Page 15 - Curriculum Visions Dynamic Book

The great mass of surface clays are found naturally as beds of particles that settled out in ancient lakes or shallow seas. These clays were produced as rainwater decomposed rocks on land, and the clays were then carried by rivers until they reached the still waters of lakes and seas, where they settled out. As these rocks were buried, they were compressed.
Very compressed clays are called shales, but they
are not useful in the ceramics industry. What that p industry needs are clays that can still be moulded.
The important feature of clay particles is that when lubricated by water, they are able to slip over one another yet still hold together. The water does not bind them. Instead, they are bound electrically. This happens because as the clays form, the surface of each clay particle become very slightly negatively charged.
In ordinary water there are many positively charged particles. Calcium in the water is a good source of them. It often comes in the form of tiny positively charged particles (cations). Since opposites attract, the calcium is attracted to the negatively charged surfaces of the clays. The calcium has enough charge to allow it to attach to two clay surfaces at the same time, and that is how the clays come to be held together even though they are waterlogged.
The forces holding them together are, however, not very strong. Thus the combination of particle shape and weak bonds means that the clays can easily be moulded, or formed, into new shapes.
The other two main ingredients of common ceramics making are silica (sand) and feldspar. Sand is a filler and provides reinforcing for the clay both when it is wet and once it has been fired and hardened.
Feldspars are similar to clays but also contain sodium, calcium, and potassium ions. When feldspar (as opposed to china clay) is used, it acts as a material called a flux, lowering the temperature at which the clay will sinter.
Ca Clay particle
Ca
(Above) How clays stick together using calcium ions. Calcium has two positive charges, so one charge can link with the surface of one clay, and the other charge can link with the surface of the other. This locks the particles together.
(Above) Two contrasting colours of bricks (red and grey) create an attractive decorative effect on this building.
Ca
Clay article
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