Clay lumps are collected and are wetted with water and allowed to mature for 1 or 2 days. It is kneeded well until it attains required consistancy. Sometimes fibrous materials like gobber is added in the mix.
It prevents cracks in the plaster. If plaster is to be used for outer walls, it is sprayed or painted with bitumen.
It is cheap mortar. Its durability is less. It is normally used for the construction of temporary sheds and cheap houses in rural areas.