Here we go.... i'm probably gonna repost this in my blog too (this is from something i typed up a few years ago)...
History of Synthetic Sharpening Stones
People have been passing down the knowledge of how to sharpen and polish blades with various tools for generations. To many, sharpening is a very familiar task. However, since the beginning of civilization, there has been a strong demand for sharpening tools and skills. Over time, these demands could not be met by just natural stones (including tennen toishi), and thus began the development of synthetic sharpening stones.
North America was at the forefront of inventing synthetic abrasives. In 1895, an American company, called Carborundum invented carborundum abrasives as the first synthetic abrasive material. Following that, Norton, another US company, invented their own synthetic abrasives in 1901. Since then, industrialized countries have continued to develop abrasive materials. Over the years, these abrasives have became necessary for producing steels, ships, airplanes, cars, and many other things.
The history of synthetic stones in Japan began towards the end of Taisho (the Taisho period was from August 31, 1879 December 25, 1926) to the begging of Showa (the Showa period was from December 25, 1926 January 7, 1989) in Kyoto. In the initial years, Kyotos synthetic toishi makers borrowed wood kilns being used for kiyomizu-yaki pottery. Later on, they started using threshing machines. During World War II, Hiroshima was a major center of production for synthetic toishi, and thanks to their great success at producing these stones, the popularity of synthetic toishi in japan began to rise.
Three Main Components of Synthetic Stones
Grain: the abrasive materials in the stone
Bond: the bonding materials in the stone that hold the abrasive particles
Pores: Bubble-like gaps between grain and bond that hold water and allow waste material to be washed away
Making Process of a Synthetic Stone
1. Mix raw abrasive materials together
2. Mix abrasive materials with bonding materials and stir together
3. Press the mixture into shape
4. Dry
5. Bake
6. Re-shape
7. Quality check
8. Wrapping
9. Shipping/delivery
Main Types of Toishi Production Methods
Vitrified method: Feldspar and other minerals are added to the main grain, and water is added to moisten the material. This is then pressed, dried and baked in a high heat kiln 1200-1350 degrees Celsius. This method creates stones that have great durability (and can be stored in water). If kept well, this type of stone should last very long time. Among synthetic stones, this is the oldest production process and is still in a common use.
Resinoid method: Phenol, benzenol, and other plastic are added to main grain as bonding material. This material goes through the same process as vitrified stones. However, the baking is done at a significantly lower temperature (about 200 degrees Celsius). Compared to vitrified stones, resinoid stones are more effective at high speeds and therefore are great for large sharpening wheels. This method can creates softer stones (sometimes with a bouncy feeling), and is often used in the creation of super fine shiage toishi (or finishing stones).
Magnesia method: A type of cement is added to main grain, and formed to shape. The material is then dried out at room temperature. This type of stone is often used to sharpen thin blades. Recently, magnesia stones have also been used for cutting and sharpening of non-steel items, such as other stones and glass.
Main Types of Toishi Used Today
Arato (from about 60-600 grit): Used to form blade shape, repair major damage, and change the geometry (or angle) of cutting edge.
Nakato (around 1000 grit, but can range from 700-2000): Used to erase scratches from the arato, and touch up cutting edge (everyday maintenance).
Shiageto (3000+ grit): Used refine the cutting edge and put a very sharp edge on the blade.
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Maintenance of Toishi
Vitrified stones should be washed thoroughly and stored after the usage (they can be stored in water). Resinoid and magnesia stones should be washed and dried thoroughly, and then must be stored in the shade. Also, resinoid and magnesia stones would be better kept if wrapped in cloth, and kept away from extremely dry or warm areas, since those conditions can change the bonding material in these stones.