Ceramics

What is silicate?

Silicate is a combination of silicon and oxygen formed when rocks break down in a process called weathering. Silicate makes up the bulk of clay, which is a key component in ceramics.

Properties of ceramics

The term 'ceramic' describes a number of materials that have the following properties:

• Resistance to heat
  
  
• Low chemical reactivity
  
  
• Low density compared with metals
  
  
• A hard, durable surface resistant to abrasion
  
  
• A high melting point

See image 1.

Most ceramic items are made of clay, which can contain different-sized particles. Porcelain is a type of clay, also known as kaolinite, with very fine, white particles.

Wet clay can be moulded into shapes. When water is added to clay, the water molecules slip between the silicate layers, allowing the layers to slide over one another. When the water evaporates, the clay hardens and loses it ability to be moulded.

When clay is fired (heated to a high temperature), the silicates undergo a chemical change that hardens the clay into a glass-like substance and binds all the components in the clay together, making it water resistant. The higher the temperature, the more glass-like the clay. Porcelain has a high firing temperature of about 1400°C, accounting for its less porous nature.

Glazing

Even after being fired, sometimes the product is still porous, that is, able to absorb water. An application of finely ground glass called glaze makes the clay more water resistant. When the ceramic item is fired again, the glaze melts to form a glass barrier that is non-porous. Glaze is usually shiny and, with the addition of various metals, provides colour to the item. See image 2. 

Other ceramics

Pottery is just one type of ceramic item. Other examples that fit the properties of a 'ceramic' item include: bricks; cement; concrete and glass; which are all commonly used building materials.

A distinct disadvantage of conventional ceramics is its brittle nature, which means it is likely to shatter if acted on by a force. However, because of the many beneficial properties of ceramics, modern ceramic materials contain silicon and zirconium compounds, zirconia (ZrO₂) or silicon nitride (Si₃N₄) making it strong and resistant to thermal shock (big changes in temperature).