Alkali is in general terms referring to any 'basic' substance i.e. any material with a pH greater than 7, as opposed to 'acids'.
In ceramic contexts 'alkali' usually refers to hydroxides and carbonates of sodium and potassium. Alkalis act as fluxes in ceramics and glass production-enabling silica to melt at lower temperatures.
Alkali materials, such as sodium and potassium compounds, are used in ceramics for various purposes, mainly in the formation of glazes and the modification of clay bodies. They play a multifunctional role in ceramics, influencing the physical, chemical, and aesthetic properties of the final products. Their careful selection and proportioning are essential for achieving the desired characteristics in ceramic pieces, whether they are artistic or functional.
Here is a breakdown of their uses:
1. Formation of Glazes:
Fluxing Agent: Alkali materials act as fluxing agents in glazes, helping to lower the melting point of the other components. This allows the glaze to melt and form a glassy surface at the temperatures reached in the kiln.
Durability: Alkali in glazes helps in forming a durable and resistant surface, protecting the ceramic piece from wear and tear, and making it suitable for functional uses like tableware.
2. Modification of Clay Bodies:
Plasticity: Alkali materials can influence the plasticity of the clay, making it easier to work with. This is particularly important in processes like throwing on a potter's wheel.
Firing Characteristics: The presence of alkali materials can affect the firing characteristics of the clay, influencing factors like shrinkage and warping.
3. Color and Aesthetics:
Color Response: Alkali materials can affect how glazes react with various colorants and oxides, influencing the final appearance of the ceramic piece.
Surface Finish: The use of alkali can impact the glossiness, matte effect, or other surface finishes of the ceramic piece, contributing to its aesthetic appeal.
4. Chemical Stability:
Resistance: Alkali materials in the glaze can improve the chemical resistance of the ceramic items, making them resistant to acids, bases, and other chemicals.
Consistency: Helps in maintaining the consistency and stability of the glaze composition during the firing process.
5. Glass Formation:
Contributor to Glass Phase: Alkali materials contribute to the formation of the glass phase in ceramics, which is crucial for the development of glazes and the vitrification of clay bodies.