Tim Willey

Tempering, is simply the addition of non-plastic materials to the clay body to adjust its working, firing and practical characteristics.

In open firing, tempering plays a vital part in controlling the clay’s tendency (on initial rapid heating) to blow apart, and similarly, (during rapid cooling) controlling it’s tendency to crack (dunt).

The two principle considerations for open firing are:
Tempering Materials and Particle Size Distribution.
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Sand is a very common tempering material owing to its relative abundance and it’s naturally graded particle size. However sands have a very variable mineralogy and some sands present there own particular problems. Here is a x 20 image of a local beach sand. Grading is obviously very consistent and so to is the smooth and rounded grain structure.

This particular sand is predominantly composed of silica and as such is subject to the quartz inversion. At 573c it will suddenly increase in size by about .45% and on cooling at 573c it will contract, again by .45%.

Below we we see how this has might adversely affect the ceramic fabric.
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This image shows a (x 20) broken section of open-fired ceramic. The tempering was the beach sand shown above. The rounded grains have very little grip, and so when combined with the mechanical stresses of the quartz inversion (particularly on the cooling cycle) the grains become detached from the surrounding material. The result is a very weak body.

Even if the sand was of a much sharper structure, the quartz inversion can strain the whole fabric of the ceramic to the point of dunting (cracking on the cooling cycle).

It should be stressed, that silica sands present few problems when used in modest amounts, but in relation to open firing, we are sometimes dealing with over 50% tempering material. Also, with the low temperatures involved in open firing, it does not share the mechanical resilience of higher-fired ceramics.
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This x20 image shows the far more more textured grains of grog - a product made from crushed and graded ceramic material.
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In this broken ceramic shard, we can see how the grog sits tightly in the surrounding matrix. Also, as grog has a near identical rate of expansion and contraction to the surrounding fabric, it imposes very little additional stress.
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Organic temper (which effectively burns out after firing) has been used consistently for tempering open fired bodies.

It does have great benefits: It is easy to source, its often fibrous nature gives pre-fired strength and also aids workability. Organic temper is also very useful at opening the texture of the body so allowing gasses (mostly steam) to escape in the early stages of firing.

In large amounts however, it creates many post-firing voids and the ceramic fabric becomes frail and unusable.

This image (x20) shows the rather flaky and open texture of an organically tempered body. The materials used in this case were coffee grounds and jute fibres.
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Shell temper (or shell-sand temper) is in many ways an ideal choice for tempering open-fired bodies. It is easy to source and prepare, It exhibits a plate-like particle section which gives workability and strength, and its expansion/contraction rate is very closely matched to the predominant clay minerology.
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This image shows oyster shells after grinding and grading. (1mm to dust)
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Here at x20 magnification is a section of fired fabric showing the plate-like oyster shell temper, very securely locked in place and demonstrating a strong overlapping bond. It is also interesting to note that, after working (in this case coiling) the flat particles tend to align themselves to the working plane.
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Shell temper is composed almost entirely of calcium carbonate and as such it can cause problems with lime popping. In this image of a North Devon Jug there is evidence of a nodule of limestone, which after firing, has produced the characteristic, crater-like, signature of lime popping.

The mechanism that causes lime popping is fairly straightforward and easy to identify: The calcium carbonate CaCO3 (shell, limestone, chalk etc.) on firing converts to lime CaO which after firing combines with the moisture in the air and hydrates to Ca(OH)2. as it hydrates it also expands. Lime popping can occur at any time after firing, which, for obvious reasons, is very disconcerting!
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So why has shell temper (and shell sand temper) been used, and even sought after, for millennia. The simple answer is that it functions so well in every other respect that it’s worth mitigating its one significant shortcoming!

Firstly it should be stated that lime popping is a phenomenon which is strongly related to the size of the calcium carbonate particle. The bigger the particle the greater chance of popping - it could be assumed that on a small scale, the ceramic fabric simply resists the modest forces involved. Also the conversion of calcium carbonate to lime increases with temperature, and is most evident above 750c

Secondly, and this is less widely understood, the use of sea water in the clay body has a mitigating effect on lime popping. Published research has shown that salty water was known to potters for thousands of years as an essential additive to shell/limestone tempered pottery, but we only have rather conjectural theories of what mechanisms are at work.

One theory is that the sodium in the salt would fuse with the calcium carbonate and somehow prevent conversion to lime.

In this image, an oyster-shell tempered test-piece has been mixed with salt water, and, even after weeks of exposure to humid conditions, shows no evidence of lime hydration.
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This image shows powdered calcium carbonate after firing to 850c. After saturating with water the sample immediately reacted with a strong exothermic effervescence and rapid expansion. The calcium carbonate has clearly converted to CaO (lime).
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Here, the calcium carbonate has been dampened in a 4% saline solution (approx. equivalent to sea water) and again fired to 850c. After cooling and soaked with fresh water the sample remained stable. Clearly it has not converted to lime.

I’m still not sure of the mechanism involved here, but it is interesting to note that salt melts at approx. 800c which coincides with the temperature that calcium carbonate starts converting to lime.

I’m guessing, but could the formation of sodium carbonate and calcium chloride be part of the answer:
(2 NaCl + CaCO
3 Na2CO3 + CaCl2) ?
Note: Open-firing is dangerous - I always wear PPE appropriate for hot working.