Research i need help?

i'm working on a research entitled "potential use of oyster shells in ceramic production" where oyster shells serve as an additive material to make ceramics more durable. The steps in making our ceramics tell us that we need first vigan clay and ball clay. First step is to mix them. Then, soak them in water overnight. The third step is to knead the mixture. This step bothers me because as i have heard from some people, they say vigan clay is so called as that to indicate that it's hardened..so how will i be able to knead the clay as based from the steps?what is vigan clay anyway?