BOND CLAY.
The problem of making a refractory brick from native clays is based upon the fact that "the purer the clay the more in fusible." Our purest clays are flint-clays, which are probably refractory by reason of their structure as well as their compo sition. These then make an admirable basis for the brick. As they are non-plastic, their successful use compels the addition of a small amount of plastic clay, and on the choice of this clay all depends. A fine-grained, sandy clay, hard in its native state, and plastic when ground up in water, makes the best bond ; it is needless to add it should be pure. The more aluminous a clay is, the more will it shrink on burning, and if the clay which has been used to incorporate the non-plastic part should shrink materially on burning, it would loosen the bond between the pieces of hard clay and make the whole fabric unsound. Therefore, the clay fit for a bond is one in which the natural shrinkage is at a minimum ; this condition is found in a fine-grained, sandy clay. It is ignorance of this point, which seems so simple, that has caused the failure of so many patent mixtures for refractory materials. It has seemed to each man in succession who has approached the subject, that as pure kaolin is infusible, and pure sand is infusible, and as these bodies represent respectively our ideal of plasticity and non-shrinking qualities, a proper mixture of the two should produce the most desirable results. But, when such a mixture is heated, the enormous shrinkage of the kaolin loosens the bond of the whole body and makes it weak and fragile.
If, then, a pure, sandy and plastic clay can be found, the bond is one likely to be satisfactory ; but the main trouble is in a lack of purity, for if a clay fills the other conditions required, it is liable to be impure like a stoneware clay.
Sufficient attention has not been given by fire-brick manu facturers generally to the structure of the fire-brick as it relates to the size and distribution of particles of flint-clay, which forms the base of first-class brick, and to the bond clay that cements the particles together. Before it can be determined what is required, it must first be ascertained how or why a brick fails or wears away in the furnace. A practical manager in one of
the largest steel works using the Siemens furnaces, and who has given close attention and much study to this subject, says, that where the particles or small pebbles of flint-clay in the brick are of good quality they do not fuse or melt, and that it is the bond that holds the thicker particles that runs—the coarse particles floating away on the fluid bond-clay. He also says that large particles should not, for this reason, be in the brick, and the flint-clay should be prepared or screened so as to represent in size ordinary bird-shot, nothing larger, and it should be as free from dust as possible, using as little bond clay as is necesrary to make a sound brick. When the clay is so prepared another fault arises, which is this : The object of the wet-pan is not to grind what is already sufficiently fine, but only to mix and toughen the clayey mass so that when the. clay is sufficiently mixed it should be immediately taken out of the pan ; otherwise, by longer grinding, these particles are rendered too fine, as the extra grinding, in place of improving the quality of the mixture, depreciates it. To achieve this equality in the size of the grains of the clay an improvement might be made in the way the material is screened. Perforated sheet-iron screens are almost in general use, over which the clay as it comes from the dry-pan slides over the dust, and a portion of the small pebbles go through the screens. On put ting your hand into the chute which conveys the screenings back to the dry-pan, you will find that if passed through a hand-screen one-half of the material ought to have been passed to the bin, being already sufficiently fine. To obviate this ob jection there should be used a circular revolving screen, twelve wires to the inch, fixed slightly lower at one end than the other, so that, as the clay is delivered into. the higher end, the revolving screen will throw it from side to side, entirely remov ing every particle of the required size, the rough screenings falling out at the lower end.