|Monthly Tech-Tip |
In ceramics some clays of are of such exceedingly small particle sizes that they can stay in suspension in water indefinitely. But unlike common colloids, clays have a secret weapon.
Colloidal particles are so small and light that they do not settle in water. Milk is colloidal. In true colloidal suspensions the movement of water molecules is enough to keep them from settling. Bentonite contains colloidal particles, but it also contains larger ones which also stay in suspension. Why? Because clay particles are flat and they are electrolytically charged (having opposite charges on the faces and edges). This phenomenon encourages a house-of-cards orientation in the water. Because of this, clay particles that are larger-than-colloidal can behave in a colloidal way. This phenomenon is so effective that the system can be loaded with a majority of non-colloidal, non charged particles within the structure and they can also be held in suspension. In the case of ceramic glazes, electrolytes can be added to the system to enhance this phenomena.
Materials can be ground to nano-sized colloidal particles in a ball mill, for example, and exhibit colloidal behavior purely as a product of their size. A liquid can be tested to demonstrate if it is a solution or colloidal by shining a beam of light through it. Its degree of visibility is an indicator (although low specific gravity colloids can be fairly clear).
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with that same material ball milled for an hour (right). The 10 mesh designation is a little misleading, those are agglomerates. When it is put into water many of those particles break down releasing the ultimates and it does suspend fairly well. But after 24 hours, not only has it settled completely from the upper section but there is a heavy sediment on the bottom. But with the milled material it has only settled slightly and there is no sediment on the bottom. Clearly, using an industrial attrition ball mill this material could be made completely colloidal.
This is VeeGum T, a processed smectite clay (similar to bentonite, extremely small particle size). I have propeller-mixed enough powder into water that it has begun to gel. How long does it take for them to begin to settle? Never. This sat for a month with no visible change! That means it is colloidal.
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.