Clay minerals are present in a wide variety of forms. They belong to a large family and can be characterised by their layered, crystalline structure. There are three main members within the clay family, known as
with other transitional forms occurring within these. Examples of these include kaolin, mica, and smectite, respectively. These different clay mineral groups vary with regards their chemical make up, and hence all behave slightly differently when they come into contact with water. This behaviour is an important determining factor, with regards to selecting a suitable clay for making up clay plaster. This is because it has direct implications for how much the clay will expand and contract with water. This has a direct correlation to how much a clay plaster will shrink as it dries, how much cracking will occur in its dry state, and how it will interact with water once applied to the wall as a protective finish.
To understand which of these clays are most suitable for use in clay plaster, and to understand how clays behave when they come into contact with moisture, it's good to know some of the more basic chemistry involved in their functioning. Clay minerals derived from feldspar consist predominantly of microscopic particles of alumina and silica. These molecules are shaped like plates, and are alternately stacked one on top of another. The alternate stacking creates an electrostatic charge, which attracts water into the spaces between the platelets. This is why clay is considered to be hydrophilic or, 'water-loving'. When present, this water acts as a bridge between platelets, bonding them tightly together to form a cohesive structure.
The specific presence and organisation of the platelets determines a clay plaster's tendency to attract and absorb water into its structure. These differences can be exhibited by the different behaviours between the main clay groups mentioned above. A kaolinite, for example, consists of a larger surface area will have more open attachments and therefore a greater abilility to enter into chemical bonds. This will provide a greater capacity to take on more water between the platelets. When the spaces between the platelets are filled with these fine films of water, the platelets have the ability to slide over one another. This is why clay feels so smooth to the touch and is easily worked and molded when wet. Clay also expands when all of these spaces between the platelets are filled.
Clay plaster has the ability to relinquish any absorbed moisture as quickly as it was taken on. This is through the process of evaporation. When the water evaporates from the body of the clay, the platelets are pulled closely to one another, hence the characteristic nature of clays to shrink when they dry. Beneficial to anyone working and sculpting with clay however, the platelets will remain in the same shape that they have been molded into even, when they dry.
Unlike other binders used for plasters and renders, such as lime and cement, clay does not undergo a chemical transformation as it dries and cures. This means that clay plaster can be indefinitely rewetted and reworked, and ensures that it can be infinitely recycled as a building material. It does, however, also mean that though good for areas of high humidity (water vapour), they cannot be used on walls that come into direct and prolonged contact with liquid water.