A spherical ball of clay, 2 in. in diameter

A spherical ball of clay, 2 in. in diameter, has an initial moisture content of 10 wt%. The diffusivity of water in the clay is 5 A? 10–6 cm2/s. At time t = 0, the surface of the clay is brought into contact with air such that the moisture content at the surface is maintained at 3 wt%. Estimate the time for the average moisture content in the sphere to drop to 5 wt%. All moisture contents are on a dry basis.