This is a necessary operation in the manufacture of enameled brick. At the same time it leads to much trouble and difficulty. An unpressed piece of clay is easy to enamel and have it smooth and perfect, for, even if laminated or filled with air cavities, there has been no distortion of laminations or compression of the air, and when the surface is moistened by the slip or enamel there is a very slight or no tendency to blister, but repressing seems to magnify every defect of structure in the brick. The laminations are distorted and the contact between laminated faces seems to be ruptured. The laminated condition is certainly much more apparent to the eye, as can easily be seen by breaking a few unpressed brick and a few pressed brick. In a pressed brick the air in the larger air cavities is much compressed and leads to large blisters after dipping, and, besides this, there seems to be a number of minute air cavities, or it may possibly be simply an expansion of some of the particles of clay itself that leads to a rough, irregular face. These latter irregularities are very small, but still they are great enough to show quite a difference in the appearance of a pressed and unpressed brick after dipping. Pin holes are much more prevalent in the pressed than in unpressed brick.
The blanks may be repressed either by hand or machinery. The best hand represses are probably the Perfection Press, made by C. W. Raymond & Co., Dayton, O., and the Titley Screw Press. Either of these presses would be much improved by some device for an automatic adjustment of indent that would make every brick of equal thickness. The adjustment of thickness cannot be done with escape holes or vents, as this causes a flow in the clay while under pressure. This flow leads to a stratified arrangement of the clay, which will show afterwards in the enameled face. The effect of this flow will be somewhat, reduced by subsequent repressing in a tight box, but will not be entirely removed.
The best power press for this class of work on the market, up to 1894, was the Perfect Press, made by Pullan & Mann, Leeds, England. This press is arranged with a driving pulley that allows the press to stop automatically at each complete revolution, or at the finish of each brick, allowing the finished brick to be removed and the new blank to be placed in position without danger of the hands of the operatives being caught A very small motion of a lever serves to immediately start the press. This press is arranged with an automatic movable indent, which is pressed back by the clay in the brick to a greater or less extent, depending upon the amount of clay in the brick. After the indent is pressed back it is locked in position by a double wedge, and a slight additional pressure is given, with the indent fixed. On the up or delivery stroke the wedges are loosened and the indent is forced out by a heavy spring and is in position for the next brick. There is one criticism that can be made of this press, and that is that the indent-forming plate is out, or prepared to make a large indent. This large indent-forming plate strikes the brick and unduly distorts it and tends to crack it. It would be better if a press could be made in which the main plunger were brought down to within a trifle of the proper thickness, and the indent-forming plunger then forced into the brick up to a certain pressure and locked, then a slight additional pressure given with indent and main plungers locked together. The ultimate capacity of the Perfect Press is about 6,000 bricks per day, but about 5,000 is a good day's work. It requires one man and two boys for its proper operation.
The precautions to be taken are the same whether the press is a hand or power press, but, if a sizing press, there is no necessity of weighing or otherwise bringing the blanks to uniform size. The blanks should be struck on the faces that are to be enameled with a paddle, and afterwards smoothed up with a knife. The condition in which the bricks should be pressed is somewhat different for each and every clay. The blanks should be, as a rule, as stiff as they may be not to crack in pressing, and yet to press up well in the corners. The pressure should be sufficient to make sharp corners, but should not be excessive, else longitudinal cracks will result As little oil should be used as possible, and this should be as largely composed of low test kerosene as possible. Where low test kerosene alone will do the work it is advisable to use it, but in some cases a heavier oil will have to be mixed with it
Care should be taken to keep plungers well fitted, so that there will be as little finning as possible. When finning does occur the offbearing boy should place the brick on a table, and, with his offbearing paddle, turn the fin down on the flat side of brick. This will be found better than attempting to remove the fin, as the turned-down fin will afterward be removed with the cleaning brush, leaving a clean, sharp edge, whereas a removal of the fin while the brick is soft is apt to leave a ragged edge.
The two main troubles to be avoided at the press are cracks in brick and down corners. These are due to almost directly opposite causes, and in some clays it is very difficult to avoid one without running into the other. The softer the blank the less the tendency to crack, but the greater the tendency to drag at the corners. A brick body with plenty of grit in it is less apt to drag at corners than a pure clay body.
The method of handling and transporting the brick from the press to the enameling room and on through the other departments will vary with the amount of daily output. With a large daily output a car system would undoubtedly be advisable, but with a small output a simpler method can be used to advantage. A method with which the writer was thoroughly familiar proved to be a good one. This plan required level and smooth floors throughout all departments, from press to kiln. It consisted of boards placed on skids that would raise them about four inches from floor. These boards were placed alongside of press and filled with sixty brick. When full, a two-wheeled cart or truck, with large wheels, about three feet in diameter, with bent axle, and pick-ups suspended from said axle, was run over the load. By pulling down on the handles of truck the load was lifted from floor and wheeled to any part of the building and set down. This is a cheap plan, as the investment is chiefly in boards. Three or four trucks will suffice for the handling of quite an output - one at presses, one or two for the dippers and one for the kiln gang.