Ask A Scientist© General Science Archive

name         Ivor L.
status       educator
age          60s

Question -   Dear Sir or Madam
I write articles for potters and ceramic artists.
I wish to know how plasticity develops. The popular notion is that
crystals of Kaolinite slide over each other lubricated by water.
But there seem to be an anomaly.
Plastic clay can vary from between 70% to 80% mass (weight)of clay with
the balance water. But this represents a volumetric ratio of about 50% to
70% water with the balance clay. So the clay should fall apart and become
totally fluid. It can be encouraged to do this with substances such as
sodium silicate in a process called deflocculation.
I wish to know if the water becomes a solid with an ice structure under
the influence of the electronic forces of the clay crystals above the
freezing point of water, even though this seems to be thermodynamically
untenable.
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The system you describe is well known in the paper industry, where papers
are often coated with clays and other "pigments" to improve their ink
receptivity and other properties, so what you are asking is a known
phenomenon.

When you start adding water to dry clay, the water is absorbed and firmly
bound by the clay particles, that have very polar surfaces. As you are well
aware, "mud balls" result. The water you have added at this point is not
"free" to lubricate flow between particles. Also at this point a lot of
"bridging" between clay particles occurs so that the whole mess is a rock,
more or less.
The dry clay is a powdery sponge, and has a large capacity to absorb and
bind a lot of water as you indicate. The water is not "frozen" in the sense
of ice formation, but it is "immobilized" by absorption onto the clay
particles and cannot provide any lubricity to the mixture. As you add more
water and STIR  the mud a point is reached where the mixture becomes fluid
(this can happen quite abruptly). However, the fluid mixture is unstable and
over time settles out.

Now enters the sodium silicate (water glass). This substance belongs to a
large class of materials called dispersants, which, as you have said,
deflocculates the clay particles. Many dispersing agents are used in the
paper industry; your selection of sodium silicate is probably due to the
fact that the objects will be baked at high temperature and you don't want
any volatiles from decomposition of the dispersant. But FYI, polyphosphate
salts are also used, and they are also non-volatile.

The dispersion process is a complex one. Briefly, here is what happens:

     1. The dispersant preferentially absorbs on the clay displacing water,
which then becomes "free" to provide a fluid medium for the clay particles
to move around in.

     2. The surface of clay contains both positive and negative charges, so
the surface is not so highly charged on a macroscopic scale. In contrast,
the silicate anions are very highly negatively charged, so when they absorb
on the surface of the clay, the particles repel one another (remember, like
charges repel). The sodium ions more or less just roam around. So now the
clay particles are pushing one another apart. This increases the inter
particle distance making it easier for the particles to move past one
another. That is, the "mud" becomes fluid at a much higher concentration of
clay than is possible without the dispersing agent. The dispersing agent
also adds "bulk" to the surface, which prevents the particles from
approaching one another. This is called "steric stabilization" and accounts
for the stabilizing properties of some dispersants that are electrically
neutral, rather than charged, like sodium silicate.

     3. The dispersed clay also becomes more resistant to settling, because
in order to settle the clay particles must start agglomerating -- sticking
together -- the negatively charged surfaces and the physical bulk of the
dispersing agent(s) stops, or at least slows the process down.
Furthermore, if/when settling does occur, it is "soft settling" which is
easily redispersed by stirring, rather than "hard settling" where the
settled clay forms a "brick" in the bottom of the container.

As evidenced by the length of this explanation, you correctly perceive that
pigment dispersion is a topic to which I have given much attention
professionally.

Vince Calder
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