern methods have been employed with a due regard for speed and economy in turning out the work. Hence it has been thought best to confine our remarks chiefly to the principles involved in the work, and to tell in a common sense way, how the work is done; what forces are employed; how they are handled, and how the various formulae may be made and varied to suit the conditions of the work or to overcome the difficulties that may arise in continuous working of solutions.

It has seemed better to do this than to give a large number of formulae which, although they may be perfectly adapted to a special purpose, contain in themselves no real information, but have to be taken blindly, like a physician's prescription for a fever. It must not be forgotten that the chemistry of plating has undergone

4 INTRODUCTION.

strict investigation in the last five years by thoroughly educated and competent men, who have found abundant opportunity for their labors in the many large platingworks now used in connection with manufacturing establishments, where the finishing of the products is of the highest importance, and owing to the quantity of goods turned out any defect in the composition or management of the baths is of the highest importance. The labors of these chemists have been chiefly effective in eliminating from the formulae in common use those chemicals which, from their nature, were antagonistic to the chief ingredients, or which by their decomposition left detrimental or inert matter in the solutions. As an instance of this we may mention the substitution of chromic acid for bichromate of potash in battery fluids; the purchase and use of the carbonate of copper and zinc in brass solutions instead of compounding them in the old way from the sulphates, etc. In all cases the effects of the alterations have been to simplify and greatly improve the keeping and working qualities of the solutions, and in many cases the improvement has been very great. These new methods of making solutions have been industriously circulated by the dealers in plating apparatus and supplies, until now the chief source of trouble in plating has shifted from the plating to the polishing department, and

INTRODUCTION. 5

while the chief difficulty formerly lay in the solutions, nine out of ten cases of trouble now may be traced to the improper preparation of the work before it reaches the bath, or to careless and hasty finishing after it has left it. Therefore, it has seemed better to deal with the operations of plating in a general way, detailing the principles involved, and allowing the operator to make his own baths from general directions, rather than to follow the empirical methods formerly employed, which gave no hint of the source of any difficulty, and left the plater unable to help himself when trouble was encountered.

It will be readily seen from the above remarks that plating has been stripped of about all the mystery that formerly surrounded it, to the great chagrin and vexation of a certain class of workmen who formerly found an easy way of getting an income by the sale of certain very important and mysterious formulae, without which, they solemnly assured their intended victim, good plating could not be done. In fact, it is now generally recognized that good, durable plating depends chiefly upon the degree of care and skill employed in the application of well known methods of doing work, while the question of profit depends altogether upon the personal dexterity of those who do the work, and a rapid and skillful plater holds the same relation to his fellow

6 INTRODUCTION.

employes that a rapid and skillful machinist does in his trade.

With this explanation we think the reader will sufficiently grasp the scope and purpose of this work. It does not pretend to teach everything, but only to prove useful in aiding the workman to a general understanding of the subject which will enable him to consult other books intelligently, or to help himself where other guidance is not available.

The little pamphlet. " Practical Points on the Deposition of Metals," which was the first attempt to give practical information in a general way has been so warmly received that it encourages me to hope that more on the same subject will be appreciated by those who are desirous of acquiring skill in the art of plating.

J. H. VAN HORNE.

Chapter I. The Management Of Batteries

The first requisite in attempting to do electroplating in a small way is to understand the battery and to select one that will give an electric current of the proper intensity and quantity for the required time, without too much care and attention on the part of the workman. Were he provided with measuring instruments, so that he could readily determine when his current was changing in quantity and power, the choice of a battery would not be of so much importance; but volt meters and ampere meters are too expensive to be possessed by the average man who does plating in a small way, and he is necessarily obliged to depend on theory in arranging his forces and judge of the results by the appearance of his work in the bath. Hence it is important that he should have an understanding of the nature of the action in the battery and be able to maintain the requisite conditions from the appearance of the battery itself.

O MODERN ELECTRO PLATING.

Electricity. — Without attemping to give too close a definition, electricity may be defined as a force or energy which is the result of a displacement of the normal balance of forces between two elements connected with one another by a conducting medium. This difference in the balance of force is called the potential of its elements and if two elements having different potentials, are connected together and placed in a fluid which will produce chemical action upon one or both, the result will be a flowing of energy through the connection to the element having the lowest potential. This will be kept up as long as the chemical action continues and the connection between the two elements remains unbroken. It will be readilv seen, that owing to the varying potentials of the different elements, the varying facility of the conductors used to connect them, and the varying intensity of chemical action in the solution employed, the electrical current will vary in strength (or voltage) in different batteries, and in quantity, according to the size of the elements and the freedom with which they are attacked by the solution.

