Nickel sulphate 5 partsAmmonia sufficient to neutralise the nickel salt.Ammonium tartrate 3.75 partsTannin 0.025 partsWater 100 parts
Nickel sulphate 5 parts
Ammonia sufficient to neutralise the nickel salt.
Ammonium tartrate 3.75 parts
Tannin 0.025 parts
Water 100 parts
The nickel sulphate and ammonia are dissolved in half the water, the ammonium tartrate in the other half with the tannin. The solutions are mixed and filtered at about 40° C. This solution works well at ordinary temperatures, or slightly warm, with a current density of ten ampères per square foot. In an experiment made for the purpose I found that plating may go on for an hour in this solution before the deposit begins to show signs of flaking off. The deposit is of a fine white colour.
The resistance of the bath is rather high and rather variable, consequently it is as well to have a current indicator in circuit, and it may well happen that five or six volts will be found requisite to get the current up to the value stated. For nickelling small objects of brass, such as binding screws, etc., it is very necessary to be careful as to the state of polish and uniformity of their surfaces before placing them in the plating bath. A polished surface will appear when coated as a polished surface, and a mat surface as a mat surface; moreover, any local irregularity, such as a speck of a foreign metal, will give rise to an ugly spot in the nickelling bath. For this reason it is often advisable to commence with a coat of copper laid on in an alkaline solution and scratch-brushed to absolute uniformity.
An examination of the work will, however, disclose whether such a course is desirable or not; it is not done in American practice, at all events for small brass objects. These are cleaned in alkali and in boiling cyanide, which does not render a polished surface mat, as weak acid is apt to do, and are then coated with a current density of about ten ampères per square foot.
In spite of what is to be found in books as to the ease with which nickel deposits may be polished, I find that the mat surface obtained by plating on an imperfectly polished cathode of iron is by no means easily polished either by fine emery, tripoli, or rouge. Consequently, as in the case of brass, if a polished surface is desired, it must be first prepared on the unplated cathode. In this case, even if the deposit appears dull, but not gray, it may be easily polished by tripoli and water, using a cork as the polisher. Scratch-brushing with brass wire, however, though possibly not with German silver wire, brightens the deposit, but discolours it. When the deposit becomes gray I have not succeeded in polishing it satisfactorily.
Soldered brass or iron may be satisfactorily coated with nickel by giving it a preliminary coating of copper in the cyanide bath. On the whole, I recommend in general that iron be first coated with copper in the alkaline bath, scratch-brushed, and then nickel-plated, and this whether the iron appears to be uniform or not. Much smoother, thicker, and stronger coats of nickel are obtained upon the copper-plated surface than on the iron one, and the coating does not become discoloured (? by iron rust) in the same way that a coating on bare iron does. The copper surface may be plated for at least an hour at a density of ten ampères per square foot without scaling.
Scales or circles divided on brass may be greatly improved in durability by nickel-plating. For this purpose the brass must be highly polished and divided before it is nickelled.
The plating should be continued for a few minutes only, when a very bright but thin coat of nickel will be deposited; it then only remains to wash and dry the work, and this must be done at once. If the nickel is deposited before the scale or circle is engraved, very fine and legible divisions are obtained, but there is a risk that flakes of nickel may become detached here and there in the process of engraving.
142. Miscellaneous Notes on Electroplating.
Occasionally it is desirable to make a metallic mould or other object of complex shape. The quickest way to do this is to carve the object out of hard paraffin, and then copy it by electrotyping. Electrotype moulds can be made in many ways. The easiest way perhaps is to take a casting in plaster of Paris, or by means of pressure in warm gutta-percha.
In cases where the mould will not draw, recourse must be had to the devices of iron-founders, i.e. the plaster cast must be made in suitable pieces, and these afterwards fitted together. This process can occasionally be replaced by another in which the moulding material is a mixture of treacle and glue. The glue is soaked in cold water till it is completely soft. The superfluous water thrown away, one-fourth part by volume of thick treacle is added, and the mixture is melted on the water bath; during which process stirring has to be resorted to, to produce a uniform mixture.
