Annealing (Sax. anaelan, to heat), a process of softening and toughening certain metals and glass by heating them, and then cooling them very slowly. In working some of the metals under the hammer, or in rolling them into plates, or in drawing them out into wire, they become hard and brittle, so that the process cannot be continued without restoring them to their former condition. This is done by annealing. It has sometimes to be often repeated in drawing out a single plate of brass or aluminum. The jarring motion to which the wheels of railway cars and their axles are subjected changes in time the soft, fibrous texture of the iron into a crystalline structure approaching that of cast iron. They become brittle, and can be restored only by working them over and annealing. Intense cold produces a similar effect when the change of temperature is very sudden. The tempering of steel is an artificial hardening of the same nature. Cast iron may be chilled and become as hard as steel, but brittle. It may be annealed (with a slight change in the composition at the same time) and form malleable castings - even cast-iron nails that will clench. - The subject is more particularly interesting as it relates to the changes effected by temperature in glass.

When this is melted and shaped into articles which are allowed to cool in the air, the glass becomes too brittle for any use. The exterior cools first and forms a contracted crust, which shelters the interior particles; so that these continue longer in a semi-fluid state, and are prevented from expanding, as glass does in cooling, and uniting with the rest to form a homogeneous mass. The inner parts are thus constantly tending to expand. If, on the contrary, the glass is placed in a hot oven, and this is allowed to cool very slowly, the particles of glass appear to assume a condition of perfect equilibrium of cohesive force without tension, so that the mass becomes tough and elastic. The extreme effect of sudden cooling is very curiously shown in the philosophical toys called Prince Rupert's drops and the Bologna phial. The former, which were shown by Prince Rupert to Charles II. in 1661, are little pear-shaped lumps of glass, with a curved stem, formed by dropping melted glass into water. Most of the particles burst to pieces, but some assume this form.

When taken out of the water they will bear a smart blow without breaking, the effect being spread equally throughout the whole body; but if a little piece be broken off the end of the stem, they will fly into fragments with a sort of explosion. Dr. Ure explains this phenomenon by referring it to the tendency of a crack once formed in the glass to extend its ramifications in different directions throughout the whole mass. The same effect is observed in the very large sheets of plate glass used for shop windows: once cracked, they are seen in time to fall to pieces, the cause no doubt being imperfect annealing. The Bologna phials are made of unannealed glass, 3 or 4 inches long and about 1/8 of an inch thick. No regard is paid to their shape. They will bear a pretty hard blow with a hammer handle on the outside, or a small bullet may be dropped into one without breaking it; but if a sharp fragment of sand, or small piece of stone, be dropped in, the glass will burst in pieces, generally at once, but sometimes after a considerable interval.