In article , Rusty Mase
wrote:

On Mon, 05 May 2003 18:53:15 GMT, animaux wrote:

I don't know why anyone would calcine it, either.


Interesting question, Victoria, and I do not know the answer, and
besides, you need to get on with getting ready for the film crews.

Rusty Mase




Field Roebuck says it is calcined to get rid of impurities
(http://froebuck.home.texas.net/toppage7.htm). It is then sintered to
fuse the diatomaceous particles to generate particles of more uniform size
suitable for filtration.

The very important distinction however is that in its natural form DE is
amorphous silica, but when heated can form several crystaline forms. The
crystaline forms are definitely dangerous.

As you suggest the danger of silicosis cancer etc. has been documented for
workers in the DE industry. E.g. in a study by Park et al. and published
in Occup Environ Med 2002 Jan;59(1):36-43, "Exposure to crystalline
silica, silicosis, and lung disease other than cancer in diatomaceous
earth industry workers: a quantitative risk assessment." The authors
conclude: "The excess lifetime risk for silicosis at the current PEL was
75/1000. CONCLUSION: Current occupational health standards for crystalline
silica permit risks of lung disease other than cancer far in excess of
what is usually considered acceptable by the Occupational Safety and
Health Administration (a lifetime risk of less than one in a thousand
deaths)." Please note the current PEL (permissible exposure limit is .05
milligram crystabolite per cubic meter). The same authors had earlier
concluded an increased cancer risk of magnitude.

In terms of material that is commercially available I do not know how much
of the material is amorphous (presumed safe) and how much is crystaline
(demonstrably dangerous) in the forms that are sold & I do not necessarily
want to find out.


Roland