To preserve mixed paints in pots from ‘skinning over’ or drying up, they should be kept constantly covered with water; or, what is better, with a thin film of linseed oil.
Brushes, when out of use, may be preserved in a similar manner to mixed paints. When dirty, or required for a paint of another colour, they may be cleaned with a little oil of turpentine, which may be either preserved for the same purpose another time, or may be allowed to deposit its colour, and then used to thin down paints as usual. In no case, however, should it be thrown back into the cistern or pan with the pure ‘turps.’
Paints, Flex′ible. Prep. Take of good yellow soap (cut into slices), 21⁄2 lbs.; boiling water, 11⁄2 gall.; dissolve, and grind the solution whilst hot with good oil paint, 11⁄4 cwt. Used to paint canvas.
Paints, Vitrifi′able. See Enamel, Glaze, Stained Glass, &c.
PALLA′DIUM. Pd. A rare metal discovered by Dr Wollaston in the ore of platinum, in 1803.
Prep. 1. A solution of the ore of platinum in aqua regia, from which most of the metal has been precipitated by chloride of ammonium, is neutralised by carbonate of sodium, and then treated with a solution of cyanide of mercury; the white insoluble precipitate (cyanide of palladium) is next washed, dried, and heated to redness; the residuum of the ignition (spongy palladium) is then submitted to a gradually increased pressure, and welding at a white heat, so as to form a button, in a similar manner to that adopted with platinum. Prod. Columbian ore of platinum, 1%; Uralian do., ·25% to ·75%.
2. The native alloy of gold and palladium (from the Brazils) is submitted to the operations of quartation and parting, the nitric acid employed being of the density of 1·3; the silver is next precipitated from the solution by means of a solution of common salt or dilute hydrochloric acid, and the decanted supernatant liquid, after evaporation to one half, is neutralised with ammonia, and concentrated so that crystals may form; these (chloride of palladium and ammonium) are cautiously washed in a little very cold water, dried, mixed with borax, and exposed in a crucible to the strongest heat of a powerful blast furnace, when a solid button of pure palladium is formed.
Prop., &c. Palladium closely resembles platinum in appearance, fusibility, malleability,
and ductility; but it is less dense, and has a rather more silvery colour than that metal; it is freely soluble in aqua regia, and is slowly attacked by nitric acid, but the other acids exert little or no action on it; heated to redness in the air, a very superficial blue or purple film of oxide forms on the surface, which is again reduced at a white heat. It melts at 156°—Wedgwood. Sp. gr. 11·3 to 12·1 (11·8—Wollaston; 12·14—Vauquelin). It readily unites with copper, silver, and some other metals, by fusion.
Tests. 1. The neutral solutions of palladium are precipitated in the metallic state by ferrous sulphate, dark brown by sulphuretted hydrogen, olive by ferrocyanide of potassium, and yellowish white by cyanide of mercury.—2. A drop of tincture of iodine placed on the surface of metallic palladium, and then evaporated by the heat of a spirit lamp, leaves a black spot. By the last two tests palladium is readily distinguished from platinum.