A quant.i.ty slightly in excess of the computed amount of cyanide is dissolved in distilled water, and this is cautiously added to the solution of the silver nitrate till precipitation is just complete.
The supernatant liquors are then drained away, and the precipitate dissolved by adding a sufficiency of the remaining cyanide; this process is a.s.sisted by warming and stirring.
An allowance of about one-tenth of the whole cyanide employed may be added to form "free" cyanide, and the solution made up to the strength named. It is advisable to begin with the cyanide in a moderately strong solution, for the sake of ease in dissolving the precipitate.
This solution will deposit silver upon articles of copper or bra.s.s immersed in it even without the battery, but the coat will be thin.
The solution is used cold, with a current density of about 10 to 20 amperes per square foot. The articles to be silvered are scratch-brushed, washed, and electroplated, till they begin to look undesirably rough. They are then taken out of the bath, rebrushed, and the process continued till a sufficiency of silver is deposited.
Four grammes weight of silver (nearly) is deposited per ampere hour.
It is best to use a fine silver anode, so that the solution, does not get contaminated by copper.
In most factories it is usual to "quicken" the objects to be silvered before placing them in the electrolysis vats, because the deposit is said to adhere better in consequence of this treatment. I have never found it any improvement for laboratory purposes, but it is easy to do. A dilute (say 2 per cent) solution of cyanide of mercury is required containing a little free cyanide. The objects to be "quickened" are scratch-brushed and dipped into the cyanide of mercury solution till they are uniformly white; it is generally agreed that the less the mercury deposited the better, so long as a perfect coating is obtained. The objects are rinsed after quickening, and put in the depositing bath at once.
The mat surface of silver obtained by electrolysis of the cyanide is very beautiful--one of the most beautiful things in nature--shining with incomparable crystalline whiteness. So delicate is it, however, for so great is the surface it exposes, that it is generally rapidly deteriorated by exposure to the air. It may be protected to some extent by lacquering with pale lacquer, but it loses some of its brilliancy and purity in the process. The deposit is generally scratch-brushed or burnished down to a regular reflecting surface.
-- 133. Cold Silvering.
A thin but brilliant coat of silver may be readily applied to small articles of bra.s.s or copper in the following way. A saturated solution of sodium sulphite (neutral) is prepared, and into this a 10 per cent solution of nitrate of silver is poured so long as the precipitate formed is redissolved. A good deal of silver may be got into solution in this way. Articles to be silvered need only to be cleaned, brushed, and dipped in this solution till a coat of the required thickness is obtained.
I must admit, however, that the coating thus laid on does not appear to be so permanent as one deposited by simple immersion from the cyanide solution, even though it is thicker. The cyanide plating solution will itself give a good coat of silver if it is used boiling, and if a little pota.s.sium cyanide be added.
For purposes of instrument construction, however, a thin coat of silver is seldom to be recommended, on account of its liability to tarnish and its rapid destruction when any attempt is made to repolish it. For these reasons, nickel or gold plating is much to be preferred.
-- 134. Gilding.
This art deserves to be much more widely practised than is usual in laboratories. Regarded as a means of preserving bra.s.s, copper, or steel, it is not appreciably more "time robbing" than lacquering, and gives infinitely better results. Moreover, it is not much more expensive. Strange as it may seem, the costliness of gilding seldom lies in the value of the gold deposited; the chief cost is in the chemicals employed to clean the work, and in interest on the not inconsiderable outlay on the solution and anode.
The easiest metal to gild is silver, and it is not unusual to give base metals a thin coating of silver or copper, or both, one after the other, before gilding, in order to secure uniformity. To ill.u.s.trate the virtue of a thin layer of gold, I will mention the following experiment. About three years ago I learned for the first time that to "clean" the silver used in a small household required at least an hour"s labour per diem. I further ascertained that most of this time is spent on the polishing part of the process.
As this seemed a waste of labour, I decided to try the effect of gilding. In order to give the proposal a fair trial I gilt the following articles: half a dozen table spoons and forks, a dozen dessert forks and spoons, and a dozen tea spoons. These were all common electroplated ware. They were weighed before and after gilding, and it was with difficulty that the increase of weight was detected, even though a fine bullion balance was employed. On calculating back to money, it appeared that the value of the gold deposited was about threepence. a.s.suming that an equal weight of silver had been accidentally dissolved by the free cyanide during the plating--which is unlikely--the total amount of gold deposited would be worth, say, sixpence.
