Many methods of whitening yellowed ivory have been proposed from time to time, and the more reliable of them are given below in the order of their simplicity as near as possible. In a few cases the ivory is directed to he exposed to sunlight; this should be done always under glass, which prevents the formation of cracks.

(1) Immerse the ivory in a very weak solution of sulphurous acid, and rinse in clean water.

(2) Boil with a paste of burned pumice-stone and water; expose to sunlight.

(3) Expose to the fumes of burning sulphur. The air should have free access to the ivory.

(4) Immerse for one hour in a saturated solution of alum in water, rub with a woollen cloth, and wrap in linen to dry.

(5) Immerse in water containing a very little chloride of lime, or in water impregnated with chlorine.

(6) Place in a thin paste of lime and water. Gently heat, and when white remove, dry, and finally polish.

(7) Brush with a solution of 1 oz. of nitric acid in 10 oz. of soft water. Rinse in clean water, and expose to sunlight.

(8) Wash with soap and water, and place, whilst wet, in sunlight, continuing the washing two or three times a day until bleached.

(9) Rub with fine pumice powder and water, and whilst still wet expose to sunlight. If unsuccessful, apply the pumice powder again.

(10) Support the ivory a little above the bottom of a shallow glass vessel by strips of zinc, pour in spirit of turpentine, and expose to the sun for three days.

(11) Remove grease by treating with a solution of common soda, and immerse the ivory in peroxide of hydrogen, to which liquid ammonia has been added in the proportion of 1 pt. of the former to 1 oz. of the latter. Gently heat for from twenty-four to thirty-six hours, remove, and dry slowly in the open air; rapid drying may split the ivory.

(12) The Artus process is to place the ivory for two days in a solution of 23oz. of carbonate of soda in 90oz. of water contained in a glass or porcelain vessel. Well wash in pure water, and then immerse in a solution of 34oz. of sulphite of soda in 91 oz. of water. In five or six days' time add to the solution a mixture of 2 oz. of hydrochloric acid and 11 oz. of water. Cover the vessel containing the ivory for from twenty-four to twenty-six hours, and then remove the bleached ivory, afterwards well washing it in clean water.