Types of Slate
Pennsylvania slate quarried and worked by PBBS falls into two types, Clear and Ribbon.
Clear slate is solid, one color slate, which is all from one “bed” or formation. It is blue-gray to blue black in color. When sealed, oiled or otherwise treated, it is approximately “jet black”. Ribbon slate is formed by Mother Nature when two or more formations were compressed to make one stone from more than two beds. The contrast looks like a ribbon embedded between lighter colored (clear) beds of stone. It is actually more than one bed joined to make one solid stone with ribbon like lines in it. The width of the ribbons varies. Similar to an ornamental dark band, a ribbon may be wide, narrow and sometimes double. When sealer is applied, the ribbon effect is emphasized.
Please click on the images below to see an enlarged view.
Types of Finishes
Natural Cleft – a finish unique to slate compared to other rock or stone types, a natural and pleasingly textured surface created when freshly quarried slate is manually split into sheets. The fan of the cleft is unique to slate and starts where the first hit of the chisel started the split.
Rough Textured Natural Cleft
Honed finish – Extremely smooth surface created by removing machine marks using wet diamond tools or stones to hone the slate to a soft, beautiful and smooth finish. Honing is the equivalent to polishing other stone like granite or marble, but as slate does not have a crystalline structure; it is honed and not polished. It is like a satin finish.
Sand Rubbed – A texture created by hand rubbing sand with water on the face of the stone, the result gives slate a surface that is like 80 grit sand.
Machine finish – Gauged to thickness but machine marks remain the on surface of slate from the gauging process.
Options for Edge and Fabrication of Slate (see also Countertops)
Definitions – Terms
Bevel Slope – means full area of slab as on a wash on a counter top. Can also be used for a threshold (thresholds require a sketch.)
Gauging – means gauged to thickness
Finished edges that are available on slate include:
Radius – same as nose when talking about an edge. Radius – circular cutting such as a round corner or a counter top with a round edge.
Smooth sawn – cut with a diamond blade with some visible saw marks
Honed – all saw marks are polished out leaving a smooth surface
Nose – rounded edge , done with sand paper or diamond tooling ( standard is 1/8″ radius)
Pencil Edge – slight round , just to remove sharpness off the edge and reduce likelihood of chipping
Bevel – 1/4″ x 1/4″ is done on the top edge of a stone and a slight nose on the bottom, 1/8″ or other is available on some applications. Chamfered –same as beveled
Full Bullnose – Full is a full rounded edge according to the thickness
1/2 Bullnose – is top edge rounded half the thickness of the stone
Corners – can be rounded. Examples are 1/8″ radius, 1/4″ radius, etc., sketch may be required or a template.
Countersinking – holes may be countersunk, but do not have a flat bottom.
Notching and Circle Cutting – can be straight, circle, inside, outside. Template or sketch may be require. Cut outs – for sinks or other purposes require a template with accurate measurements.
Drilling – hole made with a masonry drill bit. Large holes may be considered circle cutting.
Grooves – drip, bondage, or drain boards
Rabbeting and Slots – is a special application, must call or have specs.
Splay cuts and miters – require sketch or template
Stops and Returns – require sketch or template
Sealing of Slate
All finishes of slate can be used in their natural state or sealed with an organic or chemical sealer. Sealing slate renders it impervious to oil or other chemical marks such as would be encountered with a kitchen counter, mineral buildup as in a bathroom shower stall, or other undesired markings which would result from daily use. Roof shingles are never treated, but even outdoor patio pavers can be sealed with an appropriate product. Shown are samples of various types and finishes of worked slate with and without sealer, which can be more or less “wet” in appearance.