The first process required to begin the production of clay bricks or roof tiles is to make available the raw materials required for the process. The major material used in manufacture are clays and as such brickworks will have been built as close to a clay quarry as possible although some clays are still trucked in to aid in varying colours and technical characteristics.Once the clay has been won it is then taken to a stockpile where it is tipped and levelled. It is usual practice to layer a stockpile with different types of clay won from different parts of the
quarry to ensure consistency. Dependent upon the body formulation it is possible to add other materials from recycled or secondary sources to this stockpile. These stockpiles are then left open to the elements for a period of time ranging from weeks to years although nowadays with the costs involved companies do not want to pay for clays which are not being used to make products.
Once the stockpile has been weathered the clays are then taken to be prepared for manufacturing – primary crushing, secondary crushing and high speed rolls. Once the final clay body is ready it can take one of two different routes, either direct to the forming machines or placed in sour houses.
The final clay mix will be transported to a separate storage facility preferably undercover and sheltered from the elements. It is thoroughly soaked with water to induce plasticity and the clay is then allowed to lie in these storage areas for several weeks, months or longer as required. During this period of rest the water spreads by means of capillary attraction and the clay undergoes the necessary changes, eventually becoming a homogeneous mass ready for the pug mill. This can be a long time consuming process and dependent upon the size of your sour house may not allow the full souring process to be completed.
WHAT ARE THE ALTERNATIVES TO CLAY SOURING?
The principal of clay souring is to improve workability/plasticity of the finished clay body prior to formulation. The alternatives to which could be
- For stockpiling and then subsequent weathering the natural elements will help to homogenize the clays, remove unwanted soluble matter and break down large agglomerates. However these clays will still require to be crushed and ground (and even blended with other raw materials) prior to use and as such may lose any plasticity gained.
- The use of finer clays or more plastic clays could be seen as an alternative way to overcome souring but these too are either hard to source, too expensive or have been seen to give rise to other production problems further down the process line.
- The final body recipe may be very clay rich having a high shrinkage so the addition of grog may be essential to reduce cracking especially during drying. These could either be an inert material to act as a filler to reduce shrinkage or a porous material to open up the body. These alternatives include sands, shales, pumice, PFA or fired ceramics.
- Clay conditioners have been proven to be a good replacement for souring. They work in the same way as water in they spread via capillary attraction lubricating the inner clay particles and increasing workability. They act as a wetting agent as they reduce the surface tension of the aqueous phase and promote more homogeneous mixing. This in turn gives improved particle flow reducing friction during shaping and increased strength in both wet and dry phases. It has also been seen that the addition of a clay conditioner to clays that have not been soured led to a reduction in water addition which can help in reducing not only costs of manufacture but drier energy costs too.