Question

What happens when dried pots are subjected to firing?

Short answer

Answer: The main chemical changes during the firing process include dehydration of clay minerals to form metakaolin and the reaction of metakaolin with other silicates to form crystalline compounds like mullite, a process known as vitrification. The primary physical changes include shrinkage of the pots, increased hardness, decreased porosity, and improved resistance to moisture. These changes result in a denser, harder, and more durable final product, suitable for various applications.

Step-by-step solution

Explain the initial state of dried pots

Before firing, the pots are made from clay, which is a natural material composed mainly of fine-grained minerals like kaolinite, halloysite, and montmorillonite. The clay is mixed with water to make it pliable and easy to shape. Once the pot is formed, it is allowed to dry, during which the water evaporates, and the clay particles bond together, giving the pot its shape and strength.

Describe the firing process of dried pots

Firing is the process of heating the dried pots at high temperatures, usually between 800 to 1300 degrees Celsius, depending upon the type of clay and desired properties. It is typically done inside a kiln, where the temperature can be controlled accurately. The firing process can take several hours to a couple of days, depending on the thickness of the pots and the temperature profile.

Explain the chemical changes during the firing process

When the dried pots are subjected to firing, several chemical changes occur as the temperature increases. One of the main changes is the dehydration of the clay minerals, where the remaining water in the structure is driven off. This process is called dehydroxylation, and it results in the formation of metakaolin, an amorphous (non-crystalline) substance: \[ Al_2Si_2O_5(OH)_4 \rightarrow Al_2Si_2O_7 + 2H_2O\] The metakaolin begins to react with other silicates in the clay, forming new crystalline compounds like mullite: \[ 3Al_2Si_2O_7 \rightarrow 2(3Al_2O_3 \cdot 2 SiO_2) \] This transformation, known as vitrification, further strengthens the pot and increases its density. Other minerals in the clay may also react to form different compounds, contributing to the pot's unique properties and appearance.

Explain the physical changes during the firing process

Along with the chemical changes, the firing process also results in several physical changes in the pots. The most notable is the shrinkage of the pots, as they become denser and more compact due to the formation of new crystalline structures. This shrinkage can range from 5% to 15%, depending on the type of clay and the firing temperature. Additionally, the fired pot becomes much harder, less porous, and more resistant to moisture, making it more durable and suitable for various applications.

Describe the cooling process and final outcome

After the firing process is complete, the pots are allowed to cool down slowly to room temperature. This cooling process ensures that the pots do not crack or break due to sudden changes in temperature. The final fired pot exhibits improved properties like increased hardness, durability, and resistance to moisture, making it suitable for everyday use or artistic purposes. The firing process may also give the pots unique colors and textures, depending on the type of clay used and any additional materials or glazes applied before or during firing. In conclusion, when dried pots are subjected to firing, they undergo a series of chemical and physical changes that result in a denser, harder, and more durable final product.