Voltage is the measure of strength or intensity of the current and depends upon the difference of potentials of the elements and the kind of chemical action between them. It is the same for the same combination, regardless of the size of the elements. Thus, a battery the size of a thimble has the same voltage as one the size of a door, if the elements and solution are the same. We have not the space to explain this at length, but will simply state that the volt is the recognized unit of the measurement of strength of electric currents.

The Ampere is the unit of measurement of the quantity of currents. Amperage depends on the size of the elements; and the available amperage depends on the size of the conductors and the freedom of action between the elements. Amperage is consumed by doing work, or by the heating of insufficient conductors, or by undue resistance in the battery, just as power is consumed in turning steel, or in running shafting, or overcoming the resistance caused by friction of boxes on a shaft that is run without oil. Strictly speaking, if the voltage or intensity of the current be sufficient to do the work required, then the amperage is the force used to do the work, and it is destroyed by that work and the chemical or electrical resistance, just as mechanical power is consumed in running a lathe or doing any other work. From this, it follows, that in order to operate

IO MODERN ELECTRO PLATING.

•economically, extreme care should be taken that the connections be large enough to carry the current easily; that the solutions be kept in perfect order, both in the battery and the plating vat; and that all joints be kept bright and firm so as to insure perfect contact and offer no resistance to the passage of the current.

The current always flows from the element having the highest potential (called the + or positive pole) along the wire and through the solution in the plating vat, to the other wire, and thence to the negative pole, carrying with it in passing through the solution, particles of metal from the anode and depositing it on the article to be electroplated (called the cathode); hence care should be taken to always get the cathode affixed to the negative (■ — ) pole of the battery, in order that it may receive the deposit.

Electrical Resistance is that property of conductors (wires, solutions, objects, etc.) by which they tend to reduce the intensity of a current passing through them. The practical unit of resistance is the ohm. The number of amperes of current flowing through a circuit is equal to the number of volts of electro motive force, divided by the number of ohms of resistance in the •entire circuit, that is from positive pole clear through

MODERN ELECTRO PLATING. II

wires, solution and battery, back to the starting point. Thus it will be seen that if the resistance be greater than the voltage of one cell will overcome, no current will flow, and the voltage must be increased to such an amount as will allow the desired quantity of current to pass. This is done by coupling cells in various ways, which will be explained at length further on.

The resistance of a conducting wire is directly proproportional to its length, and inversely proportional to the square of its diameter; hence it follows that the short and large wires cause less loss of current than smaller and longer ones.

In all batteries the resistance increases with the distance between the elements, and decreases when the immersed surfaces are increased. The resistance is also increased by the bubbles of hydrogen liberated at the positive pole sticking to it in great numbers. Hydrogen is a non-conductor and prevents the action of the solution on the metal. When this takes place to such an extent as to stop chemical action altogether, no current will pass and the battery is said to be polarized.

These remarks are intended to aid in the intelligent selection of batteries, etc., those who, having to deal with such apparatus, yet have never had the opportunity to study an electrical treatise. We are often asked.

What is the best battery? We can only answer: There is no best battery; that is, no battery is suited to all kinds of work. That which is best in one case may be worst in another. The suitability of a battery for any special purpose depends on what is called its constants, i. e.? electro-motive force and internal resistance. In order to be really perfect a battery should fulfill the following conditions:

i. Its electro-motive force should be high and constant.

2. Its internal resistance should be small.

3. It should give a constant current and must'therefore be free from polarization, and not liable to rapid exhaustion, requiring frequent renewal of material.

4. It should consume no material when the circuit is open.

5. It should be cheap and of durable materials.

6. It should be manageable and, if possible, should not emit corrosive fumes.

No single battery fulfills all these conditions, however, and, as we have already intimated, some batteries are better for one purpose and some for another. Thus, for telegraphing through a long line of wire, a considerable internal resistance is of no great consequence, as it is but a small fraction of the total resistance in circuit.

MODERN ELECTRO PLATING.

For electric gas lighting or other low resistance circuits, on the other hand, much internal resistance would be, if not absolutely fatal, certainly a positive disadvantage. The most reliable batteries for electroplating work are the Daniell, Gravity, Bunsen, Smee and Carbon, which, we will accordingly describe in their order.

