Chief Constituent of Brick Producing Clay

Brick producing clay is typically a mix of various materials, each contributing different properties to the final brick. Understanding the main constituents is important for predicting the quality and characteristics of bricks.

The primary constituents found in good brick earth are:

• Silica ($\text{SiO}_2$): Usually present in large proportions, typically 50% to 60%. It prevents cracking, shrinking, and warping of raw bricks but excess can make bricks brittle.
• Alumina ($\text{Al}_2\text{O}_3$): A chief constituent, usually ranging from 20% to 30%. Alumina is responsible for the plasticity of the clay, which allows it to be molded. Excess alumina can cause shrinkage and warping during drying and burning.
• Lime ($\text{CaO}$): Present in small quantities, typically up to 5%. It helps fuse silica and alumina during burning, making the brick durable and hard. Excess lime can cause bricks to melt and distort or lead to the formation of quicklime lumps that expand after burning.
• Oxides of Iron ($\text{Fe}_2\text{O}_3$, $\text{FeO}$): Usually 5% to 6%. They act as a flux and give the brick its characteristic red color when burnt in oxidizing conditions. Excess iron oxide makes bricks dark and brittle.
• Magnesia ($\text{MgO}$): Present in very small quantities, typically less than 1%. It can impart a yellow tint and helps reduce shrinkage.

The question specifically asks for the chief constituent present in the range of 20% to 30% in every brick producing clay. Based on the typical composition percentages listed above, alumina fits this description, being crucial for plasticity and present within that specified range.

Therefore, the chief constituent of every brick producing clay (20% to 30%) is alumina.