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
Ribbon Grade -
Ribbon slate is characterized by an
ornamental dark band which, in the case of window sills, usually
runs along the length of the sill. A ribbon may be wide, narrow
or even double. In a wide sill, there may possibly be more than
one ribbon. Note: Although Ribbon grade is not recommended
for exterior use, with proper pitch and the ability to dry, a ribbon
sill may be serviceable for over twenty-five years or more. Ribbon
slate is supplied in Machine Gauged, Sand Rubbed or Honed 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 use in
installations where stills are installed in multiple window runs.
The variation of facial thickness with regard to the edge plane
is more pronounced when seen in a run of sills rather than a single
still.) Machine Gauged Finish is a smooth, even finish which
approaches the honed finish. However, slight swirl marks are usually
visible. 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.
- Hone & Arris Edge is a honed front edge and either
a 1/8" chamfer or a 1/8" radius on the top front edge
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.
Arris Edge is the 1/8" chamfer or 1/8" arris on
the top front edge without honing the front edge. 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.
Window sills are usually supplied in 3/4", 1", 1 1/4",
1 1/2", & 2" thicknesses. 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 Machine gauged
or Honed finish. Impregnators usually do not as radically
alter the color of the slate and give a more satin finish. As with
sealers, Machine gauged and 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