CaO (Calcium Oxide, Calcia)

Notas

Calcium oxide (CaO), also known as calcia or quicklime, functions as a critical alkaline earth oxide in ceramic and glass chemistry. While pure CaO is highly refractory with an extremely high melting point, it acts as a potent flux when combined with alkalis like soda or potash. Its fluxing activity is most pronounced in medium to high-temperature ranges, typically becoming active around 1100°C and demonstrating significant potency by cone 10. In glaze formulations, CaO enhances surface hardness, scratch resistance, and chemical durability, particularly in lead or alkaline-based systems. It possesses an intermediate coefficient of thermal expansion, and in non-lead glazes, it can assist in mitigating crazing. However, caution is required, as excessive concentrations at high temperatures can cause body volatility. CaO is known to promote devitrification and crystal growth; while this can be used to create decorative matte surfaces or visual texture at high concentrations (e.g., 30%), it can also lead to unwanted crystallization if cooling is slow. Its impact on color varies; it often encourages yellow crystalline development in iron-bearing oxidation glazes and can soften or bleach certain pigments like cobalt. Various mineral sources supply CaO, including calcite, marble, limestone, and wollastonite, the latter of which is often preferred for superior uniformity. Because it is not a stand-alone flux, its performance is highly dependent on its synergy with other glaze constituents.