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Taming wild clay

 

It sounds a bit dramatic, but ‘wild’ clay seems the best term we have to distinguish clay in the form it is found in the ground from 'manufactured' clays that are blended from multiple sources, and processed, to produce clay with particular working, firing and colour characteristics for different types of ceramic work. In practice, of course, this is a rather simplified distinction: almost certainly early potters would, if necessary, have mixed clays from different local sources to give the material better working or firing characteristics ¹, and modern manufactured (should that be ’tame’?) clays are usually based on one or two particular naturally occurring clay types, with other materials blended in — albeit prepared on a more industrial, and less local, scale. Nonetheless, pre-industrialisation, transporting and blending even local clays involved considerable work and using a single local clay, either through necessity or preference, would have been the norm ²; in other words, most clay was 'wild'. The pottery work at Lossenham is built on the idea of exploring and experimenting with the clays occurring on the farm, both from historical research and contemporary art perspectives. The first step in doing either of these, however, is to consider what preparatory processing might be needed to make them usable. Here I want to outline the sort of approach to clay preparation we have found effective and, from the historical perspective, which might correspond to the methods a medieval potter could have used.  

 

Most naturally occurring clays will contain other, non-clay, materials, such as sand, other grits, larger stones, and fragments of organic material such as plant roots. The ideal would be to find a clay deposit that is relatively free of such inclusions and which can, therefore, be taken directly from the ground and mixed, wedged and kneaded to a working consistency without further preparation. Such deposits are not common but, remarkably, we do have some clay where this is more or less the case: the inclusions are few and generally either large enough to be picked out in the kneading process, or small enough not to cause a problem when throwing. Of course, some do get missed, and then appear when forming a pot, usually at an inconvenient stage, or place! However, if they are not too frequent I have found it is usually possible to dig out the unwanted inclusion and fill the cavity with clay. It can leave some unevenness in the clay, which makes throwing a bit tricker, but often the pot can be saved ³.

 

A stone emerging in the rim of a pot during throwing

 

The same problem beset medieval potters. We know from examples of wasters ⁴ and our own experience (image below) that larger inclusions such as stones, if missed in the kneading or throwing processes can result in damage to a pot during firing. It might seem inefficient to run this risk, but the occasional loss of a pot has to be set against the time saved by minimising the preparation of the clay.

The consequence of firing a cooking pot with a stone in the clay

 

For many clays, however, we have too many inclusions and impurities to either risk them causing damage in the kiln or try to throw a pot with them in the first place. If we were hand building (using coiling or some sort of slab building approach) then the making stage would be less problematic, but throwing is particularly sensitive to unevenness in the clay, especially where pot walls are being thrown as finely as they were in many medieval pots. With care it is possible to throw clay with some inclusions in, but it is all too easy for a stone to tear out of the clay wall, or create unevenness in the clay which is hard to recover from. For such reasons, with many of the clays available to the medieval potter (and us) some sort of preparatory processing of the clay would have been required.

 

As far as we know, medieval potters would have had no mechanical aids for clay preparation. If a draught animal were available, it might have been used to crush dry clay by treading it, or using a simple rotary mill ⁵. However, for the most part, they would have had to rely on using the power of the sun, of water, and of gravity, to do as much of the work as possible.

 

There are two approaches to turning freshly dug clay into a suitable material for making pots: dry processing and wet processing. The first involves letting the clay dry in the sun, then crushing it as finely as possible, sieving the crushed clay to remove any larger material, then mixing the fine clay powder with water to produce a workable plastic clay. This method is probably most effective in warmer and more arid environments, where the clay is already quite dry when dug, dries quickly in the sun, and where sparing use of water might be important. It has the advantage of being quicker than wet processing, but requires more manual work (crushing, sieving and mixing) ⁶.

 

An alternative approach, and the one we have adopted, is wet processing. The first stage is the same: we take about six sacks of freshly dug clay, and spread it on a tarpaulin (outside when the weather is dry, under cover if not), occasionally turning it, and let the clay lumps dry out. If the lumps are large we break up the larger pieces as they start to dry out.

 

Freshly dug clay drying outside

 

Once dry, the clay is added to a large tank with enough water to more than submerge all the clay. The clay is left to slake (the rapid absorption of water by the dry clay causes it to break up) for several days, with daily stirring to help break up any lumps and ensure the slurry doesn’t settle into a solid mass in the bottom of the tank. Sometimes addition of further water is necessary. If the slurry is too thick and the specific gravity too high, then some of the materials we want to settle out only do so very slowly, if at all.

