ANNEALING.
"Toughness is one of the essential characteristics of a good paving brick. It rests very largely with the burner whether or not the brick from his kilns are tough or brittle, True, the proper kind of clay must be used, for it is not to be asked of the burner that he will produce good tough brick unless the brick given him to be burned are made of clay selected be cause it contained the elements necessary to make the required brick if it be properly handled. Yet so much does proper management of the kiln influence the toughness of the brick that the success of the enterprise rests upon the skill of the burner in annealing the brick, and thus giving them the neces sary toughness. The best of clay may be selected, the most im proved machinery employed, the best methods of management adopted, the brick may be carefully dried and go into the kiln without a crack or check, and yet improper management of the kiln at the time of closing may make the brick rotten, dead, shaky, checked, cracked, and wholly unfit for the purpose for which they were intended.
" When the kiln has been properly burned and the burner pronounces it finished, the process of annealing begins.
" The one principle involved or end to be attained is slow cooling.
" Every observing burner has noticed that pieces taken from the kiln while red hot and cooled rapidly in the air or in water present a cracked and crystallized appearance upon being frac tured, and that they are easily broken and are devoid of the metallic ring that is so desirable. Annealing is a process that is arranged for by the last acts of the burner before leaving the kiln. It is in no way performed during the firing of kiln, the raising of the heat nor the holding of the temperature while the heat is being driven through the kiln, but is strictly an act of cooling.
" It is well known that the particles of a body change their positions somewhat during the expansion attendant upon heat ing. If the body is slowly cooled the particles may return to their former position of stability, but if cooled rapidly, they are arrested in the positions that are more or less strained.
" There is a constant tendency while in this strained state to change position, just as a strained spring or bow tries to return to its original position. A blow or shock tends to shatter the piece by breaking the tie at some point and thus destroying the equilibrium. Glass manufacturers recognize this, and by annealing, which consists in re-heating and cooling slowly, allow the particles to assume positions less strained and more stable, and consequently the glass is more tough and not so easily broken.
" This strained state of the particles of a body that has been rapidly cooled and the brittleness attending may be made clearer to the mind by considering the conditions that exist in a bow that is bent or a spring that is strained. When the strain is carried to the limit, it is observed that just before breaking the spring or bow reaches a hard, stiff condition and seems to stop bending, and, as it were, tries to resist further flexure, as if it knew that to go farther would be fatal.
" Now, this is the condition of the particles of the brittle body that has been cooled rapidly. If the cooling process has been too sudden, the body cracks, as seen in the tile or brick that has been taken from the fire and cooled quickly in the air or in water. When the bow has reached the limit, it is in a condi tion that it may be broken by forcing it a little farther, it is in a brittle state. Whereas, if it is not bent at all, but is in its normal position, it may be said to be tough, and may be bent considerably in any direction without breaking. In cooling an object slowly, the particles are allowed to assume their normal positions and are at ease, and the object may be said to be tough. If cooled rapidly, the particles are arrested in a strained condition, and it may break if forced a little farther, that is, it is brittle."