4. Porcelain enamel is sometimes referred to as A. "glass lining" B. kaolin C. vitreous enamel D. Both A and C.

The answer is Both A and C. It is also known as glass lining and vitreous enamel.

Explanation:

Porcelain enamel is sometimes referred to as glass lining and vitreous enamel. It is a thin layer of ceramic or glass applied to a substrate of metal, which is used to protect surfaces from chemical attack and physical damage.

They can modify the structural characteristics of the substrate and improve the appearance of the product.

Porcelain enamel coating is used in applications like smooth, very durable, scratch and chemical resistant coating is needed. Industrial porcelain enamel is also known as glass lining are used in industrial rather than artistic and applications.