THE MANUFACTURE OF DRY-CLAY BRICK.
THE term dry-clay as applied to the manufacture of brick is to a certain extent a misnomer ; it would be more truthful to say semi-dry-clay brick, for the clay for the manufacture of brick by the dry-clay system is gathered with a view to obtain ing it as dry as possible, usually by plowing shallow, allowing the clay to dry in the sun, and then gathering it and storing it in sheds. There is, however, even after the greatest expendi ture of care, a sufficient quantity of dampness developed after passing the clay through the crushers and the pulverizers to enable one to press it into a ball by the pressure of the hand, which could not be done if the clay was perfectly dry, in which condition no amount of pressure developed by modern machin ery would press it into a perfect and solid brick if the clay were entirely dry and free from water. Brick of this character are not a mud or tempered-clay brick ; therefore, for the purpose of the present description, we will call it a dry-clay brick, as it is usually spoken of and termed in the trade. In times past there was, and even now there is, a great deal of opposition in certain quarters directed against the manufacture of brick by the dry-clay system, but this opposition, it must be conceded, has not the same basis that existed some fifteen or twenty years ago against this new system of brickmaking, which was then seeking to introduce itself into the trade. The introduction of the system of dry-clay brickmaking into the city of St. Louis, where it is probably now better developed than at any other point in the United States, was attended with many disad vantages and drawbacks owing to the prejudice at that time prevailing. It was then quite a common thing to see written in specifications that "no dry-clay brick will be permitted to be used in the work described." It is hard now, however, to pick up a specification prepared by the architects in and near St. Louis that does not explicitly specify that " dry-clay brick shall be used," or else that " the brick employed are to be equal in quality to dry-clay brick." In the manufacture of brick by the dry-clay system, vastly more depends upon the manipulation and treatment of the clay and the burning of the brick than is the case with reference to brick made from tempered clay. It is quite true that there are some clays which are naturally unsuitable, and which no amount of manipulation or experience in treating can be made suitable for making dry-clay brick. It is, however, the opinion of the writer that where one manufacturer fails on this account a dozen manufacturers fail for a lack of the proper knowledge of how to manipulate and treat the clay in its various stages, and in the burning. It is an axiom that different clays require different treatment, hence it does not matter how much experience a per son may have had with the manufacture of dry-clay brick in a given place, for if the same person embarks in this industry in a new location there would be much both to learn and to unlearn, and a perfect knowledge of the new clay would be obtainable only after experience, close observation, and whole or partial failure in treating the clay.
In the manufacture of brick by the dry-clay system much more depends upon the burning of the brick than is the case with brick made from tempered clay. In other words, the writer desires to say that as between insufficiently burned brick of the two kinds, those made from tempered clay are much the best for durability and strength, for with the dry-clay brick the fire must supply that quality imparted by water and mixing in the tempered-clay brick, namely, 'a thorough kneading together of the entire body of clay of which the brick is composed. Brick made from dry clay, notwithstanding the fact that they may be subjected to enormous pressure in their formation, are at best no more than bodies of fine granulated matter, which cannot have perfect cohesion until after it has been submitted to a heat sufficient in its intensity to fuse the fine granulated atoms together into one solid annealed and homogeneous mass, in which case the body of granulated clay is converted into a good and perfect brick. The writer has now before him sam ples of six brick of the different kinds, all made from dry clay; four of these brick are well burned, one is partially burned, and one is not burned at all. On breaking the partially burned or salmon brick there are disclosed the infinitesimal atoms or gran ules of which the brick is composed. The appearance of this brick after being broken is similar to that shown on breaking the unburned or green brick, and in fact the partially burned brick presents a more porous condition than the unburned brick, be cause in the case of the partially burned brick, the heat to which it was subjected was sufficient to destroy a large portion of the vegetable matter contained in the clay of which the brick was made. But the heat to which this partially burnt brick was sub jected was not sufficient in degree to melt or fuse the granulated atoms together in one solid mass, such as is found to be the case with hard-burned brick, which improve in quality as they ap proach complete vitrification. In burning dry-clay brick it is not unusual to find some of them in the kiln that have been completely vitrified, and when broken very much resemble the appearance of flint or glass. Brick of this character, however, are the exception, as it is not the purpose or desire of the man ufacturer to burn the brick to such a degree of hardness, and the brick so burned commonly occupy a position in the kiln in which they come in direct contact with the flames.