Slate
A highly metamorphosed clay rock, consisting ing essentially of clay. The particles are so mechanically arranged that it splits into plates that are independent of the layers of deposit, and are of a blue, green, gray, or black color. Its hardness prevents it from injury when exposed to the weather; it is therefore well adapted for roofs of houses, etc., and is in great demand for enamelling mantels and other objects, being by this means made to imitate the most expensive marbles at slight cost. Extensive quarries of this substance are worked in Cornwall, Wales, Ireland, and Scotland; also in Vermont and other States in this country.
Adhesive slate absorbs water readily, and is highly adhesive. Aluminous slate yields alum. Bituminous slate is impregnated with bitumen. Hornblende slate contains feldspar, and is used for flagging. Hones are made from slate, also pencils; and the slate-clay which consists of silica and alumina is suitable for fire-brick. See ALUM.
Slicker--sometimes called "sleeker" and "smoother"--is a moulder's tool that usually has some special shape given to it on one side. The other side has a finger-piece or handle, by which means the slicker is worked upon the sand and made to impart smoothness and finish to the surface. These tools are of cast iron, brass, and steel, highly polished on the face side, and are known as corner, elbow, pipe, button, flange, bead, and web slickers, etc. Considerable dexterity of hand and eye must be acquired before the moulder can use these tools creditably, and only the most skilled workmen should be allowed to use them indiscriminately. Inferior artists are apt to linger too long over the work, and their unpractised eye and lack of taste invariably end in producing lines that bear no resemblance whatever to the original pattern; but on such parts as offer few difficulties they will smooth the surface with such frequency that the alumina in the sand is worked into a clayey skin on the surface, which, if it be not already loosened, will more than likely shrink and break away in scabs when the metal covers it. See SCABBED CASTINGS.