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Tumbling of rocks as a lapidary technique for rock polishing usually requires a plastic or rubber-lined barrel loaded with a consignment of rocks, all of similar or the same hardness, some abrasive grit, and a liquid lubricant. Silicon carbide grit is commonly used, and water is a universal lubricant. The barrel is then placed upon slowly ...
Rotten stone, sometimes spelled as rottenstone, also known as tripoli, is fine powdered porous rock used as a polishing abrasive for metal smithing, historically for the grinding of optical lenses and in woodworking. It is usually weathered limestone mixed with diatomaceous, amorphous, or crystalline silica.
The initial shaping and facet placement may be done using laps with grits of 220, 600, 1200. The polishing step, however, requires grits of a much higher grade, such as 8,000, 14,000, 50,000 and even 100,000. This grit is also embedded into a metal lap, but sometimes applied manually to the lap during polishing.
The term is based on the word "whet", which means to sharpen a blade, [3] [4] not on the word "wet". The verb nowadays to describe the process of using a sharpening stone for a knife is simply to sharpen, but the older term to whet is still sometimes used, though so rare in this sense that it is no longer mentioned in, for example, the Oxford Living Dictionaries.
Grit size ranging from 2 mm (the large grain) (about F 10 using FEPA standards) to about 40 micrometres (about F 240 or P 360). These minerals are either crushed or are already of a sufficiently small size (anywhere from macroscopic grains as large as about 2 mm to microscopic grains about 0.001 mm in diameter) to permit their use as an abrasive.
The term "Millstone Grit" was also adopted in South Wales where rocks of similar age and lithology are found though the Millstone Grit Series of this region has recently been formally renamed by the British Geological Survey as the Marros Group. The thickest bed of sandstone within it was known as the Basal Grit and this has now been renamed as ...