"Derek Broughton" wrote in message
...
angela.copley wrote:

I am considering bentomat but cannot locate a uk retailer. Does anyone
have this information?

Bentonite is just a fine clay. Quite likely with a different name on your
side of the pond :-) I'd talk to landscapers.
--
derek

From a potter's point of view - different clays are not the same fish....

from digitalfire.com

Bentonite is the most plastic, impermeable, and fine grained common clay
material used in ceramics.

Raw bentonite is generally a pale green, buff, cream, or grey material
composed of the clay mineral montmorillonite. Its origin can be traced to
ancient volcanic eruptions where fine volcanic ash particles were carried by
winds and deposited in discrete layers which altered over time from the
glassy state to claystone.

There is a huge variation in the chemistries of bentonites, it is impossible
to specify an average. The analysis shown only attempts to represent the
amounts you might find in a common variety.

In North America, bentonites are mined in Montana, Wyoming, South Dakota and
Saskatchewan and used in things like porcelain, toothpaste, tablets,
cosmetics, oil drilling mud, oven cleaners, insecticides, putty, paint, ink,
paper, polishes, cleansing agents, explosives, detergents, plastics and
rubber.

Fine particle size: Bentonite is colloidal and typically 10 times finer than
ball clay. It can have a surface area of almost 1000 square meters per gram
(50 times that of kaolin, 5000 times that of silica flour).

Plasticity: Because of their active electrolytic behavior and fine particle
size, bentonites exhibit extremely high plasticity (and associated high
shrinkage). In pottery and porcelain clay bodies additions of only 2% can
produce marked improvements in workability and dry strength.

Bentonite is far too plastic to prepare test specimens (e.g. for drying,
strength and shrinkage evaluation). However, a mix of 10-20% virgin material
with 80-90% calcined can be extruded and formed (test specimens will still
shrink to a very small size).

Thixotropy: Bentonite is valuable in preventing suspensions from settling
over time because it tends to gel. Thus, it is common to see bentonite in
glaze recipes. While typical industrial thixotropic agents employ various
mechanisms bentonite works by charge attraction. Charges develop on the
surfaces and edges of dispersed particles and give rise to a stable
'house-of-cards' structure that can be disrupted by shear stress. However
when the stress is removed, the structure reestablishes itself.

Swelling: Most bentonites expand (as much as 15 times) when added to water.
This characteristic is valuable in thickening liquids and slurries and is
another contributing factor to maintaining suspensions. Bentonite is used in
large quantities in the gas and oil drilling industries to suspend high
specific gravity slurries which are used as a medium to float out the chunks
of rock cut by the drill bit.

Chemically inert, Inorganic, Non-irritating: Formulations which are not
fired are not altered chemically by bentonite additions. Bentonite does not
support organic growth. Thus it is suitable as a carrier for personal care
products like hand cream and cosmetics.

Binder: Bentonite binds particles together in ceramic bodies to make them
stronger in the green or dry state. Its minute particles fill voids between
others to produce a more dense mass. Adding bentonite to glazes imparts
better dry strength and a harder and more durable surface.

Suspender: Maximum suspending benefit can be achieved by blunging bentonite
with the water before adding the other dry materials (to insure that every
particle is whetted on all sides). However, this can be difficult even with
a propeller mixer. If necessary dry mix the bentonite with the silica or
feldspar then blunge thoroughly (for hours if possible).

Firing: Standard grades typically vitrify (around Orton cone 6-10) to grey
to deep red coloration. However soluble salts can be so high that they form
a glaze on pure test specimens. Utility grades often contain granular iron
material that causes specking in clay bodies. Still a white body can often
tolerate a few percent bentonite without firing significantly darker,
although it is important to use a finely ground ceramic grade to avoid
specking (finer than 200 mesh).

There are a number of white firing and highly refined bentonites produced
for the ceramic industry. However they tend to have much less plasticity and
are many times more expensive.