After three years" continuous use the gilding is still perfect, except at the points on which the spoons and forks rest, where it is certainly rather shabby. Meanwhile the "gold" plate only requires to be washed with hot water and soap to keep it in perfect order, a much more cleanly and expeditious process than that of silver cleaning.
-- 135. Preparing Surfaces for Gilding.
Ordinary bra.s.s work--rough or smooth--may for purposes of preservation be dipped, scratch-brushed, and gilt at once. Seven years ago the writer gilt the inside of the head of a copper water still, and simply scratch-brushed it; it is to-day in as good order as when it was first done. If it is intended to gild work from the first, with the view of making an exceptionally fine job of it, "gilding metal," i.e. bra.s.s containing one to one and a quarter ounces of zinc to the pound of copper may be specified. From its costliness, however, this is only desirable for small work.
Iron and steel are generally given a preliminary coating of copper, but this may be dispensed with though with no advantage--by using a particular process of gilding.
Base metals, zinc, pewter, lead, etc, are first coppered in a cyanide of copper solution, as will be described under the head of Copper-plating. If it is intended to gild soldered articles, the preliminary coating of copper is essential.
The most convenient vessel for holding a gilding solution is undoubtedly one formed of enamelled iron. Particularly useful are the buckets and "billies" (i.e. cylindrical cans) made of this material.
These vessels may be heated without any fear of a smash, and do not appear to be appreciably affected by gilding solutions--at all events during several days or weeks. The avoidance of all risk of breakage when twenty or thirty pounds" worth of solution is in question is a matter of importance.
Under no circ.u.mstances is it desirable to use anything but the purest gold and best fused cyanide (called "gold" cyanide) in the preparation of the solutions. The appearance of a pure gold deposit is far richer than of one containing silver, and its resistance to the atmosphere is perfect; moreover, in chemico-physical processes one has the satisfaction of knowing what one is dealing with.
-- 136. Gilding Solutions.
The strength of solution necessary for gilding bra.s.s, copper, and silver is not very material. About one to two pounds of "gold"
pota.s.sium cyanide (? 96 per cent KCN) per gallon does very well. The gold is best introduced by electrolysing from a large to a small gold electrode. One purchases a plate of pure gold either from the mint or from reliable metallurgists (say Messrs. Johnson and Matthey of London), and from this electrodes are cut.
The relative areas of the electrodes do not really much matter. I have used an anode of four times the area of the cathode. The solution is preferably heated to a temperature of about 50 C, and a strong current is sent through it, say twenty amperes to the square foot of anode. The electrodes must be suspended below the surface of the solution by means of platinum wires. If the gold plates are only partly immersed, they dissolve much more rapidly where they cut the surface, possibly on account of the effect of convection currents, though so far as the writer is aware no proper explanation has yet been given.
After a time gold begins to be deposited on the cathode in a powdery form, for which reason it is a good plan to begin by wrapping the latter in filter paper. The process has gone on for a sufficient time when a clean bit of platinum foil immersed in the place of the cathode becomes properly gilt at a current density of about ten amperes per square foot.
The powdery gold deposited on the cathode while preparing the solution can be sc.r.a.ped off and melted for further use, or the whole cathode may now be used as an anode. The platinum foil testing cathode may also be "stripped" by making it an anode, and is for this reason preferable to German silver or copper, which would contaminate the solution while the "stripping" process was in progress.
For general purposes a current density of say ten to fifteen amperes per square foot may be used, but this may be considerably varied, so long as the upper limit is not greatly overpa.s.sed. During gold-plating there is a considerable advantage in keeping the electrodes moving or the solution stirred.
After immersing the cleaned and scratch-brushed articles, depositing may go on for about three minutes, after which they are removed from the bath and examined, in order to detect any want of uniformity in the deposit.
The articles should be entirely immersed; if this is not done, irregularity is apt to appear at the surface. Platinum wires employed as suspenders, and coated along with the articles to be gilt, may also be cleaned without loss by making them anodes. If, on examination, all is found to be going on well, reimmerse the cathodes, and continue plating till they appear of a dull yellowish brown (this will occur in about four minutes), then remove them, rinse and scratch-brush them, and replace them in the bath.
When a second coat appears to be getting rather brown than yellowish brown, i.e. of the colour of wet wash-leather, the removal, followed by scratch-brushing, may be repeated, and for nearly all laboratory purposes, the articles are now fully gilt.
