Formula: Al2Si2O5 (OH)4
Alternate Names: Plastic Vitrox, P.V. Clay, P. V. Clay, Plas Vitrox
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|PCE - Pyrometric Cone Equivalent||17|
P. V. Clay is a feldspathic semi-abrasive mineral consisting of clay, mica, feldspar and silica (Moh hardness of 5). It is a product of the hydrothermal alteration of intrusive rhyolite and is mined from a substantial deposit in the Mojave Desert of California.
PV Clay has a low loss on ignition (approx 3%), high silica content, good casting properties, low iron content and a long firing range. When fired by itself it produces a pinkish color as it begins to vitrify around cone 6 and then progresses to a very glassy white by cone 10 (so bodies employing it can be very white burning). The relatively high potash and soda content give PV Clay a lower PCE than any other commonly available white burning plastic material. The particle size is significant (the finer the grade, the better vitrification achieved). The material is also plastic (not as plastic as a typical pottery clay, but it is formable). Thus, in a ceramic casting slip or plastic clay body, PV Clay has the unique capability of promoting plasticity from the clay portion, vitrification from the feldspar portion, fired stability and glaze frit from the quartz portion and whiteness from the low iron content.
This makes it useful in a very wide range of clay body types, both vitreous and non-vitreous. It is viewed by some as "nature's pure porcelain".
-It is very useful in artware casting (in high talc recipes). It imparts resistance to glaze crazing (because of its high silica content), resistance to dunting (because the silica is fine grained), white color and good casting properties. The popular "California Artware Body" recipe is 3 parts California Talc, 1 part PV Clay and 2 parts ball clay.
-PV is a very practical material for use wall tile bodies (because of the whiteness, plasticity and high silica content).
-It is practical for floor tile bodies (because it vitrifies the body when fired higher).
-It is possible to make a stoneware body of only ball clay and PV (it supplies the feldspar, silica and white burning kaolin) that is very plastic, fires white and matures below cone 10.
The flux content, low iron and plasticity of PV Clay made it a practical glaze material also. A simple 50:50 mix of PV Clay and Gerstley Borate was used widely as a transparent glaze at cone 5-7. A mix of 80:20 PV and calcium carbonate produces a cone 10 transparent glaze.
P.V. Clay is also used as a filler in rubber products, as a mild abrasive in polishes and cleansers, and as an extender for coatings, compounds, and other industrial products.
The manufacturer, Protech Minerals, claims that the material is maintained to constant uniformity chemically and physically by careful mining, close particle size control and cleanliness in milling and rigid laboratory control.
Particle Size Distribution
(typical Sedigraph Method)
40 91.0 (below 40 microns)
Specific Gravity 2.65
Dry Brightness 82
Fired Brightness 80
Tapped Density 66
Unfired Physical Properties:
Rate of Cast, Thickness 15 min
(specific gravity 1.5) 1.21 cm 1.27 cm 1.83 cm
Percent water of plasticity: 28 29 32
Percent dry shrinkage 4.2 4.4 4.8
Cone 03 Cone 5 Cone 8
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LOI 1.2 2.4 2.9 1.6 2.7 3.0 2.2 2.7 3.3
Absorption: 19.5 18.8 20.6 11.7 6.4 .6 6.0 .2 .1
Fire Shr: .5 1.0 1.0 5.2 8.1 10.7 8.0 9.9 10.1
MOR: 1051 1202 1807 3357 4482 6717 4812 6257 6900
Color light buff Brown Gray lt med med
grey gray gray
PCE Cone 18
Melting Point: 2700F
At one time, PV Clay was made by a wet milling process and was known to the trade as "Plastic Vitrox" from whence it derives its name.
Some quotations on the chemistry show more alumina and silica than is show here. For example: 1RO 1.69Al2O3 14.64SiO2.
PV Clay is an unusual material, having the flux of a feldspar and the plasticity of a clay along with enough quartz to make it a casting porcelain all by itself.
PV Clay normally this fires very vitreous. It has had some variation in the degree of vitrification.
Out Bound Links
Overview of quartz hazards in the ceramic industry and process
In Bound Links
PV Clay, Plas Vitrox, P.V. Clay, P. V. Clay