This liquid forms the moulding mixture, and it is allowed to flow round the object to be copied, contained in a suitable box, whose sides have been slightly oiled. The object to be copied should also be oiled. After some hours, when the glue mixture has set, it will be found to be highly elastic, so that it may be pulled away from the mould, and afterwards resume very nearly its original form.
One drawback to the use of these moulds lies in the fact that the gelatine will rarely stand the plating solution without undergoing change, but this may be partially obviated by dipping it for a few seconds in a 10 per cent solution of bichromate of potash, exposing it to the sunlight for a few minutes, and then rinsing it.
In order to render the surface conducting, it is washed over with a solution of a gold or silver salt, and the latter reduced in situ to metal by a suitable reagent. A solution of phosphorus is the most usual one (see Gore, Electro-metallurgy, p. 216). Such a mould may be copper-plated in the sulphate bath, connection being made by wires suitably thrust into the material.
Plaster of Paris moulds require to be dried and waxed by standing on a hot plate in melted wax before they are immersed in the plating bath. In this case the surface is best made conducting either by silvering it by the silvering process used for mirrors, or by brushing it over with good black lead rendered more conducting by moistening with an ethereal solution of chloride of gold and then drying in the sun.
The brushing requires a stiff camel's-hair pencil of large size cut so that the hairs project to a distance of about a quarter of an inch from the holder. The brushing must continue till the surface is bright, and is often a lengthy process.
The same process of blackleading may be employed to get a coat of deposited metal which will strip easily from the cathode.
In all cases where extensive deposits of copper are required, the growth takes place too rapidly at the corners. Consequently it is often desirable to localise the action of the deposit. A "stopping" of ordinary copal varnish seems to be the usual thing, but a thin coat of wax or paraffin or photographic (black) varnish does practically as well.
I do not propose to deal with the subject of electrotyping to any extent, for if practised as an art, a good many little precautions are required, as the student may read in Gore's Electro-metallurgy. The above instructions will be found sufficient for the occasional use of the process in the construction of apparatus, etc. There is no advantage in attempting to hurry the process, a current density of about ten ampères per square foot being quite suitable and sufficiently low to ensure a solid deposit.
§ 143. Blacking Brass Surfaces. —
A really uniform dead-black surface is difficult to produce on brass by chemical means. A paste of nitrate of copper and nitrate of silver heated on the brass is said to give a dead-black surface, but I have not succeeded in making it act uniformly. For optical purposes the best plan is to use a paint made up of "drop" black, ground very fine with a little shellac varnish, and diluted for use with alcohol. No more varnish than is necessary to cause the black to hold together should be employed.
In general, if the paint be ground to the consistency of very thick cream with ordinary shellac varnish it will be found to work well when reduced by alcohol to a free painting consistency.
A very fine gray and black finish, with a rather metallic lustre, may be easily given to brass work. For this purpose a dilute solution of platinum tetrachloride (not stronger than 1 per cent) is prepared by dissolving the salt in distilled water. The polished brass work is cleaned by rubbing with a cork and strong potash till all grease has disappeared, as shown by water standing uniformly on the metal and draining away without gathering into drops.
After copious washing the work is wholly immersed in a considerable volume of the platinum tetrachloride solution at the ordinary temperature. After about a quarter of an hour the brass may be taken out and washed. The surface will be found to be nicely and uniformly coated if the above instructions have been carried out, but any finger-marks or otherwise dirty places will cause irregularity of deposit. If the process has been successful it will be found that the deposit adheres perfectly, hardly any of it being removed by vigorous rubbing with a cloth. If the deposit is allowed to thicken — either by leaving the articles in the solution too long or heating the solution, or having it too strong — it will merely rub off and leave an irregular surface.
This process succeeds well with yellow brass and Muntz metal, either cast or rolled, but it does not give quite such uniform (though still good) results with gun-metal, on which, however, the deposit is darker and deader in appearance.
A book might be written (several have been written) on the art of metal colouring, but though doubtless a beautiful and delicate art, it is of little service in the laboratory. For further information the reader may consult a work by Hiorns.
