The relative mass loss data was fit to a phenomenological surface erosion model (Equation ) in which 𝜕𝑚 𝜕𝑡 is the change in mass with time, 𝑚 is the instantaneous mass, 𝑘 𝑑 is the specific surface degradation rate, 𝐴 𝑠 is the surface area, and 𝑉 is the volume .