The coating of gold deposited from a hot cyanide solution is spongy in the extreme, and if the maximum wear-resisting effect is to be obtained, it is advisable to burnish the gold rather than to rely upon the scratch brush alone.
If the area of the cathode exceeds that of the anode the solution is said to grow weaker, and vice versa. This may be remedied in the former case by an obvious readjustment; the latter introduces no difficulty so far as I know except when plating iron or steel.
The student need not be troubled at the poor appearance of the deposit before it is scratch-brushed. Heavy gold deposits are almost always dull, not to say dirty, in appearance till the burnisher or scratch brush is applied. On the other hand, the deposit ought not to get anything like black in colour.
The following indications of defects may be noted--they are taken from Gore. I have never been really troubled with them.
The deposit is blackish. This is caused by too strong a current in too weak a bath. This may be remedied to some extent by stirring or keeping the cathode in motion. The obvious remedy is to add a little cyanide of gold.
The gold anode gets incrusted. This is a sign that the bath is deficient in pota.s.sium cyanide. The gold anode gets black and gives off gas. The solution is deficient in cyanide, and too large a current is being pa.s.sed.
If a bright surface is desired direct from the bath, some caustic potash (say 2 per cent) may, according to Gore, be added, or the articles may be plated only slightly by using a weak current and taking them out directly they show signs of getting dull. By a weak current I mean one of about five amperes per square foot.
The deposit is said to be denser if the solution be heated as directed; but the bath will gild, though not quite so freely when cold.
To gild iron or steel directly, dilute the bath as above recommended some five or six times, add about 1 per cent of pota.s.sium cyanide, and gild with a very weak current (say two or three amperes per square foot) in the cold. Frequent scratch-brushing will be found requisite to secure proper adherence.
It is generally recommended to gild bra.s.s or German silver in solutions which are rather weak, but in the small practice which occurs in the laboratory a solution prepared as suggested does perfectly for everything except iron or steel. The scratch-brushing should be done over a large photographic developing dish to avoid loss of gold. It is a good plan to rinse the articles after leaving the bath in a limited quant.i.ty of distilled water, which is afterwards placed in a "residue" bottle, and then to scratch-brush them by hand over the dish to catch fine gold. When any loose dust is removed the articles may be scratched in the lathe without appreciable further loss.
Silver-gilt articles tend to get discoloured by use, but this discoloration can be removed by soap and water. After long use a gold cyanide bath tends to alter greatly in composition, In general, the bath tends to grow weaker, from the fact that there is a strong temptation to gild as many articles at once as possible.
It is therefore a good plan to keep a rough profit and loss account of the gold in order to find the quant.i.ty in solution. Fifty dwts. per gallon (or 78 grms. per 4.5 litres) is recommended. A gallon of solution of this strength is worth about eleven pounds sterling in gold and cyanide, and a serviceable anode will be worth about 10 pounds. (Fine gold is worth nominally four pounds four shillings and eleven pence ha"penny per oz.) Gold may be easily obtained containing less impurity than one part in ten thousand.
-- 137. Plating with Copper.
Copper may be deposited from almost any of its salts in reguline form, the sulphate and nitrate being most usually employed. In the laboratory a nearly saturated solution of sulphate of copper with 1 or 2 per cent of sulphuric acid will answer most purposes. A current density of, at most, fifteen amperes per square foot may be used, either for obtaining solid deposits for constructional purposes or for calibrating current measuring instruments by electrolysis. A copper anode is of course employed.
When coppering with a view to obtaining thick deposits it is a good plan to place the electrodes several inches apart, and, if possible, to keep the liquid stirred, as there is a considerable tendency on the part of copper deposits to grow out into mossy ma.s.ses wherever the current density exceeds the limit mentioned. As the ma.s.ses grow towards the anode the defect naturally tends to increase of itself, hence the necessity for care. The phenomenon is particularly marked at the edges and corners of the cathode.
If the deposit becomes markedly irregular, the best plan is to stop the process and file the face of the deposit down to approximate smoothness. In coppering it is of the utmost importance that the cathode be clean and free from grease; it must never be touched (by the finger, for instance) from the time it is scratch-brushed till it is immersed in the plating bath. Any grease or oxidation tends to prevent the copper deposit adhering properly.
A copper deposit oxidises very easily when exposed to the air.
Consequently if the surface be required free from oxide, as, for instance, when it is to be silvered or gilt, it must be quickly washed when withdrawn from the coppering bath, scratch-brushed, and transferred immediately to the silvering or gilding bath.