Understanding the impact of soluble salts
Naturally occurring soluble salts are found in several ceramics raw materials, including plastic clays, kaolin and feldspars.
But it’s the ‘unknown’ salts – those introduced during production – that can have a dramatic impact on product quality.
Plaster moulds, for examples, can yield calcium and sulphate ions, while process water can introduce sodium, magnesium, chloride, carbonate, bicarbonate and nitrate ions, in addition to calcium and sulphate.
Sulphate and chloride anions and calcium and magnesium cations can have a dramatic influence on the viscosity and plasticity of ceramic systems when they are present at sufficiently high levels.
What is the effect of soluble salts during ceramics manufacture?
If high levels of sulphate are present and not controlled, very high viscosities can result, making ceramics processing almost impossible.
Faults during fabrication and in use can include:
- While drying the ware, a white or coloured deposit known as ‘scum’ can occur, caused by the soluble salts being transported to the surface with evaporating water. When glazes are then applied, the salts can prevent the glaze from adhering properly – leading to a fault known as ‘peeling’ or crawl’
- Efflourescence can occur when bricks are exposed to weather. As soluble components are brought to the surface by capillary action, a white crystalline deposit can form.
- Soluble salt migration can also occur in glazed wall tile, setting up expansion forces and causing glaze cracking
The interaction between the many soluble components present is extremely complex and impossible to analyse exactly. However, by controlling the amounts and types of soluble materials that occur within a ceramic composition, a better control of stability can be achieved.
An added challenge for ceramics manufacturers is the fact that amounts of soluble salts do not remain static over time. Under certain circumstances they can increase dramatically. This phenomenon of ageing in ceramic systems will naturally reflect in the viscosity and deflocculant requirements of bodies containing such components.