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Volclay Bentonite

Alternate Names: Volclay 325 Bentonite, Dioctahedral smectite

Oxide Analysis Formula
CaO 0.65% 0.10
Na2O 2.57% 0.35
MgO 2.67% 0.56
FeO 0.35% 0.04
Fe2O3 3.25% 0.17
Al2O3 21.08% 1.73
SiO2 63.02% 8.79
Oxide Weight 784.55
Formula Weight 831.44


A micronized sodium bentonite.
The 325 mesh version of this is most suitable for porcelains and a possible substitute for the more expensive HPM-20 Volclay.

Dioctahedral smectite, an expanding layer silicate.
Gray and cream-colored clays (montmorillonites) for use as suspension agents, viscosifiers, gellants and binders.
Has lower iron content that HPM-20 bentonite.
However while they company refers to their 325 mesh product, the data sheet says that 90% passes 325 mesh?

Related Information

Could these bentonite particles cause specking in a porcelain?

The stated particle size of a material and fired appearance can both be misleading. For example, these are Volclay 325 bentonite particles fired to cone 8 oxidation. They are from a water washed sieve analysis test, the oversize particles from a 325 mesh screen (left) make up 2% of the total and 1% are from the 200 mesh screen (right). Although the 325 particles appear ominously dark, individually they are likely to small to produce apparent fired specks in a porcelain. However 200 mesh sizes can produce visible fired specks, but that fraction of oversize does not have nearly as high iron or flux content. Still, the finer darker particles could agglomerate, it might be better to use a cleaner bentonite to plasticize a porcelain.

Two plasticizers, two results

A comparison of the plasticity of Volclay 325 Bentonite:Silica 25:75 (top) and Hectalite 200:Silica 50:50. Both are mixed with silica powder. The latter (a highly refined bentonite) is much less plastic even though it is double the percentage in the recipe.


Materials Hectalite 200
Materials Hectabrite
Materials HPM-20 Volclay Bentonite
Materials Fuller's Earth
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.
Volclay 325 Data Sheet


% Passing 325 Mesh Wet90%
Apparent Viscosity (cps)12 cps minimum @ 6.25% solids
pH for dry powder8.5-10.5 @ 5% solids
% Passing 200 Mesh Wet97%

By Tony Hansen

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