Glassmaking involves two main steps, heating and mixing raw materials to produce molten glass, and forming the molten glass into the desired shape. Most glass then receives further treatment to produce the final product.
Glass manufacture begins with the careful selection and measurement of raw materials. The most important raw material is sand. Other raw materials used depend on the type of glass being made. Broken glass, called cullet, is usually added to the raw materials. It promotes the melting of the raw materials as they are heated.
Most cullet is waste from glass-forming operations; some is obtained from recycled glass products. The raw materials and cullet are heated until they have melted into a spongy mass full of bubbles.
The temperature of the melt is then increased to make it more fluid, allowing the bubbles to rise to the surface and escape. The glass at this stage is clear and homogeneous. When the glass is withdrawn from the furnace its viscosity is too low for it to hold any form. As the glass is cooled, it flows less easily and can be formed. This is done using one of the following processes: machine blowing, drawing, floating, pressing, or rolling.