Manganese dioxide (MnO2) is a highly versatile compound that serves multiple purposes in ceramic glazes, such as imparting color, creating textures, and influencing chemical properties. Here are its principal applications within glazes:
Colorant
· Brown and Black Shades: Manganese dioxide is widely utilized to generate brown and black hues in glazes. For instance, in a transparent lead - based glaze, incorporating approximately 7% of manganese dioxide can yield a deep, rich brown color.
· Purple and Violet Tones: In environments with high alkali content (KNaO) and low alumina levels, manganese dioxide has the ability to produce purple and violet shades. This effect is particularly noticeable when it is combined with cobalt.
· Bronze Effects: When mixed with copper oxide, manganese dioxide can create a striking metallic bronze appearance on the glaze surface.
Texture and Surface Effects
· Speckling and Crystallization: When the concentration of manganese dioxide exceeds 5%, it has a tendency to precipitate and crystallize. This process results in the formation of speckled or crystalline patterns on the glaze surface, adding unique visual interest.
· Matte and Iridescent Surfaces: Depending on the specific glaze composition and firing conditions, manganese dioxide can play a crucial role in the development of matte or iridescent surfaces, enhancing the aesthetic appeal of ceramic pieces.
Chemical Properties
· Flux and Refractory Behavior: At temperatures below 1080°C, manganese dioxide functions as a refractory material, increasing the viscosity of the glaze melt and making it more resistant to flow. However, when the temperature rises above 1080°C, it dissociates to form MnO, which then acts as a flux. MnO reacts with silica, leading to the creation of a variety of colors.
· Avoiding Blistering: Since manganese dioxide releases oxygen at around 1080°C, glazes containing this compound should be fired outside this temperature range. This is necessary to prevent the occurrence of blistering and other surface defects.
Safety Considerations
Manganese dioxide poses significant health risks. In its dry state, it can be inhaled, causing respiratory problems. When in suspension, it can be absorbed through the skin, potentially leading to neurological disorders. Therefore, ensuring proper ventilation during the firing process is of utmost importance to effectively remove harmful gases and safeguard the health of those involved in ceramic production.
