Technical Ceramics Pyrophyllite wonderstone
Wonderstone is a uniquely pure form of Pyrophyllite Loosely termed technical ceramics , mined some 300 kilometers west of Johannesburg and has been used by sculptors and carvers ever since the primitive inhabitants – (probably the Khoi and the San) roamed the area. Dozens of the flat stone surfaces sport images of non-human figures, probably in dance postures. One of these hybrid creatures has an antelope head and human legs, while other engravings are of an identifiable eland and a buffalo.
Wonderstone is a Machinable mineral which, after heat treatment, assumes many of the attributes of fired ceramics. It can be turned, milled, tapped, sawn or drilled to a high degree of accuracy. Components can also be pressed from powder.
These will display the following characteristics:
- Excellent thermal shock resistance
- Extremely low thermal expansion
- Excellent chemical resistance
- Good electrical insulation
- Retaining strength up to maximum operating temperature of 1200 ºC
- Melting point is at 1630 ºC
- Very good heat retention and good thermal insulation
Wonderstone can be held to closer dimensional tolerances than any other ceramic body that is not ground after firing. This allows for rapid prototyping and pilot scale productions. It has been used for coil forms and bars, electrical application parts, gas burner tips, welding cups and atomising nozzles. Wonderstone powder is used as a filler in train brakes.
When heat treating the material certain precautions need to be observed:
- Sections need to be less than 13 mm otherwise thermal stresses could cause cracking.
- The heating rate should not be greater than 50 ºC per hour till 400 ºC then 100 ºC per hour to a maximum of 1300 ºC then held for an hour at the final temperature.
- When fired to a range 800 ºC – 1000 ºC expansion of 1.7 – 2% takes place and over this temperature a gradual shrinkage to within 101% of the machined size as the material densifies.
As you have no doubt arrived at the conclusion that Pyrophyllite only just creeps in to the category of Technical Ceramics ! due to its natural formation and flaws within the material.
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