The Daniell, Fig. 1, consists of a glass or stoneware jar, containing a cylinder of copper surrounding a porous clay cup, in which stands a cylinder of zinc. At the upper part of the copper sheet is a pocket of perforated copper, which is filled with crystals of sulphate of copper. The object of the pocket is simply to hold the sulphate up to the top of the solution, so that it will dissolve more readily, and any other method would do as well.

In charging this battery, the glass vessel and the porous cup are filled with water, and crystals of sulphate of copper are put in the pocket. If wanted for immediate use, a small quantity of sulphate of zinc may be dissolved in water and added to the

Fig. 1.

porous cup; if not wanted immediately, the battery may be short circuited by connecting the zinc and copper elements by a piece of copper wire, and it will attain its full strength in ten or twelve hours. A little sulphuric acid dropped in the porous cup will answer just as well. if sulphate of zinc is not on hand. The chemical action of this battery is as follows: The zinc decomposes the water, forming oxide of zinc and liberating the hydrogen. The oxide of zinc attacks the sulphate of copper, depriving it of the acid, which fcrms sulphate of zinc, and leaving it as oxide of copper; the oxide of copper is thereupon attacked by the hydrogen, which combines with the oxygen and forms water, while the metallic copper falls to the bottom as a fine powder. It will thus be seen that action is simple and continuous, no fumes are given off, and all that is required to maintain the action is a regular supply of copper sulphate to keep the fluid in the outer jar, near the point of saturation. The most prominent fault of this battery is the tendency of the copper to rill the pores of the cup, and thus decrease the action of the battery. It can be partially prevented by coating the bottom and about a quarter of an inch of the sides of the porous cup with wax, and brushing off the deposit of copper on the cup as fast as it is formed. The battery should not be allowed to stand on open circuit without the zinc element being removed, and the sulphate of zinc solution in the cup should not be heavier than 250 B. nor lighter than 15° B. If these precautions are observed, the battery should give a constant and free current as long as any zinc remains. Its electro-motive force is about 1.07 volt, and a gallon cell will give about one-half ampere, when in good order, on a short circuit. Its internal resistance varies, but should not be allowed to exceed three to five ohms.

The Gravity Battery, In consequence of the trouble caused by the precipitation of the copper on the porous cell in the Daniell battery. Cromwell F. Varley, in 1854, while experimenting, found that the difference in specific gravity between solutions of sulphate of copper and sulphate of zinc was sufficient in itself to entirely separate them, the copper solution lying at the bottom of the cell, and the zinc solution remaining superposed upon it. He accordingly dispensed with the porous cup, placed his copper element at the bottom, and the zinc near the top of a glass jar, and thus originated the gravity battery of today, It is the simplest, most reliable and constant form known, and has displaced all others for closed circuit work, requiring a low voltage.

i6

MODERN ELECTRO PLATING.

such as telegraphing, etc. Its voltage, when first set up. is 1.07, running down under constant work to .90, and a gallon cell will give one half ampere on short circuit. The form of cell shown in Fig. 2 is known as the "crowfoot," on account of the manner in which the zinc '(positive) element is spread out, to expose a large surface of zinc to the solution. It is the form used for telegraphing, and, therefore, can be readily obtained any where. Of course, other forms, shapes and sizes can be made at the option of the workman. To set up this battery, the copper strip, being unfolded so as to form a cross, is placed at the bottom of a jar, the zinc is suspended from the top as shown, and clean water, containing one-tenth of a saturated solution of sulphate of zinc is added, until it nearly touches the zinc. Sulphate of copper crystals are then added until, if the battery is meant to be continually used, they nearly cover the top of the copper strip. If the battery is not intended for continual use, it will be found more advantageous to use but a few ounces of sulphate of copper, as the more concentrated the solution, the greater is the tendency to local action. The sulphate of zinc may be dispensed with if the battery is not required for immediate use; in this case, the latter should be short circuited, and left so for several days. The need of blue vitriol will be indicated by the discoloration of the lower stratum of the liquid. It is best to keep the line marking the two solutions about half way between the zinc and copper. Should the sulphate of zinc become too concentrated, a portion of it should be removed by means of a syringe qv cup, and its place supplied by water. To determine when this is necessary, a hydrometer may be used. Below 150 the solution is too weak; above 250 it is too strong, and should be diluted. If the battery is taken care of from month to month, it should not require a thorough cleaning more than once a year. When this is done the deposits formed upon the surface of the zinc should be scraped off, the jars washed and the liquids renewed as in the beginning.

