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Pyrophyllite

A Soft Hydrous Aluminum Silicate

Formula: Al2O3.4SiO2.H2O
Alternate Names: Pyrophillite

OxideAnalysisFormula
Al2O328.30%1.000
SiO267.70%4.060
LOI4.00
Oxide Weight346.01
Formula Weight360.42
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.

Pyrophyllite is a low expansion mineral. The individual particles expand less on heating (and therefore contract less on cooling) than other refractory particles (like silica). Thus, during firing, pyrophyllite particles counter the shrinkage of the plastic components surrounding them (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. Pyrophyllite has a second expansion-lowering mechanism: Its presence in stoneware and porcelain bodies encourages the development of more mullite, this both increases firing strength and reduces thermal expansion. In this case, the individual particles of pyrophyllite react and transform from their original state.

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). However, this will of course mean that glazes may need to be adjusted (or they will craze). Often times, glaze fit takes priority, and it is actually better to leave the silica content at 20-25% (silica increases thermal expansion and improves glaze fit).

When substituted for feldspar pyrophyllite will decrease body maturity, firing shrinkage and thermal expansion. Strength may also be increased (even though the body is less vitreous).

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


By Tony Hansen

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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