On Laboratory Arts Part 29

CHAPTER IV

ELECTROPLATING AND ALLIED ARTS

-- 127. Electroplating.

This is an art which is usually deemed worthy of a treatise to itself, but for ordinary laboratory purposes it is a very simple matter--so simple, indeed, that the multiplicity of receipts as given in treatises are rather a source of embarra.s.sment than otherwise.

The fundamental principles of the art are:-

(1) Dirty work cannot be electroplated.

(2) Electroplated surfaces may be rougher, but will not be smoother than the original unplated surface.

(3) The art of electroplating being in advance of the science, it is necessary to be careful as to carrying out instructions in detail.

This particularly applies to the conditions which determine whether a metallic deposit shall come down in a reguline or in a crystalline manner.

-- 128. The Dipping Bath.

An acid dipping bath is one of the most useful adjuncts to the laboratory, not only for cleansing metals for electroplating, but for cleaning up apparatus made out of bits of bra.s.s tube and sheet, and particularly for quickly cleaning binding screws, etc, where it is necessary to ensure good electrical contact.

The cheapest and most satisfactory way in the end is to make up two or three rather large baths to begin with. The gla.s.s boxes of storage batteries do very nicely for the purpose, and being generally ground pretty flat at the top, they may be covered by sheets of patent plate gla.s.s, and thus preserved from the action of the air.

First Bath. A 30 or 40 per cent solution of commercial caustic soda.

Objects may be cleansed from grease in this bath by heating them as hot as is consistent with individual circ.u.mstances, and plunging them into it.

It is a considerable advantage to begin by removing grease from articles subsequently to be dipped in an acid bath, both because it saves time and acid, and because more uniform results are obtainable when this is done than when it is omitted. It is a great advantage to have the caustic soda solution hot. This is always done in factories where nickel-plating is carried on, but it is inconvenient in the laboratory. The articles after dipping in the alkali are swilled with water, and may even be scrubbed with a brush, so as to remove greasy matters that have been softened but not entirely removed.

Acid Bath. A convenient bath for laboratory purposes is made by mixing two volumes of strong commercial nitric acid with one of strong sulphuric acid in a cell measuring, say, 12 X 10 X 15 inches.

Copper or bra.s.s articles are dipped in this bath for a few seconds, then rinsed with water, then dipped again for a second or two, or until they appear equally white all over, and then withdrawn as rapidly as possible and plunged into a large quant.i.ty of clean water.

Care must be taken to transfer the articles from the bath to the water as quickly as possible, for if time be allowed for gas to be evolved, the surfaces become mat instead of bright.

In order to save acid it is advisable to make up a third bath, using those odds and ends of acids which gradually acc.u.mulate in the laboratory. Sulphuric acid from the balance cases, for instance, mixed with its own volume of commercial nitric acid, does very well.

The objects to be dipped receive a preliminary cleansing by a dip in this bath, the strong bath being reserved for the final dip. Sheet bra.s.s and drawn tube, as it comes from the makers, possesses a really fine surface, though this is generally obscured by grease and oxide.

Work executed in these materials, cleaned in alkali, and dipped in really strong acid, will be found to present a much better appearance than work which has been filed, unless the latter be afterwards elaborately polished.

On no account must paraffin be allowed to get into any of the baths.

When the final bath gets weak it must be relegated to a subordinate position and a new bath set up. A weak acid bath leaves an ugly mottled surface on bra.s.s work.

-- 129. A metallic surface which it is intended to electroplate must, as has been mentioned, be scrupulously clean. If the metal is not too valuable or delicate, cleaning by dipping is easy and effectual. The following notes will be found to apply to special cases which often occur.

(1) Silver Surfaces intended to be gilt. These are first washed clean with soap and hot water, and polished with whitening. They are then dipped for a moment in a boiling solution of pota.s.sium cyanide.

A 20 per cent solution of common commercial cyanide does well, but the exact strength is quite immaterial. The cyanide is washed away in a large volume of soft water, and the articles are kept under water till they are scratch-brushed.

