When studying the sorption for timber at 20 deg. C and one plots the equilibrium moisture content in volume % against the relative humidity of the surrounding air from 0% to 100% relative humidity one finds that the adsorption and desorption curves are different (i.e. they don't coincide). Can someone please explain to me the reasons for this observation?
The behaviour you are describing is called sorption hysteresis and is a known property of wood. At any given temperature, the value of the E.M.C. will depend on whether it was attained by sorption or desorption. As for the reasons behind this behaviour, it appears to remain an open question for the time being, but it is most likely ultrastructural in nature (i.e. not anatomical). If you would like more data on this phenomenon, there are a number of articles in forestry journals that look into the subject, altough they tend in general to be more concerned with it's practical impact than on its origin.
I totally agree with Sir Leonardo. The difference between adsorption and desorption curves is normal because of Hysteresis phenomena. After first drying procces the wood lose some absorption area on its structure (for example; that sites has OH groups). So, after drying, if you put the wood in humide area you will find lower abosorption.
Good Luck
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