What exactly is clay?

So you’ve heard that some minerals like kaolin and bentonite are called clays while others are not clays. What exactly does this mean? What makes one mineral a clay and another not?

It all started when folks tried to explain the components of the soil. They found some soils held water well but others did not – the water drained away quickly. The minerals that retained the water were then called clays.

Early man quickly discovered that soils high in clay could be formed into shapes, so they started making pots out of this soil. These pots hardened when baked in an oven. This was the birth of CERAMICS!

These days we have a much more scientific definition of clay which goes something like this:
“Clay is a term used to describe a group of hydrous aluminum phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals, that are typically less than 2 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminum oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and tendency toward high plasticity”

This narrows it down quite nicely. It makes it easy for us to decide which mineral is clay and which is not clay. It must conform to the following five points:

Clay is always an aluminium silicate

(You can see the above was written by an American, they never could spell aluminium right). So a mineral like talc is not a clay because it is magnesium silicate. However the clays kaolin, pyrophyllite, bentonite and ball clay are aluminium silicates.

Clay is a phyllosilicate

This describes the platy, layered structure of the mineral. Under the microscope it looks a bit like phyllo pastry.

Clay holds water

The water is generally held between the platelets or layers of the mineral. This makes it slow-drying as it reluctantly releases its retained water.

Clay has a small particle size

It always surprised me that our kaolin occurred naturally with an average particle size of 2 microns. No milling was required, only a bit of classification (known as water-washing) to remove the larger silica and mica impurities. Now I realise this is typical for a clay.

Clay is plastic

This is the reason that all ceramic articles are made of clay – it can hold a shape because it is ‘plastic’. It can be formed and jigged, pressed and moulded into cups and plates, tiles and toilet bowls. What a useful thing a clay is!

But clay articles can break easily, hence the term “feet of clay”. This saying comes from the Bible and describes an idol with a hidden weakness or fatal flaw.

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