These machines are an important element in the preparation process. They ensure proper homogeneity amongst different types of clays and additives, as well as a controlled water addition. The closeness of the knives produces a paddle-against-paddle effect, squeezing and kneading the clay, resulting in an improved plasticity of the mass. The mixing knives are designed in such a way that their inclination can be adjusted. Also their position is adjustable to compensate for the wear and to make them last longer, thus reducing maintenance costs.