ANNEALING, or NEALING. A pro cess applied in the manufacture of glass and some metals, to prevent the particles arranging themselves in that condition which produces a brittle quality. When a glass vessel is allowed to cool immedi ately after being blown, it will often bear to be struck violently on the outside with a stone or hammer without injury ; but if a small piece of glass be gently dropped into it, flies to pieces either immediately or after a few minutes—a slight scratch is often sufficient to break it. This curi ous phenomenon of unannealed glass is illustrated in the Bolognaphial, and Prince Rupert's drops. The former will bear a pistol bullet to fall on it from a height of two feet without harm ; but if a grain of sand drop into it, it flies to pieces. These phials lose this property by age. Rupert's drops are glass melted off a rod, and allowed to drop into cold water : many will burst in the water, but those which escape show the unanneal ed property remarkably. They may be struck with a hammer on the bulb-end unharmed ; but snapping off an extrem ity of the capillary tail, makes the whole drop fly to pieces so small as to be fine dust. If the drops and the phials be heated and allowed to cool slowly, they lose this property. By sudden cooling, the particles of glass have not time to arrange themselves in that form which constitutes stable equilibrium, and hence on the slightest disturbance at favorable points, an attempt is made at re-arrange ment. Glass forms one of the few ex ceptions to the law that bodies contract in size in passing from the fluid to the solid form. When it is allowed to cool slowly its molecules arrange themselves in a fibrous form so that freedom of mo tion or elasticity of the whole mass re sults ; and the substance can propagate vibrations from one extremity to the other. But when melted glass is sud denly cooled, the solidification of the sur face occurs so fast that the particles of the interior are enclosed before they have had time to arrange themselves in this elastic condition. The cohesion of
the particles is but slight, a partial force overturns it. Drinking glasses which are unannealed and happen to be thick, are easily broken on that account when they are thrown into vibrations. While annealing, the glass is kept at a condition nearly fluid for several hours, by which means the interior particles can expand and crystallize regularly. Mr. Pcllat has proved that this arrangement of particles does take place : he found that two tubes of equal size (forty inches long), the one which was annealed contracted one-six teenth of an inch more than that which was cooled in the air. Glasses which are exposed to rapid transitions of tempera ture, may be annealed better than is done in the glass-house, by placing them in a vessel of cold water and raising it to the boiling point, and keeping it so for some hours. .Lamp-shades ought to be heated alwa3P in this way.
When metals have been hammered out very much they become brittle, and they will crack if it be carried much far ther : their malleability is restored by the workman annealing them, or heating them red hot. The annealing seems to remove the closeness of the proximity of the particles of metal which the hammer hardening had produced. In the manu facture of some articles of steel after they have been hammered into shape, they are annealed to allow of their being cut by the scissors or file. In wire drawing, in the rolling and flattening of vessels, it is necessary to anneal occasionally, otherwise the metal would become too hard and would not extend. Iron and steel are occasionally annealed in the open fire, and left to cool gradually 15y simple removal. This oxydizes the sur faces and destroys the steeling.
Soft metals, such as tin and lead, are annealed by being dipped into hot water.