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National Standard 325 Bentonite

Alternate Names: SELECTCHEM 325

Description: Wyoming Sodium Bentonite

Oxide Analysis Formula
SiO2 65.00% 9.01
Fe2O3 1.80% 0.09
Al2O3 20.90% 1.71
MgO 2.50% 0.52
Na2O 2.40% 0.32
CaO 0.90% 0.13
K2O 0.30% 0.03
TiO2 0.15% 0.02
LOI 5.00%n/a
Oxide Weight 783.10
Formula Weight 824.32


A finely ground Wyoming bentonite used in industrial coatings, paper, ceramics and household products.

They quote trace amounts in the chemistry for Cr (0.02), Mn (0.06) and V (0.03).
They quote a quartz content between 2 and 5%, illite 2-4%, smectite 85-91%.

We have quoted one of two analyses received from them (this one is closer to toalling 100).
We have estimated the LOI since that was not provided.

Related Information

National Standard Bentonite fired to 1650F in a crucible

The powder was simply put into this cast crucible and fired. Sintering is just beginning.

Two bentonites that should not look this similar

The powders of HPM-20 bentonite (left) and National Standard 325 bentonite (right) fired to cone 6. Both have sintered into a solid mass. The HPM-20 is much more expensive because of the extra grinding done to make it micro-fine (for non-ceramic uses). However, its data sheet shows an Fe2O3 content double that of the National Standard material. That means the latter should be firing to alot lighter color. But they seem very similar.

HPM-20 and National Standard bentonites fired to cone 9

The powders of HPM-20 bentonite (left) and National Standard 325 bentonite (right) were fired in cast crucibles to cone 9. Both have sintered into a solid mass and have shrunk as a unit away from the walls of the crucible. Considerable shrinkage has occurred since cone 6.

Soluble salts as a white powder on a commercial bentonite

A dry lump of bentonite with solubles as a powder

Bentonite is a super-plastic clay. This block of it took months to dry, the material really holds on to its water! It shrunk to about half the size and, of course, broke up into many pieces in the process (because bentonite has such a high drying shrinkage). That white powder is calcium sulphate, it is soluble and comes to the surface with the water as the clay dries. The finer the manufacturer grinds the material, the more salts are liberated. In most ceramic applications for commercial raw bentonites, these soluble salts are not an issue (but the iron content certainly can be). The reason these salts can be tolerated is that bentonite is normally employed in bodies and glazes in the 1-5% range.


Typecodes Clay Other
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Materials HPM-20 Volclay Bentonite


% Passing 325 Mesh Wet 95%
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