The grinding of the clay has been attempted by various methods, including crushing-rolls, pulverizers and disintegrators of various kinds, and by other mechanical appliances. For hard clays none of these have proved successful, and the only method to give satisfaction is the grinding-pan, either wet-pan, dry-pan, spiral pug or chaser-mill. The latter is not much used now for grinding clay for fire-brick making, although it is a very suitable machine for some other branches of clay-working. The method of preparing the clay depends largely on the mode of manufacture adopted. Up to the present time the moulding of fire-brick is almost entirely accomplished by the hand method, which requires the clay to be tempered and made up quite soft.
There are three distinct methods of accomplishing this, viz.: The wet-pan process, the dry and wet-pan combined, and the dry-pan and pug-mill combined. The methods of grinding and tempering are somewhat various. The "Fire Brick Mill," as used in England, is unknown in the United States.
The methods in use in America are the wet-pan process, where the grinding and tempering are done together in a solid bottom pan ; the dry-pan and wet-pan combined ; and the dry pan and pug-mill combined. Other methods of grinding, such as the "ring-pit," for example, have from time to time been tried, but have failed of adoption to any extent.
The wet-pan is found in most common use where a hard, flinty clay and considerable calcine are used. In charges con taining considerable amounts of both these bodies and only a little plastic clay, such hard and intimate mixing is the only way in which the structure of the brick can be made sound. This wet-pan process is used entirely in the Sciotoville district, Ironton and Logan, Ohio. The clay is dumped into the pan in the rough, water is turned on, and the charging is ground and tempered until the attendant judges it to be fine enough. There are some objections to this method of preparing fire clay, as as is next to impossible to get the clay of a uniform grain, as some of it is ground too fine, while another portion may be too coarse. Then, again, the process is slow, and for an extensive works a great many pans are required to prepare enough clay, the usual quantity for each pan being a day's work for one moulder, which consists of 4,00o brick, with a few extra thrown in for waste. The pan is usually fitted with a pipe from the engine, from which water can be had by turning a faucet. Either hot or cold water may be used ; it probably makes no difference which, in the quality of the brick, but the former makes the work of the moulder much more endurable.
The wet-pan method has one great advantage which is not possessed by either of the other processes, and that is that each moulder may be on a different grade of brick, requiring a different mixture of clay ; and by having a vet-pan to serve each moulder, several different mixtures can be in preparation at the same time, whereas in either of the other processes only one kind can be made at once, and, therefore, all the moulders must work on the same grade of brick.
In the dry and wet-pan combined, the tempering is done in a wet-pan.
As the clay has been already ground by the dry-pan, a few revolutions of the wet-pan are sufficient to mix the clay with water and bring it to a proper temper for moulding. The clay is usually taken out by hand, either with a loose, ordinary spade or shovel, or by a level-shovel fixed for the purpose. A special style of pan is used by the Union Mining Co. of Mount Savage, Md. In this pan the rim is loose and does not revolve. In the rim is a gate which opens into the pan. When a charging of clay is put into the pan and is suffi ciently tempered, the gate referred to is opened and the clay is automatically discharged, and the pan is then ready for another charging of clay. This is an ingenious plan and a great im provement upon the ordinary pan. The clay after being pre pared is carried to the tables of the various moulders by an endless belt, where the brick are to be moulded by hand.
The dry-pan and pug-mill mixer combined is the style of grinding and preparing fire-clay least adapted to general use of all three ways, but is a cheap and useful method in some cases. The pan is very similar to the ordinary wet-pan, but has this difference : the floor is fitted with plates cast in segments fit ting on a framework of radii beneath the pan. The plates are thus fitted into a level and continuous floor ; they are full of parallel slots or holes, which open immediately into a larger room from the underside, so that any particle of matter which passes the surface will have no chance to stick lower down. Beneath the pan is a bin into which the clay, as fast as it is re duced fine enough to pass the bottom of the pan, falls ; in this bin revolve arms, which collect continually the powder and de liver it at the foot of an elevating-belt which is at one corner of the bin. The charge is all introduced together and is run until it has all disappeared beneath the surface, or else its propor tion of calcine would not be equally distributed. All dry- pans are subjected to this disadvantage, that the softest parts go through first and the harder last, so that the powdered clay as delivered by the elevating-belt would not be strictly homo geneous ; also, the largest part of the clay goes through at once, and the longest part of the grind is devoted to getting the least of the charge through, which is a waste of energy. Another disadvantage resulting from this plan is the fact that the hard material is never rendered finer than is necessary to pass the holes in the floor, which to make the machine work at all rapidly are necessarily larger than is good for the brick. The powdered clay having been delivered into a bin above, is ready to be mixed as needed. The mixing-machine is a trough about eighteen inches wide by eight feet long by eighteen inches deep ; in it works a horizontal axis on which are fixed cutting-arms, which are arranged spirally, but at such a pitch that their action is slow in moving the clay forward. The tem pering is done by merely adding clay and water, and allowing the machine to mix it up to a paste.