On Laboratory Arts Part 33

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 quant.i.ty 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 pa.s.s 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 GLa.s.s

IN the Philosophical Magazine for July 1888 (vol. xxvi. p. 1) there is a paper by Professor Kundt translated from the Sitzungsberichte of the Prussian Academy. This paper deals with the indices of refraction of metals. Thin prisms were obtained by depositing metals electrolytically on gla.s.s 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's Polytechnik 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 the Central Zeitung is 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 PtCl4 is contemplated.

The dried salt is added little by little to rectified oil of lavender, placed on a gla.s.s 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 Baume 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 const.i.tutes the platinising liquid, and is applied as follows:-

A sheet of clean gla.s.s 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 gla.s.s is to be dried.

The dried gla.s.s is finally heated to a temperature of dull redness in a m.u.f.fle 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 gla.s.s.

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 quant.i.ty does not matter, but perhaps about 2 per cent may be mentioned as giving good results.

The gla.s.s is painted over with this solution and dried slowly on the water bath. When the gla.s.s 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 m.u.f.fle furnace. The necessary and sufficient temperature is reached as soon as the gla.s.s 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 gla.s.s is fired.