Studies of Knacke and Pohl (1959) using clay minerals showed effects of water
content on hardness of the agglomerates. The hardness of the agglomerates is proportional
to the surface tension of the pelleting fluid and inversely related to the particle
size distribution. They showed that with an increase in the filling of the pores with
water, an optimum value for hardness exists. This optimum is dependent on the surface
tension of the pelleting fluid.