FIRE-CLAYS.
Clays are termed fire-clays or refractory clays, when they re sist exposure to a high temperature without melting or becom ing in a sensible degree soft and pasty. These clays differ much in degree of refractory quality. They occur in various geological formations, old as well as recent ; but some of the best abound in the coal-measures.
All clays as they occur in nature consist essentially of hy drous silicate of alumina, and upon the presence of the water of combination depends their fictile or plastic property ; that is, their capability of being moulded into vessels or other objects when mixed with water and kneaded to a pasty consistency. All clays contain hygroscopic water, which may be expelled at C. without lessening their plasticity. When, however, clay is heated to redness, it loses not only its hygroscopic water, but also its water of combination, and as a consequence, it ceases to be plastic. In this dehydrated state it cannot di rectly combine with water and regain its plasticity, though it may absorb water with avidity. Pounded brick, for example, which is dehydrated clay, may absorb a considerable quantity of water, yet without regaining the slightest degree of plas ticity.
It is important to note that there may be great variation in the composition and quality of clay from contiguous beds in the same pit, and even from the same continuous horizontal bed in the same locality.
If we compare different clays together in respect to elemen tary composition, we find the relation between the silica and alumina to be extremely variable ; and accordingly, the for mulae which have been proposed to express their rational con stitution are very discordant. This is in great measure to be explained by the fact that in many clays a large proportion of silica exists uncombined either in the form of sand, or in much finer state of division. The grittiness of clay is due to the presence of sand.
Geologically speaking, fire-clays are of three distinct forma tions, viz., cretaceous, the carboniferous or coal measures, tertiary ; and they are all used in the manufacture of fire-brick. The first and the last of these are both of them soft and plastic. The coal measures produce both plastic and non-plastic, but the plastic is neither soft nor friable, like the other formations.
On the contrary, it is quite hard when freshly mined, though it always lacks the clean-cut fracture characteristic of the non plastic.
Fire-clays are of many colors, black, dark and light gray, blue, light green, white, both clear and deepening into a bluish or grayish tint. The flint or non-plastic clays are sometimes clear, of various shades, other times spotted, again of a dark, almost of 'a chocolate color, with veins running through it, and very beautiful. The different kinds of fire-clay vary so much in hardness that while one is dug with a spade, another has to be blasted in the same manner as rock is quarried.
There are two distinct varieties, non-plastic or flint clay and plastic clay. The former is the most refractory. They are often found in the same vein and interchangeably as regards position. It is difficult to understand how it happens that some clays exist in this flint-like condition, and that they should be so much more refractory than the other, found in the same vein, and perhaps of identical composition. The fact is, however, that the more plastic any clay becomes, either natur ally or artificially, the less refractory it is.
In the United States, superior qualities of fire-clays are found in various localities.
In the majority of instances fire-brick plants are located upon the land from which the clays are taken. There are, how ever, some notable exceptions to this rule. The works of the celebrated fire-brick manufacturers, The Harbison & Walker Co., are located in the city of Pittsburg, Pa., while the fire clays which they employ are drawn from various parts of the state of Pennsylvania. The fire-clays found at Mount Savage, Md., the " Amboy"clay of New Jersey, and the fire-clays of Farrandsville, Pa., are all highly esteemed, and are among the best fire-clays found in America. The "Amboy" fire-brick are produced from a cretaceous clay, which is first burned in a kiln, its plasticity being lost iu the process, and resulting in what is known as " cement." The second, or Mt. Savage brick, are produced from two qualities or varieties of carboniferous fire-clay, one of which has in its natural state the properties of the " cement" just mentioned.