CASTING

This technique developed during the second half of the 18th century. It is a very useful procedure for the production of objects with shaped surfaces or non-circular moulds.
The working process consists of using liquid clay, which is called "barbottina", the material is poured into plaster moulds, which reproduce the objects to be realized. The mould contains the negative model of the object to be created. Thanks to its hydro-absorbing properties, plaster absorbs the water contained in the "barbottina", thus causing the paste to partially solidify and creating a thickness of clay on the plaster mould surface. The mould must always be kept full until the desired thickness is reached. At this point, the "barbottina" in excess in the mould may be emptied, by dripping it for a long time, thus creating the vase cavity. The thickness of the solidified "barbottina", in the void mould, must be proportioned to the size of the object to be produced.
The time needed to create thickness in the mould depends on the clayish paste density, as well as on the state of humidity of the plaster, the season and the environment temperature. During the casting phase it is necessary to keep the level of "barbottina" above the edge of the object to be produced; there is usually a formation of a funnel at the mould entry, which allows the level to be kept high and is also used as a tank. After a certain time span, when clay has reached the hardness of leather, it is possible to pass on to the opening of the elements making up the mould. At this stage, the craftsman should clean the surface of the object from burrs and imperfections printed by the mould.