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.
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 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'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 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:-