Al2O3.4SiO2.H2O
A very fine-grained plastic clay of volcanic origin containing colloidal matter. Used in small amounts as an additive to clays to aid in plasticity and in glazes as a suspending and binding agent.
Bentonite 325 - A sodium bentonite. The most commonly used bentonite, and is considered a standard glaze and clay additive. It can at times be coarser than 325 mesh, and may cause some specking if used in amounts over 1%.
Typical analysis:
SiO2 58-64%
Al2O3 19-22%
CaO 0.65%
MgO 2.5-3.2%
Fe2O3 2.8-3.5%
FeO 0.2-0.4%
Na2O 1.5-2.7%
K2O 0.2-0.4%
TiO2 0.1-0.2%
Moisture 6-12%
PH 9.0-10.5
Melting point approx. 2192°F
Bentonite can be added to most glazes in amounts up to 1%. Excessive amounts can cause a glaze to become gel like. When used as a plasticizer in clay body, 1-3% is generally used. Bentonite can cause warping and cracking in clays if used in large amounts.
Bentonite should always be added to dry compositions, never to wet. Magnesium bentonites tend to swell, holding large amounts of water. Calcium bentonites are of the non-swelling variety.
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