 

Dried clay being added to the slaking tank

 

 

Water is added to about twice the depth of the dry clay

 

Clay slurry is mixed regularly over 3-4 days

 

(Apologies for the clay colour changes in the images ... the photos come from the processing of various different clays!)

 

Once the bulk of the clay is in slurry form and about the consistency of single cream or thinner, it is thoroughly mixed and left to stand for half an hour or so (the time really being dependent on how well the stones and other materials seem to be settling out). At that point we siphon the fine part of the slurry from the upper part of the tank into a settling tank — a large shallow, cloth lined ‘pond’, to form a layer 3-4 inches deep. Once the upper part of the slurry has been removed it is sometimes necessary to add more water to dilute the thicker material remaining in the tank, and repeat the mixing-settling-siphoning  process. Eventually we should have removed most of the clay and be left with a couple of inches of water, grit and stones in the bottom of the tank.

 

Clay slurry being siphoned into the settling tank

 

Once in the settling tank, the clay settles and is left to dewater

 

Stones and grit remain in the bottom of the slaking tank

 

Once the liquid clay is in the settling pond it is left to ‘dewater’ by evaporation from the surface and water soaking into the ground  underneath until it reaches a state where it is still soft, but solid enough to be cut into squares with a wooden blade and peeled away cleanly from the cloth at the bottom of the settling pond. We have experimented with mixing the clay periodically during this dewatering, or just leaving it to its own devices. Both work, but mixing does seem to speed the dewatering process somewhat and, when the weather is very warm, it helps even out the drying and prevent a hard crust of drier clay forming on the surface.

 

The time taken to dewater the clay varies considerably depending on the clay, the amount of water, and most particularly on the weather. Seven to ten days is typical in the summer, but in a wet cool autumn it can be several weeks. A cover for the settling tank is pretty much a necessity: one good hard downpour can undo many days of drying!

 

Clay almost ready to remove from the settling tank

 

Clay gathered into lumps drying a little more before removal

 

The clay is then either heaped up and covered for later preparation, or wedged and kneaded and bagged up ready to use. Depending on the amount of material discarded at the bottom of the slaking tank, we find that our initial six sacks of clay ⁷ generally make about 60-70 kg of prepared plastic clay.

The processed clay heaped up for covering and later use

 

Whilst there is a fair amount of physical work in even this process, the real work is done by the sun’s heat (drying the clay, dewatering the slurry), water (penetrating the dry clay and breaking it up) and gravity (separating the larger and heavier materials from the fine clay particles). And while we have not tried to replicate the exact equipment medieval potters might have used, there is no stage of the process which could not have been achieved using what was readily available to them (beaten earth surfaces for drying, wooden barrels or pits for slaking, shallow pits or wood/stone edged shallow pools for dewatering).

Although no records of the clay preparation in the medieval period have been found, by the seventeenth century a process essentially the same as the one described above (known as the ‘sun pan’ or ‘sun kiln’) is recorded as being used in the Staffordshire potteries ⁸. One additional element which is mentioned in that process is the use of a cloth sieve, through which the slurry was passed when being drained into the settling tank. We have also sometimes used a sieve in the same way, but we have found this is mainly useful where there are quantities of organic material (mainly plant fragments) in the clay; being less dense, such material does not settle out of the slurry like sand or stones and a sieve can be a useful way of removing the at least the larger parts of this.

Coarse sieve being used to remove larger plant fragments as the clay goes into the settling tank

 

And, on the subject of sieves, one question often raised is why not just mix the clay to a slurry and then pass it straight through a fine sieve — the method often suggested in contemporary instructions for preparing wild clay? With modern wire or plastic sieves and small quantities of clay, this does work perfectly well, and we use it for preparing smaller amounts (a bucket full, say) of clay for testing. But for the medieval (and indeed the present day) potter the main issue with sieving the slurry is about quantity. Clay slurry is not easy to sieve through a fine mesh. Its remarkable capacity for cohesion means that the sieve clogs very rapidly, even though individual clay particles are vastly smaller than the mesh size. The material has to be worked through the sieve with a stiff brush, or similar, and this can be a very slow process. Actively working clay through a sieve is also a problem if the sieve is made of cloth, or plant fibres, which would rapidly abrade. It seems unlikely that the medieval potter would have had access to wire sieves for this purpose, though by the 16th century cloth sieves are recorded as being used for straining impurities out of clay ⁹.

