Proceedings e report - Firenze University Press

SORPTION OF MOISTURE AND DIMENSIONAL CHANGE OF WOOD SPECIES USED IN HISTORIC OBJECTS
water vapour were introduced, and the respective mass increases due to the sorption and the respective
equilibrium pressures were recorded. Samples were considered to have reached equilibrium when the
weight changes were less than 0.05% in 40 minutes. The equilibrium moisture contents (EMC) were
calculated on the basis of the initial weight of the out-gassed sample.
The dimensional change of wood accompanying water vapour sorption was measured in the radial and
tangential directions using two inductive transducers of accuracy 2 μm. The dimensions of wood
specimens coincided with the principal anatomical directions in wood – radial 2 cm x tangential 2 cm
x longitudinal 0.5 cm. The measurements were taken in a specially built specimen holder placed in a
vacuum vessel. The vessel was connected to the same outgassing and water vapour dosing system
which was used to determine water vapour sorption isotherms; in fact the two measurements were
taken simultaneously. Both absorption and desorption branches of the sorption and dimensional
change isotherms were recorded. The process was fully automated and rapid - measuring of 10
adsorption and 10 desorption points took on average 15 hours.
After the measurements of dimensional changes induced by water vapour sorption and desorption,
each wood specimen was immersed in distilled water until full saturation. At the saturation state, its
dimensions were measured with a micrometer.
3. Sorption isotherms
The sorption of water on wood can be described by sigmoid shape of the type II isotherm in the
IUPAC 1985 classification [3]. It indicates the monolayer-multilayer physisorption in which an
adsorbed surface layer progressively thickens as the vapour pressure is increased up to the saturation
pressure, close to which the adsorbed layer becomes a bulk liquid. The three-parameter Guggenheim-
Andersen-de Boer sorption equation [4] is used to interpret the sorption data for wood by expressing
the equilibrium moisture content as a function of water vapour activity:
vcka m 0
va ( 0)
=
(1)
(1 − ka )(1 + ( c −1)
ka )
0 0
where a0 is water vapour activity taken equal to p/p0 where p is the experimental partial water vapour
pressure and p0 its value at saturation (thus if RH is the relative humidity expressed as a percentage a0
= RH/100), v the amount water vapour sorbed by a gram of wood at water vapour activity a0, vm the
monolayer capacity in the same units as v, c - energy constant related to the difference of free enthalpy
of water molecules in the liquid state and in the monolayer, and k - the third parameter, characterizing
the state of the sorbed molecules beyond the first layer. The usefulness of the GAB model in the wood
science has been demonstrated [5].
In the present study, the GAB constants were determined by a least-squares regression of the GAB
relation for a0 < 0.85, identified as the applicability interval. The regression was performed separately
for the adsorption and desorption branches. By way of example, the experimental data for water
vapour sorption by the lime wood are compared in Fig. 1 with curves calculated using the GAB
equation. RH scale is used on the ordinate axis of the plot. The GAB constants for adsorption and
desorption for the lime wood, as well as 20 other wood species investigated, are given in Table 1.
As one can see in Fig. 1, the applicability range for the GAB equation for the adsorption branch is
0.05