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.
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.
RelativeBolting Value of Silk, Wire, and Grit Gauze | |||
Threadsper inch Approximate. | TradeNo. of Silk. | TradeNo. of Wire. | TradeNo. 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 | ||