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or Alto Alto Relievo

alum, potash, water, paper and ounces

ALTO RELIEVO, or ALTO RILIEVO.—A term used principally in photo-sculpture. The term should only be applied when the figures project more than half their true proportions. When they project, exactly one-half, the term used is Mezzo relieve, and if less then half, Basso relieve, or, in English, ALUM.—The name given to double salts of aluminium with sulphates of potassium, sodium, and ammonium, or of other monatomic metals, as silver, thallium, etc. They crystal lize in octohedra. The alums of commerce are potash alum (formula + (1211,0) and ammonium alum )212 H,0). Large quantities are manufactured, principally at Hurlet, near Glasgow, and Whitby in Yorkshire. It is prepared by the double decomposi tion of shale containing iron pyrites FeS,, which is gently burnt, and exposed to the air in a moist state. It oxidizes and forms sulphates, and on the addition of a potash salt or ammonium sulphate to the solution obtained by water, alum crystallizes out. It is then purified by three successive recrystallizations. Ammonium alum can be distinguished frcm potash alum by its giving off ammonia when warmed with sodium hydroxide.

Alum has an astringent and acidulous taste, is styptic, and reddens litmus paper. It dissolves in about six times its own weight of boiling water, but to only about io per cent. in cold. It is insoluble in alcohol and ether.

Alum possesses the peculiar property of hardening gelatine and rendering it insoluble. Combined with citric or other acids, it serves for clearing and hardening gelatine negatives that have become discolored by the action of the developer. A small quantity of potash alum added to the silver sensitizing bath used for albumen paper prevents scum forming, and by at once coagulating the albumen causes the print to be more on the surface of the paper.

Chrome Alum (Formula, + potassium chromic salphate molecular weight, 999, is obtained as a secondary product in the oxidation of substances by potassium bichromate and sulphuric acid, and is formed in the bichromate battery.

It can be made on a small scale by dissolving 3 ounces of bichromate of potash in io ounces of boiling water and gradually adding 4,4 ounces of strong sulphuric acid, followed by half an ounce of pure alcohol added by drops. Evaporate until the solution measures about 5 ounces, and then put aside for a few days, when the crystals of chrome alum will separate out. They should be rinsed in cold water to free from any sulphuric acid that may be left.

This salt forms large dark-purple octahedral crystals, which exhibit a ruby color by reflected light. It is soluble in about to parts of cold water ; it decomposes in hot, and is insoluble in alcohol. Chrome alum is also used for hardening and rendering gelatine insoluble, for which purpose it is superior to potash alum.

Liesegang has discoveredt that if paper be impregnated with potash alum, and exposed for three minutes to sunlight, such a change takes place in those parts of the paper upon which the light has acted that they are able to slightly reduce nitrate of silver. If heated with gallic acid these parts become black, the rest remaining white.

ALUM BATH.—An alum bath may be prepared by placing a handful of potash alum in a zo ounce bottle and filling it up with water, replenishing with the latter whenever the solution is used. By this means a saturated solution is always at hand, and it is often of great use in an emergency to check the frilling of plates or the blistering.of paper.