25. Pulverization

25.    Pulverization. The reduction of any substance to dust or powder is generally performed by means of a pestle and mortar, or, on a larger scale, by stamping, grinding or milling. A few soft substances, as carbonate of magnesia, carbonate of lead, etc., may be pulverized by simply rubbing through a fine sieve; while many hard and gritty, and some soft substances, such as chalk, antimony, etc., are pulverized on a large scale by elutriation. (See No. 14 (Elutriation).) Others will only yield to a rasp or file. "Whichever method is adopted, the substance to be pulverized must be very dry, and may even require artificial drying or desiccation. (See No. 12 (Desiccation).) On the other hand, a few substances, as rice, sago, nux vomica, etc., are often soaked in water, or steamed, before being pulverized. In some cases, some other substance or intermedium is introduced to aid in the operation; thus, sugar is used in pulverizing civet, musk, nutmeg, and vanilla; absorbing the moisture which could not otherwise be readily got rid of. The addition of a very small quantity of alcohol renders the powdering of camphor easy. Gold leaf is pulverized by mixing with sulphate of potassa, and then removing the potassa by washing with water. (See also No. 2517.) Fusible metals are reduced by melting and rubbing in a mortar until cold, or by agitating when melted in a box covered inside with chalk or whiting. Glass, quartz and silicated stones require to be heated red hot and then thrown into cold water, to make them sufficiently friable for pulverization. When powdering very dusty or costly articles in a mortar, it should be covered with a loose skin of leather, fastened firmly round the top of the mortar and the pestle, to prevent loss of the dust, and possible injury to the operator's lungs. When a substance is required to be reduced to an impalpable powder, a slab and muller are used; this process is termed porphyrization.

26. Reduction

26.    Reduction. This term is applied to a process by which the oxygen is withdrawn from a metallic oxide, leaving the base in its original state. This is effected by heating the oxide with carbon or hydrogen; or by exposing it to the action of some other body which has a powerful affinity for oxygen. A portion of the metallic oxide to be reduced, is mixed with finely powdered charcoal and exposed in a crucible to the heat of a furnace. The metallic residue, which remains after reduction by this means, is usually mixed with coal dust. This is prevented by lining the crucible with charcoal dust made into a dough with clay and water, leaving a space in the middle to receive the metallic oxide, not mixed with charcoal, as in the former instance; the crucible must be covered, and then heated. The reduction in this way is slower, but the metal will be pure and free from coal dust.

When hydrogen is employed for reduction, the metallic oxide is heated to redness in a glass or porcelain tube, and subjected to a current of hydrogen gas, which absorbs the oxygen, and leaves the metal pure. Other agents are sometimes used for reducing, as tallow, oil, resin, sugar, and starch; but carbon and hydrogen are the agents generally employed.