§ 144.Sieves. —
Properly graded sieves with meshes of a reliable size are often of great use. They should be made out of proper "bolting" cloth, a beautiful material made for flour-millers. Messrs. Henry Simon and Company of Manchester have kindly furnished me with the following table of materials used in flour-milling.
Sieves made of these materials will be found to work much more quickly and satisfactorily than those made from ordinary muslin or wire gauze.
Relative Bolting Value of Silk, Wire, and Grit Gauze
Threads per inch Approximate.
Trade No. of Silk.
Trade No. of Wire.
Trade No. of Grit Gauze.
18
0000
18
16
22
000
20
20
28
00
26
26
38
0
32
34
48
1
40
44
52
2
45
50
56
3
50
54
60
4
56
58
64
5
60
60
72
6
64
66
80
7
70
70
84
8
80
80
94
9
106
10
114
11
124
12
130
13
139
14
148
15
156
16
163
17
167
18
170
19
173
20
§ 145. Pottery making in the Laboratory. —
When large pieces of earthenware of any special design are required, recourse must be had to a pottery. Small vessels, plates, parts of machines, etc., can often be made in the laboratory in less time than it would take to explain to the potter what is required. For this purpose any good pipeclay may be employed. I have used a white pipe-clay dug up in the laboratory garden with complete success.
The clay should be kneaded with water and squeezed through a cloth to separate grit. It is then mixed with its own volume or thereabouts of powdered porcelain evaporating basins, broken basins being kept for this purpose. The smoothness of the resulting earthenware will depend on the fineness to which the porcelain fragments have been reduced. I have found that fragments passing a sieve of sixty threads to the inch run, do very well, though the resulting earthenware is decidedly rough.
The porcelain and clay being thoroughly incorporated by kneading, the articles are moulded, it being borne in mind that they will contract somewhat on firing.[Footnote:The contraction depends on the temperature attained as well as on the time. An allowance of one part in twelve will be suitable in the case considered.]The clay should be as stiff as is convenient to work, and after moulding must be allowed to get thoroughly dry by standing in an airy place; the drying must not be forced, especially at first, or the clay will crack.
Small articles are readily fired in a Fletcher's crucible furnace supplied with a gas blow-pipe; the furnace is heated gradually to begin with. When a dull red heat is attained, the full power of the blast may be turned on, and the furnace kept at its maximum temperature for three or four hours at least, though on an emergency shorter periods may be made to do.
The articles are supported on a bed of white sand; after firing, the crucible furnace must be allowed to cool slowly. It must be remembered that the furnace walls will get hot externally after the first few hours, consequently the furnace must be supported on bricks, to protect the bench.
The pottery when cold may be dressed on a grindstone if necessary. This amateur pottery will be found of service in making small fittings for switch-boards, commutators, and in electrical work generally.
Pottery made as described is very hard and strong, the hardness and strength depending in a great degree on the proportion of powdered porcelain added to the clay, as well, of course, as on the quality of both of these materials.
It is a good plan to knead a considerable quantity of the mixture, which may then be placed in a well-covered jar, and kept damp by the addition of a little water.
Pottery thus made does not require to be glazed, but, of course, a glaze can be obtained by any of the methods described in works on pottery manufacture. The following glaze has been recommended to me by a very competent potter:—
Litharge
7 parts by weight
Ground flint
2 parts by weight
Cornish stone or felspar
1 parts by weight
These ingredients are to be ground up till they will pass the finest sieve — say 180 threads to the inch. They are then mixed with water till they form a paste of the consistency of cream. They must, of course, be mixed together perfectly. The ware to be glazed is dipped into the cream after the first firing; it is then dried as before and refired. The glaze will melt at a bright red heat, but it will crack if not fired harder; the harder it is fired the less likely is it to crack.
If colouring matters are added they must be ground in a mill free from iron till they are so fine that a thick blanket filter will not filter them when suspended in water. This remark applies particularly to oxide of cobalt.
