It is opaque, has an earthy texture, is about twice as heavy as water, when moistened is very ductile, adheres slightly to the tongue; and with its peculiar smell (called clayey) every one is acquainted.
115. COMMON CLAY, or POTTER’s CLAY, which is found in nearly every country of the world, is sometimes white, has a blue or yellowish tinge, or is brown or reddish.
It is the peculiar quality of this substance to become so hard by heat that it will even strike fire with steel. The ductility of clay, and its property of thus hardening in the fire, have rendered it an article of indispensable utility to mankind in all civilized countries. It is formed into eating vessels of almost every description; plates, dishes, cups, basins, bowls, and pans for keeping provisions in. For these almost any kind of clay may be advantageously used; but it is necessary to mix it with sand, for the purpose of rendering the vessels that are made of it more firm and strong. Those that are applied to culinary, and other uses in which it is requisite for them not to be penetrable by water, are covered with a glazing. This glazing, for coarse ware, is sometimes made with lead, and sometimes by throwing a certain portion of salt into the furnace. In the formation of the better kinds of earthenware, the clay is made into a paste with water, moulded into the requisite shape upon an horizontal wheel, the inside being formed by one hand of the potter, and the outside by the other, as the wheel turns round. When the pieces have been baked, they are dipped into a glazing mixture, consisting of white lead, ground flints, and water, and are exposed a second time to the fire. The different colours of earthenware are obtained by means of various kinds of metallic oxides ([21]).
The coarser kinds of clay are manufactured into bricks for the building of houses, and tiles for the covering and paving of them. These are formed in moulds of the requisite shape, afterwards dried for some time in the sun, and finally piled in kilns, and there baked to a proper degree of hardness. The earth for bricks ought to be sufficiently fine, free from pebbles, and not too sandy, which would render them heavy and brittle; nor ought it to be entirely free from sand, as this would make them crack in drying.
Clay is a substance of inestimable value for forming the bottoms of ponds, and the bottoms and sides of canals and reservoirs, to prevent the water from draining away. It also composes, in a great measure, those tenacious earths called arable soils. What is peculiarly denominated clay land is known by its holding water, and not soon drying when wetted. Such land requires much labour from the husbandman, before it can be sufficiently pulverized, or brought to a fit state for being productive of corn or grass.
116. Pipe Clay is a fine and yellowish white variety of common clay. It is very plastic, adheres strongly to the tongue; and, in a strong heat, is hardened, and rendered perfectly white.
It is of this clay that tobacco pipes are made, by the simple process of casting them in moulds, forming a hole through the stems by means of a wire, generally dipping the small end into some glazing material, and then baking them. Pipe clay is also formed into oblong pieces, dried, and employed for cleaning white woollen cloths, and for various purposes of domestic utility. It is likewise the basis of the yellow, of what is called Queen’s ware pottery. This is glazed in a manner somewhat different from that of common pottery. The glazing mixture consists of a certain proportion of carbonat of lead ([239]), ground flint, and flint glass, worked with water to the thickness of cream. The ware, before it is glazed, is baked, and thus acquires the property of strongly imbibing moisture. It is then dipped into the above composition; exposed a second time to the fire, by which the glaze it has imbibed is melted. A thin glossy coat is thus formed upon its surface, which is more or less yellow, according to the greater or less proportion of lead that has been used.
117. LOAM is a yellowish or brownish kind of clay; sometimes containing a considerable proportion of sand. It occurs in immense beds, and is found in almost every part of the world.
This substance, when mixed with straw or hair, to prevent it from cracking, is extensively used for the building of what are called mud cottages or houses. These are generally reared on a foundation of stone, or brickwork, to secure them from injury by the moisture of the earth. It is said to be the most advantageous practice to form the loam into bricks, and to dry these in the shade, and afterwards in the sun. The use of such bricks is of great antiquity. We are informed that the ancient city of Damascus, and even the walls of Babylon, were constructed of bricks made of loam.
118. PORCELAIN CLAY is generally of white or reddish white colour, sometimes inclining to yellowish or grey. When dry, it absorbs moisture rapidly; and it becomes very tenacious when kneaded.