Mat surfaces are readily produced on standard silver by dipping in hot strong sulphuric acid. The appearance of new silver coins, which is familiar to everybody, is obtained by this process.

(2) Finely turned and finished Bra.s.s Work. If it is intended to nickel-plate such work, and if it is desirable to obtain brightly polished nickel surfaces, the work must be perfectly polished to begin with. Full details as to polis.h.i.+ng may be found in workshop books or treatises on watch-making. It will suffice here to say that the bra.s.s work is first smoothed by the application of successive grades of emery and oil, or by very fine "dead" smooth files covered with chalk.

Polis.h.i.+ng is carried out by means of rotten stone and oil applied on leather.

In polis.h.i.+ng turned work care must be taken to move the file, emery, or rotten stone to and fro over the work with great regularity, or the surface will end by looking scratchy and irregular. The first process of cleaning is, of course, to remove grease, and this is accomplished best by dipping in a bath of strong hot caustic soda solution, and less perfectly by heating the work and dipping it in the cold caustic soda bath.

During this process a certain amount of chemical action often occurs leading to the bra.s.s surface exhibiting some discoloration. The best way of remedying this is to dip the bra.s.s into a hot bath of cyanide of pota.s.sium solution. If it is inconvenient to employ hot baths or to heat the bra.s.s work, good results may be obtained by rubbing the articles over with a large rough cork plentifully lubricated with a strong solution of an alkali.

If the surfaces are very soiled or dirty, a paste of alkali and fine slaked lime may be applied on a cork rubber, and this in my experience has always been most effective and satisfactory in every way, except that it is difficult to get into crevices. If the alkali stains the work, a little cyanide of pota.s.sium may be rubbed over the surface in a similar manner.

Bra.s.s work treated by either of these methods is to be washed in clean water till the alkali is entirely removed, and may then be nickel-plated without any preliminary scratch-brus.h.i.+ng. The treatment in hot baths of alkali and cyanide is the method generally employed in American factories as a preliminary to the nickelling of small bra.s.s work for sewing machines, etc.

(3) Copper either for use as the kathode in electrolysis calibration experiments or otherwise is most conveniently prepared by dipping in the acid bath, rinsing quickly in cold water, scratch-brus.h.i.+ng under cold water, and transferring at once to the plating bath. In the case where the copper plates require to be weighed they are dipped into very hot distilled water after scratch-brus.h.i.+ng, and then dried at once by means of a clean gla.s.s cloth.

(4) Aluminium (which, however, does not readily lend itself to plating operations [Footnote: This difficulty has now been overcome. See note, section 138.] ) is best treated by alkali rubbed on with a cork, or by a hot alkaline carbonate where rubbing is inexpedient. The clean aluminium is scratch-brushed under water, and at once transferred to the plating bath.

(5) Iron for Nickel-plating. According to Dr. Gore (Electra-metallurgy, p. 319) the best bath for cleaning iron is made as follows: "One gallon of water and one pound of sulphuric acid are mixed with one or two ounces of zinc (which of course dissolves); to this is added half a pound of nitric acid." The writer has been accustomed to clean iron by mechanical means, to deprive it of grease by caustic alkali, and to finish it off by, means of a hard scratch brush. This process has always worked satisfactorily.

(6) Articles soldered with soft solder containing lead and tin do not readily lend themselves to electrolytic processes, the solder generally becoming black and refusing to be coated with the electro-deposit. Moreover, if soldered articles are boiled for any length of time in caustic alkali during the preliminary cleansing, enough tin will dissolve to form a solution of stannate of potash or soda--strong enough to deposit tin on bra.s.s or copper. A method of coppering soldered articles will be described later on.

-- 130. Scratch-brus.h.i.+ng.

This process is generally indispensable, and to its omission is to be traced most laboratory failures in electroplating. Scratch-brushes may be bought at those interesting shops where "watchmakers' supplies"

are sold. It will be well, therefore, to purchase a selection of scratch brushes, for they are made to suit particular kinds of work.

