12 I Terracotta tile project. These drawings would then be used as the basis of a 'jointing' scheme, breaking the elevation down into individual blocks. The aim of this exercise is to create the highest repeat factor possible with blocks of a size and shape which will not cause difficulties in manufacture. Constructional design is also considered at this stage and large scale details are prepared showing how the terracotta works in conjunction with the other elements of construction. This design package is then submitted, via the architect, to all other parties in the design team, main contractor, fixing contractor, structural engineer etc. for their approval. _ MODEL MAKING Working from the clay drawings which provide an exact profile to make the model, the craftsman produces a zinc template by cutting, dot by dot, punches straight through the drawing, the reverse of which is mounted on a timber frame called a 'horse'. This is designed so that it can be pushed along the edge of a bench. Underneath this profile, liquid plaster is added by degrees with the 'horse' being moved back and forth to transfer the profile shape to the drying plaster. The template is mounted on a centre point for curved blocks or fixed to a lathe for circular items. This is not a mechanical process but rather requires skilled artists to complete any ornate additions to the blocks. MOULD MAKING Again using plaster, a mould is now made around the model, the latter being coated with shellac and soft soap to prevent sticking. Sides are then built up by hand one at a time until all faces other than the back are formed. When dry the mould can be stripped from the model, which is set aside. Once reassembled without the model, the mould is dried and can then be filled with clay. CLAY PREPARATION There are many different types of clay, some more suited than others for the different type products man has devised - china for pottery and earthenware for pipes. No one clay falls under the category of 'terracotta'; rather the physical properties of a particular clay make it suitable or not. It is in the processing that the important characteristics such as colour, strength and firipg qualities are developed. First the clay must be allowed to stand outside for a period of time to allow weathering. It is then processed. One method is the 'press' technique -simply reducing the clay to a water mix slurry, which is sieved to remove unwanted particles and then pressed, being forced through nylon cloths for approximately five hours at a pressure of 90 lb/in 2 . Alternatively, the 'pan mill' method involves a mixing machine where the clay is ground through a sieve by large heavy metal wheels, with iron particles being extracted by passage under an electromagnet. CLAYWORKING There are three methods of working the clay, of which hand pressing is the most common. Clay is smeared by hand into the plaster mound, ensuring that no pockets of air are formed, to an even thickness of approximately 30 mm. Webs are formed in the hollow void to give support and maintain shape during drying and firing. The hygroscopic nature of the plaster is such that water is literally sucked from the clay, starting the process of drying and causing contraction to begin. After a few hours, the clay is stiff enough to be turned out of the mould, which is then dried for later re-use. After a day or two, the surface of the block is worked on. Using knives, leather, rubber and wooden tools the surface is smoothed off, pushing the coarser particles of clay down and bringing the finer up to the surface. This not only improves the appearance of the block but creates a 'skin' which imparts much of the wearing characteristics of the finished piece. Slip casting involves the pouring of liquid clay into the mould, again with the hygroscopic effect drawing the water out. After a predetermined time, the remaining liquid can be poured off leaving a desired thickness of set clay in the mould. Extrusion, as the name implies, is suitable for the production of many, straight shaped blocks. When such are needed, there is a considerable cost advantage in this latter method. DRYING Half of the shrinkage occurs in the drying process and so extreme care must be taken in ensuring a slow and even curing. Warm currents ofair, from underfloor heating, are allowed to circulate around and through the blocks. Sometimes a bed of rollers is set under the block to allow this often considerable amount of movement to be accommodated. GLAZING It is at this stage that the application of a glaze, when required, is carried out. Early methods included brushing on or even dipping in a trough full of liquid glaze. Today, however, compressed air is used to spray the glaze onto the block. There might be several layers of glaze applied and various techniques used to give a particular effect, such as speckling and mottling. FIRING AND INSPECTION The process of firing causes the finer, more fusible, particles of the clay in a body to turn to liquid which, when cool, then bonds the coarser particles together. This vitrification essentially ensures that the blocks will be hard wearing and durable. There are basically two types of kiln. One is a continuous type, often a tunnel kiln, which works on a set cycle of reaching full heat and then cooling, and is suitable for items of low volume such as wall tiles. Terracotta, however, with the large masses often involved, is usually fired using an intermittent kiln where the product is fired in two separate stages, the burners firing in two separate durations to reach predetermined temperatures - often as high as 1200°C. To finish there is the final inspection where attention is given not only to accuracy of size and colour but the detection of any occurrence of copper or iron contamination. Spares, at the rate of one in twenty, are usually allowed during manufacture to ensure that replacements are in hand should blocks be damaged in transit or on site. lbs tock Hathernware Ltd, Normanton on Soar, Loughborough LE12 5EW CONTEXT 52
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