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 amperes per square foot being quite suitable and sufficiently low to ensure a solid deposit.

-- 143. Blacking Bra.s.s Surfaces.

A really uniform dead-black surface is difficult to produce on bra.s.s by chemical means. A paste of nitrate of copper and nitrate of silver heated on the bra.s.s 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 sh.e.l.lac 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 sh.e.l.lac 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 l.u.s.tre, may be easily given to bra.s.s 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 bra.s.s 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 bra.s.s 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 bra.s.s 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 Trade No. Trade No. Trade No. of Approximate. of Silk. of Wire. 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 pa.s.sing 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.