• Materials for Ceramics

Pyrophyllite

Alternate Names: Pyrophillite

Description: A Soft Hydrous Aluminum Silicate

Notes

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).

In glazes Pyrophyllite is a better source of SiO₂ than quartz (if its iron content can be tolerated). This is because the latter is slow to dissolve into the melt, often much is left undissolved (affecting the transparency). Also, since the SiO₂ become an integral part of the glass, its theoretical expansion-lowering effect is more fully realized.

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. For sanitary ware bodies where the translucency and fired color are not important pyrophyllite can be added to reduce pyroplastic deformation (especially where talc is employed as an auxiliary flux).

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.

Related Information

Does a grog addition improve thermal shock resistance?

Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion) and made slip-cast pieces. I fired them to cone 2 (after finding that by cone 4 shock-resistant properties begin to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. Will this work for kiln shelves? Yes!

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Mechanisms

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