Pyrophyllite
A Soft Hydrous Aluminum Silicate
Formula: Al2O3.4SiO2.H2O
Because it expands when heated pyrophyllite counters the shrinkage of the plastic components in a mix (assuming that particles remain whole and undissolved to exert their influence on the surrounding matrix). Pyrophyllite-refractory clay combinations potentially have excellent volume stability and resistance to deformation at high temperatures.
In clay bodies, substitution of part or all of the silica will decrease thermal expansion (see also kyanite, mullite) while maintaining the level of maturity (although some adjustment in the feldspar will likely be needed). Pyrophyllite normally increases firing range and therefore decreases firing shrinkage. Thus if a body is too vitreous pyrophyllite can be substituted for feldspar to reduce expansion and maturity at the same time. It has the additional effect of increasing the strength of fired bodies if particles remain intact.
In electrical porcelain, large amounts (up to 95%) may be used in bodies for optimal electrical properties and high frequency characteristics (where zero porosity is not critical).
Pyrophyllite is also used as an ingredient in wall tile bodies since it has the effect of reducing moisture expansion.
Because of its stable nature and physical properties, pyrophyllite is employed in mold release and parting agents in foundry and structural clay products.
Pyrophyllite is ideal for certain types of refractories because no calcining is needed. Its physical nature also helps to reduce wear and tear on molds and machinery that handle wet and dry product.
Some grades of pyrophyllite have a higher iron content and can darken the color of porcelain bodies. Also, porcelains that are highly vitreous can take pyrophyllite particles into solution, when this happens the effect of their low expansion properties is lost on the body.
Mechanisms
Out Bound Links
In Bound Links
XML for Import into INSIGHT
<?xml version="1.0" encoding="UTF-8"?>
<material name="Pyrophyllite" descrip="A Soft Hydrous Aluminum Silicate" searchkey="Pyrophillite" loi="0.00" casnumber="12269-78-2">
<oxides>
<oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="28.300" tolerance=""/>
<oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="67.700" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="LOI" name="Loss on Ignition" percent="4.000" tolerance=""/>
</volatiles>
</material> | 
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