They are all made of bra.s.s wire, and vary both in hardness and in the fineness of the wire. The simplest kind of scratch brush consists merely of a bundle of wires bound up tightly by another wire, and somewhat "frizzed" out at the ends (Fig. 90). A more useful kind is made just like a rotating brush, and has to be mounted on a lathe (Fig. 91).

Fig. 90. Fig. 91.

The scratch brush is generally, if not always, applied wet; the lubricant generally recommended is stale beer, but this may be replaced by water containing a small quant.i.ty of glue, or any other form of gelatine in solution--a mere trace (say .1 per cent) is quite sufficient. Very fair results may be got by using either pure or soapy water. The rotating brushes require to be mounted on a lathe, and may be run at the same speed as would be employed for turning wooden objects of the same dimensions.

Since the brush has to be kept wet by allowing water or its equivalent to drip upon it, it is usual to make a tin trough over which the brush can revolve, and to further protect this by a tin hood to keep the liquid from being thrown all over the room. In many works the brush is arranged to lie partly in the liquid, and this does very well if the hood is effective.

There is a superst.i.tion that electro-deposits stick better to scratch-brushed surfaces than to surfaces which have not been so treated, and consequently it is usual to scratch-brush surfaces before electro-deposit. However this may be, there is no doubt that adherence and solidity are promoted by frequent scratch-brus.h.i.+ng during the process of depositing metal, especially when the latter tends to come down in a spongy manner.

Gilt surfaces--if the gilding is at all heavy--are generally dull yellow, or even brown, when they come from the bath, and require the scratch brush to cause the gold to brighten, an office which it performs in a quite striking manner. The same remark applies to silvered surfaces, which generally leave the bath a dead white--at all events if the deposit is thick, and if ordinary solutions are employed. In either case the touch of the scratch brush is magical.

-- 131. Burnis.h.i.+ng.

Burnishers of steel, agate, or bloodstone can be bought at the shops where scratch brushes are sold, and are used to produce the same brightening effect as can be got by scratch-brus.h.i.+ng. The same solutions are employed, but rather stronger, and the burnisher is swept over the surface so as to compress the deposited metal.

Burnis.h.i.+ng is rather an art, but when well done gives a harder and more brilliant (because smoother) surface than the scratch brush. On the whole, steel burnishers are the most convenient if in constant use.

If the burnis.h.i.+ng tools have to lie about, steel is apt to rust, unless carefully protected by being plunged in quicklime or thickly smeared with vaseline, and the least speck of rust is fatal to a burnisher. In any case the steel requires to be occasionally repolished by rouge and water on a bit of cloth or felt. The process of burnis.h.i.+ng is necessarily somewhat slow and tedious, and as a rule is not worth troubling about except in cases where great permanence is required.

The burnisher is moved over the work somewhat like a pencil with considerable pressure, and care is taken to make the strokes as uniform in direction as possible; otherwise the surface looks non-uniform, and has to be further polished by tripoli, whitening, etc, before it is presentable.

-- 132. Silver-plating.

The most convenient solution for general purposes is an 8 to 10 per cent solution of the double cyanide of silver and pota.s.sium together with 1 or 2 per cent of "free" pota.s.sium cyanide. Great lat.i.tude is permissible in the strength of solution and density of current. As commercial cyanide of pota.s.sium generally contains an unknown percentage of other salts, which, however, do not interfere with its value for the purpose of silver-plating, the simplest procedure is as follows.

For every 100 c.c. of plating solution about 7 grms. of dry crystallised silver nitrate are required. The equivalent amount of pota.s.sium cyanide (if dry and pure) is 5.2 grms, but commercial cyanide may contain from 50 per cent upwards to 96 per cent in the best fused cyanide made from ferrocyanide only. An approximate idea of the cyanide content can be obtained from the dealers when the salt is purchased, and this is all that is required.