Jongkind clay granules are made from special purified clay that is first cleaned, reduced in size and homogenised. The clay is then heated to 1100°C in long rotating furnaces, which results in the formation of expanded reddish brown granules with an open cell structure: a combination of numerous air pores with sufficient water-absorbing capacity.
An important aspect in the production of reliable clay granules is the quality of the coal used to fuel the furnaces. The end product must be free of undesired heavy metals.