Slate for counter tops, lavatory tops and vanities is usually an interior application. Slate for exterior use should conform to A.S.T.M. C-629 for Class I. Interior applications can be Class I or II, however, ribbon grade should not be used for chemistry labs.
Clear Grade – Clear slate is free of ribbons and most markings. It is a blue-gray to blue-black color and if sealed, oiled or otherwise treated, it is approximately “jet-black”. Clear slate is supplied in natural cleft, machine gauged, sand rubbed or honed finishes.
Ribbon Grade – Ribbon slate is characterized by an ornamental dark band, which in the case of counter tops, usually runs along the length of the piece. A ribbon may be wide, narrow or even double. There may possibly be more than one ribbon and they cannot be “matched”. The ribbon is not detrimental to the physical properties of the material, except in the case of chemistry lab tops, where acids and other harsh chemicals are used. With any treatment of oil, sealer, etc., the ribbon will not be as noticeable. Ribbon slate is supplied in machine gauged, sand rubbed or honed finishes.
Face Finishes – Natural cleft finish is a rough textured, split face finish. The bottom is gauged to an even plane and the thickness, although varying due to the facial deviation of the cleft, is constant within the maximum thickness specified. (NOTE: Natural cleft finish may not be suitable for kitchen counter tops, where tight joints are desired. The variation of facial thickness can result in “lippage” at joints and the edge plane will not appear uniform.) Sand rubbed finish is a smooth, even finish with a slight stipple equal to about 80 grit sand paper. Honed finish is a smooth, satin finish equal to about 240 grit. Honed finish is the usual finish for kitchen and lavatory tops. Natural cleft finish is used for exterior counter tops and interior vanity tops.
Edge Finishes – Hone & Arris Top & Bottom Edge is a honed front edge and either a 1/8″ chamfer or a 1/8″ radius on the top & bottom of exposed edges to remove the sharpness. This may be specified in conjunction with any of the facial finishes, however in the case of Natural Cleft finish, the 1/8″ radius is normally used rather than the chamfer. Sawn Edges are edges directly from the saw and are usually very smooth and sharp as they are cut with a diamond saw. Other edge profiles are subject to availability of tooling and should be investigated prior to specifying.
Thickness – Counter, lavatory & vanity tops are normally supplied in 1″ thickness. Other thicknesses are available to comply with special field requirements.
Treatments – Sealers usually darken the slate and give a glossy appearance. Natural cleft finish and sand rubbed finish may be sealed suitably, however, sealers may not adhere nor give a desirable appearance to honed finish. Impregnators usually do not as radically alter the color of the slate and give a more satin finish. As with sealers, Honed finish may not accept the application of an impregnator. Oiled finish is usually a mixture of 50% boiled linseed oil and 50% turpentine. This is suitable for all facial finishes and is renewed only as often as desired. Since slate is very dense and non-absorbing, a finish is only required to facilitate easier maintenance.