Water smoking is a method used by potters in the preparation of ceramic vessels, also known as pottery, as a stage in the preparation of an object for firing. It can be used in the manufacture of pit fired pottery or the firing of objects in a kiln. It refers specifically to the drying and preheating of an object made of clay before it is subjected to the high temperatures used to convert it into a ceramic object. Many different cultures around the world have adapted to using the water smoking method to preheat their pottery and lower risk of defects in a multitude of ways and coping with their given environments.^[1]

Archaeologist Prudence Rice defines water smoking as, "The initial phase of the firing cycle in which all mechanically held water in the clay piece is volatilized and removed by slow heating to about 120°C."^[2] This holds true in any instance of the firing process with the primarily shifting factor being temperature.

Process

After a clay body has been worked and formed by a potter, it is necessary for the piece to be fired. However, prior to firing the clay body must first be dried. Any remaining water after this initial step is what is then removed in the process referred to as "Preheating" or "Water Smoking". This process can take place in temperatures anywhere from low heat to 200°C^[3]. These temperatures can be achieved by things such as kilns, industrial humidity driers, or direct sunlight. The added temperature during the process serves to not only speed up the evaporation time but ensure all water has been removed. It is not necessary, but is used in traditional societies that do not have modern equipment as well as in more modern commercial methods in in which manufacturing time is reduced with industrial equipment. In any case, there may still exist some mechanical water within the vessel which can cause complications even if as little as 2% of the mechanical water is left present.^[4]

After allowing a clay body to dry from ambient conditions, dehydration is expedited by heating a clay body. During this step, the shrinkage water is removed first, followed by the removal of pore water. This process will also remove any water that has adsorbed to the surface of the clay body.^[5]

The shrinkage water is the water that was mechanically bound to the individual, platelet-like clay particles. This is the water that causes clay to have its plasticity (physics)plastic attributes to be worked and molded. This water resides between the clay particles and other minerals in the clay and acts a lubricant, allowing the particles to glide across and by one another.^[6] This water is mechanically bound to the clay particles through the process of adsorption. The ionic charges of the water adsorb to those of the metallic clay particles along the surface or at points of breakage on the particle surface.

After the shrinkage water is removed, about half of the water density is gone. However the pore water may still remain. This water can require low-heat from around 105°C to 110°C to remove.^[7] The pore water is called so because it is the water that remains trapped within the clay which is porous in nature. After the shrinkage water is removed the pores of a clay body will shrink and thus capture water within it's pores. The pore water is then easily able to vaporize and escape through low heating. This smoking time is typically held at the low temperature of water's boiling point for extended periods of time. Further, higher temperatures are needed in order to remove remaining water and cause changes of the clay structure.

Different Strategies

The process can differ greatly depending on the attributes of the clay being used. For finer clays, water must be removed more slowly in order to prevent hairline cracks. Some clays require much slower gradual increases in temperature, while some with higher green strength can handle a shorter firing time. For example, smectite clays have very small particle sizes. This causes them to hold more of the adsorbed water types andto lose a great amount of water at low temperatures during the water-smoking period. Compare this to clays such as kaolinite, which are not drastically affected until they reach temperatures of around 400°C to 650°C.^[8] In clays that are used for non-industrial purposes, the consistency of minerals within them may be less pure. They may contain many different types of minerals and clays. These impurities can cause the loss of water to be more gradual and can also lower the temperatures needed for drying.^[9]

Complications

Defects such as cracks in the clay objects are most likely to occur in the first part of drying time. The temperature and drying time must be carefully controlled or complications can occur. Risks of damage can be affected by heat, air circulation, and time. Some defects can include cracking, crazing, and even explosions due to steam if the water particles expand too rapidly and cause ruptures.^[10]

Non-Industrial Manufacture

Non-industrial potters without access to electric kilns must be particularly careful with the drying process. They must pay close attention to humidity, the weather, atmospheric conditions, and time of day. In specific areas, drying may also be affected by seasons. Cold climates that can damage pots by freezing the water trapped inside the clay. Depending on conditions, potters may choose to dry their pots indoors in order to maintain a more stable atmosphere.

Preheating will be performed by the potter actively placing a clay body near a cooking fire or hot kiln. This method utilizes thermal radiation to cause the pots to reach the low temperature and in these cases the water smoking time can even range to a week. In some areas the preheating step will even be done by placing the pot into direct sunlight. This method does run the risk of heating too quickly and causing cracking,but can be avoided if the paste of the vessel is particularly coarse. In areas that are more prone to wet climates and/or cool temperatures, the preheating process will take place indoors. Potters will place vessels on a shelf above a slow-burning fire in order to prevent heating from occurring too quickly.

Notes

Rice, Prudence M. (1987). Pottery Analysis: A Sourcebook. p. 464
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 115
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 98
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 102
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 91
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 91
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 103
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 103
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 105
Rice, P. M. (1987). Pottery analysis: A sourcebook. p. 152-153

References

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