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There is an expression in Brazilian Portuguese to refer to something badly done: “made in the thighs”. But where does that expression come from? The most used explanation for its origin comes from the era of slavery in Brazil. The slaves that were unable to do heavy work were responsible for producing tiles, molding the clay on their thighs. As each one had a different size and shape of thigh, the tile production was completely heterogeneous and the roof assembly was crooked, looking like it had been badly made, or “made on the thighs”!
Although many potteries still produce ceramic materials in an artisanal way, the technology is increasingly present in this segment. The production processes of tiles and bricks are very similar and include, in a simplified way, the extraction, molding, drying and burning stages.
The production begins with the extraction of the clay, popularly known as mud, from a clay pit. This material has a very thin granulation characteristic, directly linked to the final quality of the product. The crushing machine and the mixer, plus the addition of water, are responsible for making the clay mass homogeneous. However, the water quantity has to be well dosed, once the lack of it makes the molding difficult, and its excess damages the drying, which may cause cracks in the material when passing through the oven.
With the clay mass uniform and well balanced, it’s time to the extrusion, using a machine known as an extruder. It compresses and removes the air contained in the clay mass, leaving it with the appropriate texture for molding.
Next, the mass portions gain the shape of the products in which they will be transformed and go to the drying phase. Perhaps it is the most critical part of the whole process, because it is when the elimination of the greatest quantity of water occurs. The extraction of water from the ceramic mass is necessary to avoid defects in the pieces, such as cracks, breaks and deformations.
Basically there are two types of drying: natural or artificial. In natural drying, the water evaporates naturally from the pieces, according to the climatic conditions.
In artificial drying, means are used to control the temperature and humidity of the air, resulting in a faster process and, usually, with fewer quality problems in the products, once it occurs more evenly.
The reuse of heat from ovens is commonly used as a source of heat. The temperature is maintained around 212 °F and the humidity must be controlled so as not to cause quality problems to the pieces. If the relative humidity in air is high, it will be more difficult for the water trapped inside the clay body to evaporate out to the atmosphere in the form of water vapor, since the air will already be more charged.
On the other hand, if the humidity is very low, it will cause the drying and contraction in the piece’s surface. In this case, the surface dries more quickly and makes it difficult to remove the water contained inside the product, causing deformations and cracks.
Once the pieces are dry, they go through the firing process, or sintering. They are stacked inside the oven kiln, with spaces allowing the heat passage. The kilns reach around 1832 °F and are fed by small pieces of wood that are leftovers from other processes.
High temperatures cause physical and chemical changes that vary according to the temperature and time of exposure of the material. During the firing, the remaining water is eliminated and there is the transformation of the material that composes the clay, forming new complex structures that define the hardness, resistance and durability of the ceramic pieces. After the cooling and the rest time, the pieces are ready for commercialization.
Accurate measurement, control and action on the variables involved in each stage of the production process will be crucial to differentiate a good ceramic product from one of low quality, or “made in the thighs”.
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