| Alteration of raw material properties
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| There are a number of mechanisms which act to enhance the control a stoneworker has over the fracture process. The most dramatic mechanism is controlled heat treatment of rock. This treatment involved gradually heating rock to a temperature of about 250 to 350 degrees Celsius, holding it at that temperature for 6-8 hours, and then gradually cooling the rock. The entire process can take 10 hours or more, and if successfully completed it dehydrates the rock and creates microscopic flaws that facilitate fracturing. The result is that the rock becomes effectively more homogeneous, and fracture planes pass through features (such as quartz grains) that they may previously have deviated around. Stoneworkers have employed this process for thousands of years in many parts of the world, including Australia. An excellent introduction to heat treatment in the Australian context is provided by the paper of Flenniken and White cited below.
Change in the fracturing quality of the material caused by controlled heat treatment is often reflected in the production of glossy surfaces on rocks that have been heat treated. Examination of the surface of fractures under a microscope show the dramatic difference heat treating can make. For example:
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| UNTREATED. To the right is a microscopic view of the flaked surface of a piece of silcrete (ie. grainy silicified rock). This rock fractures well and was used by prehistoric people to make artefacts. However the surface texture is visibly rough under magnification. |  |
| | TREATED. To the right is a view of the same piece of rock, at the same magnification, after controlled heat treatment. The surface texture is visibly smoother because the fracture has passed through rather than around the structural features within the rock. |  |
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| Heat treatment is most distinctive when looking at cores or retouched flakes that have been heat treated mid-way through their reduction. On such specimens there is often a number of scars, with dull and rough surfaces, that were created by the removal of flakes before the object was heat treated. Other scars, with lustrous and smooth surfaces, were created by the removal of flakes after the object was heat treated. When this happens the smooth scars indicating altered raw material properties are always found to be superimposed over the top of the rough surfaces that indicate the original fracture quality of the rock. Two examples are shown below. In these thumbnail images the scars that were made after heat treatment are the lustrous ones at the top of the photographs (marked with a yellow square to identify them).
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For additional information see:
Flenniken, J.J. and J.P.White 1983 Heat treatment of siliceous rocks and its implications for Australian prehistory, Australian Aboriginal Studies 1983/1:43-48.
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Author: Peter Hiscock, Dept. Archaeology and Anthropology, Australian National University.
Feedback:  peter.hiscock@anu.edu.au.
Date Last Modified: Thursday, 1-May-97
URL: http://artalpha.anu.edu.au/web/arc/resources/intro/heat.htm
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