The basic material used to form the product as well as for construction of the kilns and their furnishings is a near white firing clay. The early London industry employed ball clays from Devon, Dorset and the Isle of Wight. As central control by the Tobacco Pipemakers Company waned and the industry became established at other centres, local deposits of coarser less plastic light coloured clays were exploited. Eventually either through exhaustion of local material or the demand for a finer product much of the industry followed the London example in the use of ball clays. Most clays as they are dug from the ground are of variable quality requiring careful preparation before use. For such a fine object as a tobacco pipe, particles visible to the naked eye are clearly undesirable. Preparation of clay for this use must therefore be carried out thoroughly. Clays were soaked to settle out larger inclusions, beaten to ensure thorough mixing and dried to the point of correct moisture content, before use. Duhamel du Monceau records the setting aside of clay containing impurities for 'raccommodage des pots' (pot repairs - he refers to the muffle as the pot or chamber) a practice that would naturally occur to an experienced tradesman. At Tweedmouth mud, much of which would have been clay, and dust was swept from between the cobbles and mixed with manure to be used for sealing the kiln. Clearly little that could be used would be wasted.
The addition to clay of organic matter and refractory mineral inclusions was already established practice before the advent of tobacco pipe-making. Primitive people working without kilns must necessarily select clay with an open texture capable of withstanding the extreme thermal shock associated with bonfire firing. From selection it is a small step to the engineering of these properties. The addition of refractory grits to a clay body has many advantages in the manipulation of the plastic material, in its performance in the kiln and in the subsequent use of the object. The grit acts as a skeletal structure giving additional support in the plastic state. This allows work to progress further and faster without pausing for the clay to stiffen. The grit also opens up the matrix assisting the passage of air and water allowing the work to dry or be dampened more quickly. In the kiln, the resulting fabric is less likely to warp or squat under load, has greater resistance to thermal shock an is less likely to spall. There is also a reduction in the rate of the shrinkage caused by loss of water. The improved resistance to thermal shock makes such a fabric ideal for cooking pots or kiln furniture. If grog, or ground fired clay, is substituted for the grit, in addition to these benefits, an increase of strength results from the greater degree of formation of mullite crystals with repeated firings. The addition of organic matter opens up the fabric in much the same way for the access or exit of water. In the kiln the organic matter burns away leaving voids. The resultant fabric is less dense with an inhibited ability to conduct heat through its mass. The open textured fabric is more resistant to thermal shock and spalling. An understanding of the chemistry is not required for the recognition and exploitation of these characteristics. That they were recognised and exploited is evident from the selective use of different mixtures for different functions within the building and furnishing of the pipe kilns examined.
A new concept was developed by the early pipemakers; that of introducing prefired pipe stems into the plastic clay for additional support. Experiment has shown (Appendix 9) that by this means a very wet fabric could be used to form a large pot or muffle, which, without this support would immediately collapse. As fabrics with a high percentage addition of grog or organic stuffing have a tendency to become short as they are worked, this facility to use a substantially wetter clay is significant. A further bonus from this practice is the ability of the prefired stems to hold together a large muffle traversed by a number of otherwise terminal cracks. In a muffle structure of the type that evolved, repeated heating and cooling would inevitably result in cracking. Whilst the muffle remained intact the cracks could be clammed up from the inside by repeated coatings of slip wash or lute. Without this important innovation it is doubtful whether the developed muffles of the late eighteenth to mid nineteenth century could have evolved.
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Last updated: Wed May 22 1996