Tim Willey

Testing Clays : Particle Analysis
It’s very useful, in assessing clays for open-firing to have some idea of particle size and distribution. Probably the best way is too pass the liquified clay through different sizes of sieves and then after 200 mesh (fine) levigate the rest to separate the fine silt from the, much finer, clay minerals.

However I’ve found that a simple settling jar provides a useful ‘rule of thumb’ guide to the particle distribution of a clay and, although it doesn’t help categories each sediment in terms of size, it does give an indication of the 3 main components.

It’s as well to bear in mind that, in the strictest sense, most clays used for open-firing aren’t clays at all, but a complex mixture of sands and silts bound together with (sometimes a surprisingly small) amount of pure clay (clay mineral). In fact the purer the clay body, the less likely it is, to survive open firing.

Below is an annotated sequence of photographs showing the settling sequence.
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The clay ‘as dug’ is broken up and dried. This is, in fact, a glacial till from North Norfolk.
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500gm of dry clay is covered with plenty of water to slake, until broken down to a sludge.
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Most of the water is drained off and the residue is stirred into a slip.
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The slip is passed through a kitchen sieve (approx. 1mm aperture size). The residue (pebbles, gravel and chalk nodules are dried and weighed.
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The sieved slip is syphoned into a 2 litre, clear plastic bottle and water added till nearly full. It is given a good shake.
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The slip is settled for at least an hour, or until separation becomes evident. Here you can see three distinct layers.
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The bottle is carefully pierced (with a hot nail held in a pair of pliers) at a point between the water and the next (clay) sediment. The bottle top is removed and the water is allowed to drain off.
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The bottle is now again carefully pierced (with a hot nail held in a pair of pliers) at point between the the two remaining sediments (clay and silt). Here I have drained it into a bowl. Although I need to know the dry weight of this sediment I can calculate this later without the need to dry it out.
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The final (bottom) silt sediment is retrieved from the bottle. Because this is often a compacted sand/silt sediment, I usually cut it out of the bottle with scissors. This is now dried and weighed. The dry weight of this sediment and the dry weight of the material captured in the first sieving are subtracted from the 500gm original weight. This will give me the dry weight of the clay sediment, so I now have 3 dry weights:
Clay: 140gm
Sandy Silt: 350gm
Pebbles, Gravel, Chalk Nodules: 19gm (to be discarded)
Expressed as an approx, percentage I have:
Clay: 28
Sandy Silt: 70
Pebbles, Gravel, Chalk Nodules: 2 (to be discarded)
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From this very simple analysis, I concluded that this ‘as dug’ clay would be a good starting point for an open-firing body. It’s contained a good range of fine and course particles, it has a low clay content, and when slaked down, slipped and the very course (1mm + ) fraction is removed, it is surprisingly plastic, once de-watered to the right consistency (see front roll-test in photo).

However, to withstand the rigours of a rapid open-firing I still needed to add an equal volume of 1mm sieved beach sand. So: 1 part by volume plastic clay : 1 part by volume beach sand (see rear roll-test in photo). This will be explained in more detail in the section on tempering. Obviously, this mix is not as plastic as the un-sanded clay, but it’s still very workable and that’s what matters. This clay body was then tested in a very rapid open-firing and it survived.

However it proved to be a weak ceramic fabric, and this was entirely due to the nature of the beach sand; a phenomenon which will be explained in the section on tempering.
So, in conclusion, this is a simple the analysis of an ‘as dug’ clay which works, after a little modification, as an open-fired body and it would therefore act as a useful comparison to other, as-dug, clays.
Note: Open-firing is dangerous - I always wear PPE appropriate for hot working.