APPENDIX
PLATINISING GLASS
IN thePhilosophical Magazinefor July 1888 (vol. xxvi. p. 1) there is a paper by Professor Kundt translated from theSitzungsberichteof the Prussian Academy. This paper deals with the indices of refraction of metals. Thin prisms were obtained by depositing metals electrolytically on glass surfaces coated with platinum. The preparation of these surfaces is troublesome. Kundt recounts that no less than two thousand trials were made before success was attained. A detailed account of the preparation of these surfaces is not given by Kundt, but one is promised — a promise unfortunately unfulfilled so far as I am able to discover. A hunt through the literature led to the discovery of the following references:Central Zeitung fuer Optik und Mechanik, p. 142 (1888); Dingler'sPolytechnik Journal, Vol. cxcv. p. 464;Comptes Rendus, vol. lxx. (1870).
The original communication is a paper by Jouglet in the Comptes Rendus, of which the other references are abstracts. The account in Dingier is a literal translation of the original paper, and the note in theCentral Zeitungis abbreviated sufficiently to be of no value. The details are briefly as follows:-
One hundred grams of platinum are dissolved in aqua regia and the solution is dried on the sand bath, without, however, producing decomposition. Though the instructions are not definite, I presume that the formation of PtCl4is contemplated.
The dried salt is added little by little to rectified oil of lavender, placed on a glass paint-grinding plate, and the salt and oil are ground together with a muller. Care is required to prevent any appreciable rise of temperature which would decompose the compound aimed at, and it is for this reason that the salt is to be added gradually. Of course the absorption of water from the air must be prevented from taking place as far as possible. Finally, the compound is diluted by adding oil of lavender up to a total weight of 1400 grams (of oil).
The liquid is poured into a porcelain dish and left absolutely at rest for eight days. It is then decanted and filtered, left six days at rest, and again decanted (if necessary). The liquid should have a specific gravity of 5° on the acid hydrometer. (If by this the Baumé scale is intended, the corresponding specific gravity would be 1.037.) A second liquid is prepared by grinding up 25 grams of litharge with 25 grams of borate of lead and 8 to 10 grams of oil of lavender. The grinding must be thoroughly carried out.
This liquid is to be added to the one first described, and the whole well mixed. The resulting fluid constitutes the platinising liquid, and is applied as follows:-
A sheet of clean glass is held vertically, and the liquid is painted over it, carrying the brush from the lower to the upper edge. The layer of oil dries slowly, and when it is dry the painting is again proceeded with, moving the brush this time from right to left; and similarly the process is repeated twice, the brush being carried from top to bottom and left to right. This is with the object of securing great uniformity in the coating. Nothing is said as to the manner in which the glass is to be dried.
The dried glass is finally heated to a temperature of dull redness in a muffle furnace. The resinous layer burns away without running or bubbling, and leaves a dull metallic surface. As the temperature rises this suddenly brightens, and we obtain the desired surface (which probably consists of an alloy of lead and platinum). It is bright only on the surface away from the glass.
I have not had an opportunity of trying this process since I discovered the detailed account given by Jouglet; but many modifications have been tried in the laboratory of the Sydney University by Mr. Pollock, starting from the imperfect note in the Central Zeitung, which led to no real success.
It was found that it is perfectly easy to obtain brilliant films of platinum by the following process, provided that the presence of a few pin-holes does not matter.
The platinum salt employed is what is bought under the name of platinic chloride; it is, however, probably a mixture of this salt and hydro-chloro-platinic acid, and has all the appearance of having been obtained by evaporating a solution of platinum in aqua regia to dryness on the water bath. A solution of this salt in distilled water is prepared; the strength does not seem to matter very much, but perhaps one of salt to ninety-nine water may be regarded as a standard proportion. To this solution is added a few drops of ordinary gum water (i.e. a solution of dextrin). The exact quantity does not matter, but perhaps about 2 per cent may be mentioned as giving good results.
The glass is painted over with this solution and dried slowly on the water bath. When the glass is dry, and covered with a uniform hard film of gum and platinum salt free from bubble holes, it is heated to redness in a muffle furnace. The necessary and sufficient temperature is reached as soon as the glass is just sensibly red-hot.
The mirrors obtained in this way are very brilliant on the free platinum surface. If the gum be omitted, the platinum will have a mat surface; and if too much gum be used, the platinum will get spotty by bubbles bursting. There does not appear to be any advantage in using lead.
It is quite essential that the film be dry and hard before the glass is fired.