Slate GEOLOGICAL CLASSIFICATION Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many constituent minerals may be dissolved, transported and reprecipitated by thermal waters. Heat and pressure may cause recrystallization. In this way, new rocks are formed, differing widely from the igneous or sedimentary types and usually much harder than either. Thus shales and related rocks may be altered into slate. The shales from which slates originate were deposited previously as clay beds. These beds of shale, at first horizontal, were tilted by subsequent earth movements and the intense metamorphism that converted these into slates folded and contracted them. Slate, then, belongs to the metamorphic group of rocks and can be defined as a fine-grained rock derived from clays and shales and possessing a cleavage that permits it to be split into thin sheets. COLOR AND VEINING The color of a slate is determined by its chemical and mineralogical composition. Gray and bluish gray color are due chiefly to the presence of carbonaceous material; many other colors are due to iron compounds. Slates containing large proportions of finely divided carbonaceous matter are black. Other colors that are found are blue-black, red, green, purple, mottled, yellow, brown and buff. Permanence of color has considerable importance, for although some slates maintain their original color for many years, others change to new shades within a comparatively short time. Some slates tend to fade under the influences of the elements. Such changes may be due to the presence of small quantities of iron-lime-magnesia carbonates, which decompose readily and form a yellow hydrous iron oxide, limonite. Therefore, slates are of two types, "unfading" and "fading". Unfading color is not a quality verifiable by any current ASTM or other test method. TEXTURE Differences in conditions of deposition often result in variations in texture of successive strata and such variations make it possible to trace folds and contortion within the quarry. "Ribbons" are dark bands, a fraction of an inch to several inches in width, intersecting blocks of slate at various angles. Cleavage and grain are other characteristics of slate that can affect its texture. Cleavage is the tendency for slate to split with ease in one direction. However, many slates have a second direction of splitting which is less pronounced. This second direction is called the grain. SOUNDNESS Slate, consisting as it does chiefly of very small over- lapping flakes consolidated under pressure, is a strong rock. Most mica slates of good commercial quality are highly impervious to moisture. FINISHES Slate's surface may be finished in a number of ways. Typical finishes for slate are: NATURAL CLEFT FINISH A cleavage face formed when the slate is split into thin sheets. HONED FINISH A satin smooth surface with no gloss. SAND RUBBED FINISH A flat, non-reflective surface. The type of finish desired bears some small relation- ship to final cost, as the smoother surfaces require more finishing and consequently more time. The most economical finish is the natural cleft. Other finishes such as bush-hammered, sand blasted and planed may also be available. Some stone finishes can affect strength and durability. Examples are bush-hammered and thermal finishes that initially reduce a stone's thickness but also make it more vulnerable to weakening from exposure to freeze and thaw cycles. THICKNESS Standard thicknesses for slate veneers are generally 3/4 inch (2cm), 1 inch (2.5cm), 11/4 inches (3cm), 1 1/2 inches (4cm), and 2 inch (5cm). Slate tiles are available in a variety of thickness ranging from 1/4 inch (.6cm) to 1 inch (2.5cm). Metric thicknesses given above are approximate; cutting can be made to exact metric measurements through conversion of English values to metric equivalents. Note that as slate is cut thinner its tensile strength is diminished.