Specific Kaolins

 

Also known as China Clay, Kaolin is typically a primary clay, meaning it is still on the site of the parent rock. There are some secondary kaolins, but even they are typically close to the site of decomposition, and thus relatively clean and non-plastic. Because of its whiteness, kaolin is invaluable in porcelain clay bodies. Its lack of plasticity often requires the addition of clean-burning ball clays or plasticizers such as bentonite to make a workable clay, though. Developing a white, translucent porcelain is a tricky balancing act between color and workability. Kaolin is typically the primary source of alumina in glazes because its whiteness has little effect on color. Some kaolins are also excellent suspenders, keeping the flint, feldspar, and fluxes from settling to the bottom of the bucket. They are often cleaner and more pure than ball clay, and so are a good choice where color control is critical.
Kaolins, being nearly all alumina and silica, are quite refractory. They stand easily at cone 20, making them an excellent material to compound very heat-resistant shapes and coatings, such as cone packs, wadding, and kiln wash.
Because of their relatively large particle size, kaolins can be a valuable addition to stoneware bodies, neatly filling the gap between ball clay and fireclay. Such an addition can make significant improvements in plasticity, workability, and strength. Adding kaolin will typically lighten the fired color of the clay body, so an addition of iron or an iron-bearing clay may be necessary.
 
Edgar Plastic Kaolin (EPK)
EPK is a water washed, secondary kaolin mined near Edgar, Florida. It is among the whitest-firing and most plastic of the North American kaolins, making it a valuable addition to both porcelain bodies and glaze suspensions. In glaze mixtures, EPK provides suspension, gelling, and application characteristics found in no other clay; at 20% of batch, EPK will hang a glaze coat on glass. EPK is also a brings very good casting properties to slip bodies. It is economical and has been consistent over the years.
 
Typical Analysis for EPK (Source: Company web site, revision 1/96)
Chemical Analysis
Si02 45.73%
Al203 37.36%
Fe203 0.79%
TiO2 0.37%
K2O 0.33%
P2O5 0.236%
CaO 0.18%
MgO 0.098%
Na20 0.059%
LOI13.91%
 
Mineral Content (X-Ray Diffraction)
Kaolinite (Al2O3-2SiO2-2H2O) - 97%
Ceramic Properties
Linear Dry Shrinkage: 5.8%
pH (10% Solids-Wt.): 5.8
Viscosity:6000 cps at 35% solids (wt.)
Water of Plasticity:26.0%
Dry Modulus of Rupture:300 psi
Fired Shrinkage Cone 11: 11.6%
Absorption - Cone 11:11.3%
Fired Color (100% clay Cone 11): 95.1
Casting Properties:
Drainage:Excellent
Type of Cast:Plastic
Water Retention: 24.81%
Other
Average Particle size 1.36 microns
Specific Surface Area 24.25 square meters per gram
 
Particle Size Distribution:
Particle Size, Microns 40  10 5  3  1  0.5  0.2
% Cumulative Undersize:100 90 78 68 49 40   20
Sieve Analysis, Water Washed:
Mesh   +60  +80 +100 +200 +325
Percent 0.0 0.0 0.1  0.8  1.5
 
6 Tile Kaolin
 6 Tile Kaolin is a selectively mined, airfloated kaolin produced by Dry Branch Kaolin in Georgia. It is offers high plasticity, white fired color, and high green strength. It is not as white as EPK; its higher titania content tends to make bodies high in Tile #6 rather ivory-colored in oxidation firings, and grey in reduction. The plasticity of 6 Tile has changed somewhat over the past several years, so testing of the current stock is recommended. The supply of truly white kaolin deposits in Georgia is dwindling, so selective mining and blending is employed by DBK.
Typical Analysis for Tile 6 (Source: DBK web site)
 
