PV Clay
Plastic Feldspar
Formula: Al2Si2O5 (OH)4
| PCE - Pyrometric Cone Equivalent |
17 |
It 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. In a ceramic casting slip or plastic clay body, PV Clay has the unique capability of promoting plasticity from the clay portion, fluxing action from the feldspar portion, and low shrinkage and stability from the non-plastic quartz portion. As a plastic material, PV clay is not as plastic as a typical pottery clay, it is quite short, but it is formable.
Its relatively high potash and soda content give P. V. Clay a lower PCE than most kaolins and ball clays (18 vs. 26-35), and thus was widely used in the ceramic industry as a low temperature flux for artware and tile bodies. Its formula is quite similar to Cornwall Stone. It has the unique ability to promote plasticity, fluxing and stability all from the same material.
Its loss on ignition (approx 3% vs 10-15% for kaolins and ball clays) and its high silica content tend to impart low shrinkage, good density, and excellent hardness at low firing temperatures in the range 1900-2100F.
It has an ideal combination of feldspathic silicates and kaolinites that permit low temperature firing in high talc bodies with exceptional resistance to glaze crazing and dunting during cooling.
It promotes whiteness and imparts outstanding casting properties to ceramic casting bodies and floor & wall tile bodies.
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.
It has good casting properties. It casts quickly, does not cut dry strength nearly as much as non-plastic materials, a light color and long firing range and resists crazing very well. PV Clay was used in the "California Artware Body" as follows:
California Talc 3 parts
PV Clay 1 part
Ball Clay Blend 2 parts
Vitreous white burning bodies can be made with high percentages of PV clay mixed with ball clay, silica and bentonite. Actually, when fired by itself it produces a pinkish color as it begins to vitrify around cone 6 and then progresses to a very white by cone 10.
Firing properties are strongly influenced by the particle size (the finer the grade, the better vitrification achieved). P. V. clay can reach vitrification at cone 8, and is even used in glazes with success. A simple 50:50 mix of P. V. 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.
The manufacturer 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. However their data sheet for this material has not changed for more than 20 years and provides little detail. Laguna clay offers a substitute claiming it does not have problems with scumming. The manufacturer does not appear to have a website.
Particle Size Distribution
(typical Sedigraph Method)
40 91.0 (below 40 microns)
30 77.5
20 67.0
15 61.5
10 56.0
7.5 51.0
5 43.0
4 40.0
3 35.0
2 31.0
1 23.5
Mineralogical Analysis:
----------------------
Flint 46%
Feldspar 34%
Kaolinite 24%
Sulphur .03
LOI 2.3%
Specific Gravity 2.65
pH 9.2
Dry Brightness 82
Fired Brightness 80
Apparent Density
(lbs./ft3) 27
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
---------------------- -------------------- --------------------
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.
Out Bound Links
In Bound Links
- (Materials - Substitute)
Plastic Vitrox - White Burning Plastic Feldspar
PV Clay, Plas Vitrox
Pictures
PV Clay fired from cone 7-11 oxidation and 10 reduction.
PV clay fired from cone 8-11 and cone 10R. Normally this fires very vitreous, but at times it is less vitreous.
XML for Import into INSIGHT
<?xml version="1.0" encoding="UTF-8"?>
<material name="PV Clay" descrip="Plastic Feldspar" searchkey="Plastic Vitrox, P.V. Clay" loi="0.00" casnumber="1302-76-7">
<oxides>
<oxide symbol="CaO" name="Calcium Oxide, Calcia" status="" percent="1.000" tolerance=""/>
<oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="0.500" tolerance=""/>
<oxide symbol="K2O" name="Potassium Oxide" status="" percent="5.000" tolerance=""/>
<oxide symbol="Na2O" name="Sodium Oxide, Soda" status="" percent="0.900" tolerance=""/>
<oxide symbol="TiO2" name="Titanium Dioxide, Titania" status="" percent="0.100" tolerance=""/>
<oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="12.700" tolerance=""/>
<oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="76.500" tolerance=""/>
<oxide symbol="Fe2O3" name="Iron Oxide, Ferric Oxide" status="" percent="0.500" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="LOI" name="Loss on Ignition" percent="2.800" tolerance=""/>
</volatiles>
</material> | 
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