 Sieving works best if a lot of water is used, but this obviously then adds to the dewatering time.

 

 

The same problem does not occur with dry clay preparation, where the crushed dry clay can be sieved out by shaking the sieve to separate out the fine clay particles, with the larger material being kept to be crushed and sieved again. It also makes sense when dry processing to make smaller quantities, as the powdered clay can be mixed with water and used immediately, rather than having the variable time of the dewatering period ¹⁰.

 

Did medieval potters use dry clay preparation? The evidence of wet clay preparation in the English country potteries of the seventeenth century suggests not ¹¹. One possible reason is the weather. Dry preparation relies rather on having consistently warm and dry working conditions, including outdoor space, something that a potter or tile maker in medieval Britain might not have been able to rely on!

 

From our preparation process we generally get a reasonably clean, and fairly homogenous batch of clay suitable for throwing. Depending on the length of time used for the final settling process before siphoning off the slurry, and the thickness (and hence specific gravity) of the slurry, the clay will still contain the finer grits or sands that may have been present in the original material. On the whole this is probably a benefit rather than a problem, giving the clay a bit more strength and openness, and we generally haven’t found it necessary to add further temper, in the form of grog or sand, to the clay body.

 

Kneaded clay ready for throwing

 

It seems quite likely that the medieval potters, in whose muddy footprints we are treading, used a broadly similar approach to preparing clay for throwing, which makes greater demands on the purity of the clay than does handbuilding or tile making. Certainly in the case of tile making — as discussed elsewhere — minimal preparation would probably have been done; perhaps none at all beyond leaving the clay out to weather, and roughly mixing it. The example of Salamancan roof tile makers, preparing two tonnes of clay at a time by roughly breaking up the dry clay, wetting it and using a horse to tread it to a usable consistency ¹², suggests a scene not hard to imagine on a monastery construction site 800 years ago.

 

Notes

1. e.g. Duller J, 2015. “A bizarre and noble craft…” A village pottery in Andalucia. Documentary video https://www.youtube.com/watch?v=6hUs5EHxsCY 4:48; Monesma, E, 2020a. The master potter. Collection of earth and firing of pieces in a wood-fired oven. Documentary video. https://www.youtube.com/watch?v=q3O95AqxWLc; Monesma, E., 2020b. Tiles, bricks and tiles with water and earth. Documentary video: https://www.youtube.com/watch?v=_iUEdp9akek : 1:25

2. Brears, P., 1971. The English country pottery: its history and techniques. David and Charles, Newton Abbot.

3. Monesma, E., dir. 2020b. (as 1 above) https://www.youtube.com/watch?v=_iUEdp9akek. (5:48)

4. ‘Wasters’ is the term used by archaeologists, when excavating a kiln site, for the remains of pots or tiles which failed at the firing stage and were discarded (though the potters would generally have made use of the wasters for making grog, building into kiln walls, and covering the top of the load in a kiln when firing.

5. e.g. Monesma, E., 2025. dir. Earth ground by donkeys “blindly”. Transformation of clay (dust and water) into ceramic pieces. Documentary video: https://www.youtube.com/watch?v=ruJ40QKBUas (5:48)

6. In addition, preparing and handling dry clay in powder form gives rise to a great deal of dust, with the risk of serious health consequences (silicosis); not perhaps something that would have been such a consideration in a medieval pottery.

7. We tend to do the the initial clay collection by approximate volume as the weight is very variable depending how wet the clay is when dug.

8. Brears, 1971. (as 2 above) p89; Brears also notes the documentation of a different process from about the same time, in which the clay is soaked, then beaten into square lumps and these are cut with a wire into thin slices, from which the stones can be picked.

9. Piccolpasso, Cipriano. The three books of the potter’s art / I tre libri dell'arte del vasaio. Translation, Introduction & Notes by Lightbown, R. and Caiger-Smith, A. 1980. Scolar Press, London.

10. Monesma, E., dir. 2026. Kneading Clay with the Feet: Primitive Pottery. An Ancestral Technique without Machines. 2026. https://www.youtube.com/watch?v=5Tll-ZbKZP0 (3:20); Lakeside Pottery Studio, dir. 2012. The Last Iranian Woman Potter Using the Ancient Technique. 30:25. https://www.youtube.com/watch?v=ueR4iwoj2FE (time 0:16); Monesma, E., dir. 2025. (as 5 above).

11. Brears, P, 1971. (as 2 above)

12. Monesma, E., dir. 2020. (as 3 above) (2:20-5:25).