Typical Chemical Analysis
SiO2 45.5%
Na2O 0.04%
Al2O3 38.1%
MgO 0.5%
Fe2O3 0.3%
CaO 0.4%
TiO2 1.4%
L.O.I. at 950°C13.8%
Typical Physical Properties
Particle Size (%<2 microns)54-65
Median Particle Size (microns)1.0-1.3
325 Mesh Screen Residue (% max.)1.0
pH (20% solids)6.5-7.5
Strength (dry M.O.R.) (psi), 100% Clay900 minimum
Cone 10 Absorption5-8%
Moisture (% Max.)3.0
Color (fired) (Cone 8), 100% Clay
Brightness (% MgO)80.3
Whiteness Index22.9
Color (fired) (Cone 8), 50/50 Clay/Nepheline Syenite
Brightness (% MgO)60.8
Whiteness Index25.9
C.E.C. (meq/100g)8.5 Min.
 
Grolleg Kaolin
A blended English china clay, Grolleg is mined in southern England and distributed by Hammill & Gillespie in North America.  Grolleg is moderately plastic, has low titania content and relatively high flux content, low shrinkage, and white fired color, making it excellent for translucent throwing or casting bodies. Note that it has twice the iron oxide of #6 Tile kaolin but 40 times less titania. This is the primary reason for Grolleg’s translucency. The high flux content also allows one to formulate a porcelain with more clay and less feldspar, making up for the lower plasticity of Grolleg.
 
Properties of Grolleg Kaolin
Typical Chemical Analysis 
CaO 0.10% 
K2O 1.90% 
MgO. 0.30% 
Na2O. 0.10% 
TiO2. 0.03% 
Al2O33 7.00%
SiO2 48.00% 
Fe2O3 0.70% 
LOI12.01%
 
Particle Size Distribution:
%>53 microns .05 max
%>10 microns 10
%<2 microns 57
Modulus of Rupture, Kgf/cm2
At 80% Relative Humidity:10.0
Dried at 110°C:25.7
Casting Rate @ 63% solids (mm2/min): 0.8
% P84 Deflocculant Required for 5 Poise Slip: 0.65
1180°C1280°C1410°C
Fired Absorption:14% 6%0.5%
Fired Shrinkage:9%4%18%
Brightness:868675
 
Kaopaque 20
Kaopaque 20 is a delaminated, water washed, low plasticity kaolin produced by Dry Branch Kaolin, offering very large particle size, exceptional whiteness, and improved drying characteristics. It is a common adjunct to 6 Tile in domestic porcelain bodies in the ratio of 2 parts #6 Tile to 1 part Kaopaque. 
Typical Chemical Analysis
CaO 0.21%
K2O 0.08%
MgO 0.03%
Na2O 0.03%
TiO2 0.66%
Al2O3 39.30%
SiO2 45.20%
Fe2O3 0.32% 
LOI12.00%
 
Velvacast Kaolin
Velvacast kaolin from DBK is a water-washed, specially fractionated coarse particle kaolin developed for casting slips. It’s physical properties are very similar to Kaopaque 20.
Typical Chemical Analysis
SiO245.4%
Al2O338.7%
TiO2 1.4%
Fe2O3 0.3%
MgO 0.2%
Na2O 0.1%
CaO 0.1%
K2Otrace
L.O.I. at 950°C13.8%
 
Typical Physical Analysis
Particle Size (%<2 microns)38-40
Median Particle Size (microns)3.8
325 Mesh Screen Residue (% max.)0.15
 pH (20% solids)4.2-5.2
 Strength (dry M.O.R.) (psi), 50/50 clay/flint50-75
 Cone 10 (M.O.R.) (psi), 50/50 clay/flint>3500
 Cone 10 Absorption (%)20-25
Moisture (%)
Lump5-10
Pulverized1-3
Color (fired) (Cone 8), 100% Clay
Brightness (% MgO)86.5
Whiteness Index15.2
Color (fired) (Cone 8), 50/50 Clay/Nepheline Syenite
Brightness (% MgO)65.5
Whiteness Index15.2
 P.C.E.34-36
 C.E.C. (meq/100g)1.0-1.5

 

 

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