As shown in the figure above, wet material is made up the solid , the liquid and the gas as a gap.
The weight of material is the sum total of the liquid and the solid.
Since the solid weight is constant unless sublimates, the weight change of the liquid turns into the weight change of the material.
The change of material mainly occurs in the constant drying period of Ⅱterm.
Related page:
1-9.How to expressions the constant drying period 
1-7.Drying characteristic curve – Relation between a drying rate and moisture content 
1-3.How to expressions the moisture in the material 
1-1.What is drying 
1-5.Equilibrium moisture content and Critical moisture content – Influence of temperature and relative humidity 
3-8.Three types of drying 3.Drying of the paint film and Membrane 
1-2.The phase shift of the ustulation 
4-5.Drying speed and angle of the hot air