In the case of plastic clays, an open mill or mixer may be used, but for non-plastic clays, or where considerable calcine is used ; it must be a closed pug-mill of good length in order to get the clay sufficiently tempered for moulding.
A process of grinding and tempering fire-clay which is much in favor in the older river works of Jefferson county, Ohio, and also at Mineral Point and Haydenville, Ohio, is theoretically the most correct of all methods in use ; but it is also the most expensive as well. Along the river the clays used are as hard and rocky as sandstone when they are newly mined ; they are sandy and apparently non-plastic, but by this treatment they develop into one of the best working clays in the State of Ohio.
The dry-pan used is of the kind previously described, and is adjusted to deliver the ground clay into an elevator. This carries it up to the top of the building and delivers it upon a screen. This screen is a box about fourteen feet long, by four feet wide, by seven feet deep ; the bottom of this box is made of sheets of perforated sheet-iron, the holes about one-tenth of an inch in diameter ; the slant is about 45 degrees, so that whatever enters the screen is sure to leave it either by passing through or by running off at the end. That which escapes from the end is carried down by spouts to the dry-pan, and is re-ground, so that a charge being introduced runs on until it is all through the sieve. That which passes the sieve is caught by a cloth or board hopper beneath, and is conducted to the tempering mill or to the bins for storing. The clay which has been screened is beautifully fine and even. The tempering mill is on the same principle as the wet-pan first described, but is of a larger diameter ; the rolls are frequently arranged to turn instead of the pan ; they are of larger diameter and less thickness than the wet-pan rolls, and weigh usually 1,800 pounds each. The pan is provided with water, and a charge is thrown in wet and ground briskly until as plastic as can be ; by this course of treatment the qualities of the clay are developed to the best possible effect.
Moulding and Pressing. In England the brick are all made of very stiff clay, in brass moulds, and are not pressed. They are perfectly solid and square, and in fact need no pressing. This method has not been adopted to any extent in the United States. The almost universal method employed in this country is to make the brick in wooden moulds, using very soft clay, or " mud," as it is technically termed ; then to spread the brick on a warm or hot floor to stiffen, and then press in a hand lever press, and return to the hot floor to finish drying.
The object of pressing is not so much to make the brick dense as to square them up and put them in better shape. This method of moulding fire-brick is followed in every section of the country, East, West and South, and will probably con tinue to be largely made in this manner for some time yet to come. In some of the older works the brick are moulded in iron moulds, the clay being tempered in " ring pits ;" a day's work in such plants being to mould and press 1,200 brick, the men wheeling their own clay from the pit to the moulding table ; a gang comprising a moulder, presser and off-bearer. Hand-pressing is almost the universal rule, but there is a de mand for something better. This is now at hand in one or two steam-power presses that are powerful, simple and easy to operate, and which effect a saving of at least half of the cost of hand-pressing. At the same time it is often claimed that there is no advantage in steam-pressing ; that hand-presses can be moved about to suit the work, and are handier, etc. There is a semblance of truth in much of this, but when really sifted down there is very little in it. No account is taken of the time that is taken up in moving the press around, nor of the fact that the brick have to be moved from the warm, or stiffening floor, to the hot, or drying floor (of which more anon), and hence but little more moving is required to press than on a stationary steam press, and with the different system that can be adopted, the saving in cost is as above stated.