If, however, the batteries are in constant use, care must be taken to keep the zincs clean and the solutions as indicated above. If the sulphate of zinc is allowed to become saturated, it will crystalize on the zinc and on the edge of the jar, gradually creeping over the edge. This should be wiped off with a damp cloth and a little oil or fat smeared over the top of the jar to prevent iS

MODERN ELECTRO PLATING.

creeping. The jars should not be disturbed, as this would cause the two solutions to mix, and they should be kept in a dry, even temperature (6o° to 8oc F.). Freezing would stop the action of the battery.

The Bunsen, or Carbon Battery, Fig. 3, consists of a glass jar containing a hollow zinc cylinder, slit on one side to allow a free circulation of the solution; within this stands a porous cup containing a bar of carbon. To charge this battery, the amalgamated zinc is placed in the glass jar,, the porous jar in the center of the zinc cylinder and the carbon in the porous jar. In the outer jar is sulphuric acid, diluted with twelve times its weight of water, and in the porous jar electropoion fluid. (See Electropoion Fluid.) The voltage of this battery is 2.028; its amperage cannot be given, as it depends largely upon the care which is given the battery, the size of the cell and the condition of the porous cups, which vary greatly in porosity and conducting power. It emits fumes of hydrogen and sulphurous acid if not in good conditiom and should not be used in the same room with line tools

Fig- s-

MODERN ELECTRO PLATING. IQ

or metal work that is liable to injury. It soon runs down, requiring recharging every day when in constant use, but it is simple to handle when understood, and is generally furnished in small outfits for nickel plating, etc., on account of its high voltage and the quantity of current given off when in good order. The zincs must be kept well amalgamated or they will polarize very rapidly and destroy the current; care should also be taken that no sediment be allowed to accumulate in the porous cup and fill its pores, thus stopping the action. It is more expensive to run than the gravity, as the zincs are eaten by the acid much faster, especially if not kept well amalgamated; but it will deliver a greater quantity of current in a given time than a gravity cell of equal size. The internal resistance of a new cell is about one-half an ohm. The plates should be removed and cleaned when the battery is not in use.

The Smee Battery, Fig. 4, consists of two plates of amalgamated zinc, between which is placed a silver plate coated with platinum, the object of the platinum being to fill the surface of the plate with innumerable fine points which aid in discharging the bubbles of hydrogen, which would cling closely to it if the plate were smooth and thus polarize the battery. This

MODERN ELECTRO PLATING.

battery is charged with a solution of one part sulphuric acid to seven of water. The plates are connected to the clamp and placed in the jar. In this battery, above all. the precaution of amalgamating the zinc should never be neglected. With an unamalgamated zinc the results are very unsatisfactory.

The voltage of the Smee, when not in action, is 1.09 volts; when in action it runs down to .482 volts; this is caused by the hydrogen clinging to the plate as described.' This was the form of cell generally used before the introduction of dynamos for electrotyping and other heavy work, and jj| it is still used to a large extent. It emits fumes of hydrogen when in action, but it is a single fluid battery and when working in large sizes, plates 12x12 inches in size are suspended in a large tank of acidulated water, first a plate of zinc, then a plate of platinized silver, then another of zinc and so on alternately, zinc and platinum, to the end. This gives great facility in handling, as any number of plates to suit the work may be placed in the tank. As there is but one tank and the plates may be placed close together or far apart as required, the resistance may be easily made to balance that in the

Fig. 4.

depositing tank, and thus the work will be performed under the most favorable conditions.

In working the Smee, or any battery for that matter, large tanks are better than small ones, provided that the plates are kept close together so as to reduce internal resistance of the battery. In the large Smee, if plates 12x12 are worked in a tank say 15x15x30, it will not be long before the sulphate of zinc, which forms and falls to the bottom, will soon commence to rise in the tank, thus shutting off the acid from a portion of the plates and reducing the quantity of current. If the same plates were worked in a tank 24x24x30, the tank might be permitted to become half full of zinc sulphate before the action would be impaired at all, and thus a much more even and constant current would be maintained; this is generally done in practice. In a gravity battery, however, the tank ought not to be deep, because the two elements should not be more than eight inches apart on account of the increased resistance caused by the separation. The tank, however, may be large enough in length and width to contain elements of the desired size, or a number of standard zincs and coppers, if such an arrangement seems desirable, either to increase the facility of handling or reduce the cost of a large number of jars, wires, connections, etc. We have seen a number of tanks made of wood, lined with lead, 10x10x60 inches, in which a single large copper element was placed at the bottom and a number of zincs hung as required from an insulated copper bar across the top. It seemed to work well and was convenient.

