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Hardness of Precious Stones

See PRECIOUS STONES.

Hard Pig-iron is distinguished by showing at the fracture a dull, grayish-white color, flaky in appearance, with more or less mottle. An extreme degree of hardness exists when the fracture shows a highly crystalline nature, with long, needle-like crystals radiating, and no appearance of graphite. See CAST IRON; GRADING PIG-IRON; SOFT PIG-IRON.

Hard Plaster is made by saturating pieces of freshly calcined plaster with water that holds in solution 12 per cent of alum. After thorough saturation the pieces are lifted from the liquid, dried, and calcined at a red heat; after which they are pulverized and sifted, and the plaster is fit for mixing. This plaster requires only about one half of the water used for the ordinary material, but is much longer in setting. When set, this hard plaster is about 50 per cent stronger than the common, and produces a fine, polished surface. See PLASTER.