Slate is composed mainly of clay or mica, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals. Most slates are gray in color and range in a continuum of shades from light to dark gray.Origin and History
Slate is a generic name for a metamorphosed mudstone or shale, a fine -grained rock that has developed a strong, planar cleavage in a direction suitable for the manufacture of roof tiles. The term does not denote a specific mineralogy and this is entirely dependent on the local composition of the mudstone from which it was formed. Typically, fine micas are present (and it is these planar minerals which align to promote the cleavage) with varying amounts of iron oxides, iron pyrite and elemental carbon in the form of graphite. Very fine-grained quartz is frequently present. Clay minerals may be present, but these are generally converted to mica during the processes of metamorphism. Individual grains within most slates are beyond the resolution of optical microscopes. Slates form in areas of continental crust that have undergone moderate burial and deformation during their geological history. Waste from cutting and trimming slates is an abundant by-product. According to Heaton (1928) this was ground to a powder and supplied in grades of varying fineness. Slate powder was used in the paint industry as an inert filler for the preparation of filling compounds for woodwork, machinery, etc.
Slate Black is a variant or common name for this pigment. Apparently a 'shale black containing 15% carbon' could bc made by calcining bituminous shale (Mayer, 1991).Source
Our slate gray pigment is derived from slate deposits in France.Permanence and Compatibility
This pigment has not been tested for lightfastness in most artist's mediums, but based upon its chemical composition it should have an equivalent lightfastness of brown ocher, umber or natural Sienna pigment. It should also be compatible with all pigments.Oil Absorption and Grinding
The oil absorption rate of slate gray is 24 grams of linseed oil for 100 grams of pigment. Slate gray is medium grain pigment that is easily hand mulled into paint.Toxicity
There are no acute or known chronic health hazards with the anticipated use of this product. Protect yourself against potentially known hazards by keeping it out of your body. Do this by avoiding ingestion, excessive skin contact and inhalation of sanding dusts. Conforms to ASTM D4236.
Rublev Colours Pigment: Slate Gray
|Colour Index:||PBk 19 (77017)|
|Chemical Name:||Hydrated Aluminum Silicate|
|ASTM Lightfastness Rating|
|Refractive Index:||Not Available|