Varying. Strength of the Current by Coupling. A few words as to coupling batteries may be of service. It should be borne, in mind that the quantity of current flowing in any circuit is the quotient resulting

Fis. s.

from dividing the voltage by the total resistance in that circuit and that the resistance may be increased or diminished by increasing or diminishing the distance between the elements of the battery and between the anode and cathode in the plating vat; also that the resistance varies inversely, as the surface of the elements immersed. Thus a plating surface of one square foot in the plating vat will offer four times as much resistance as four square feet. It thus becomes possible by increasing or diminishing the voltage of a current to keep the current flowing in the desired quantity, and by keeping the resistance in the battery about equal to that in the vats the highest economy is obtained.

For example, let us take eight cells, having a voltage of 1, and giving say y2 ampere per cell on short circuit. If we now couple them — , ■+ , — , +, — , +, — , +, we shall have the voltage of 8 and the amperage of one cell of

— Q

Fig. 6.

the same size, in other words, the same amount of current and eight times the strength of the single cell, as in Fig. 5.

This is termed coupling in series, and would be used in solutions having a high resistance and small amount of surface immersed. If, on the other hand, the solution had a low resistance and large surface exposed, so that the voltage of one cell was ample to force the current through the circuit, they should be connected +, +, +> +j +3 +, +, +, and — , — , — , — , — , — , — , — , giving

MODERN ELECTRO PLATING.

the quantity ot eight cells and the voltage of one, which amounts to nearly the same thing as if a single battery having eight times the surface of the single cell were used. This is termed coupling in multiple, Fig. 6. Similarly, if they were coupled +, — , +, — , +, — , +. — and +, — , +, — , +, — , +, — . and those two were joined as in Fig. 7, we should have the equivalent of a battery possessing a voltage of four. c and elements twice the size of the single cell. This would be spoken of as a battery of eight cells in series of four. Also four multiples, in series of two, might be arranged to give a voltage of two and quantity due to cells of four times the size of a single cell, as shown in Fig. 8. As the amperes of current passing per second depends upon the voltage, divided by the number of ohms resistance, in the circuit, it will seen that the current can be controlled by coupling and by manipulating the resistance.

Fig.

To Amalgamate Zincs. This may be very well done by first immersing the zincs in a solution of dilute sulphuric acid and then in a bath of mercury. A brush

MODERN ELECTRO PLATING.

2 5 or cloth may be used to rub them, so as to reach all points of the surface. Where a large quantity is to be amalgamated, the following will be found to be a good method: Dissolve eight ounces of mercury in a mixture consisting of two pounds of hydrochloric and one pound of nitric acid; when the solution is complete, add three pounds more of hydrochloric acid. The zinc is amalgamated by immersing it in this solution for a few seconds, quickly removing to a vat of clear water and rubbing it, as in the first case, with a brush or cloth. If the solution is kept in a covered vessel it may be used a number of times.

In all batteries in c~ which acids are used the

Fig. 8.

zincs should be kept well amalgamated and should be removed from the solution when not in use. This is very important and should not be overlooked.

Improved Electropoion Fluid* Add one part (by volume) of sulphuric acid to ten parts of water. Of 10 pounds (or pints) of the dilute acid, add from 1 to 2 pounds of chromic acid, according to the strength of current desired. Where constant action over a long time is desired rather than maximum energy, omit part or all of the sulphuric acid.

Bichromate of potash is no longer used for batteries by intelligent workers. It owes its virtues to a small amount of chromic acid which can be obtained from it by reaction. Pure chromic acid is cheaper for the same work, and is free from many of the difficulties attendant on the use of the bichromate.

Connections. Having a knowledge of the theoretical action of the battery, the next question is the connections. Cleanliness cannot be too strongly insisted upon in making joints, etc. The plater should make it an invariable rule to see that all surfaces of wires, screws, etc., through which the current must pass, be kept bright on the surfaces through which electrical contact is made. When joining wires they should be brightened with a file, or with emery cloth, and then twisted firmly together with a pair of pliers; all permanent connections should be carefully soldered and the holes and the ends of screws in binding posts should be kept bright; and if for permanent use all conducting wires should be of pure copper, well insulated. The following table shows in the last column the loss of current in wires, carrying an economical amount of current; if the