Clay is one of the five resources in Settlers V and it's needed for many buildings. It can be obtained from clay shafts with serfs but as with all raw materials they are not very successfully. They work slowly and without professionalism.
A clay pit can be upgraded two times, so that more miners are able to work inside and increase the productivity of it. With the exploration of the pulley you can construct a clay gallery. With the knowledge in chemistry you can upgrade it to a clay mine .