Sorption experiments

Ten specimens were selected to clarify the ad- and de-sorption isotherms between 0% and 100% RH. The adsorption curve between 0% RH and 85% RH is shown in the graph below. The experiment continues.

The sorption between 40% RH and 60% is shown in the graph below. The specimens started at 50% and completed the RH cycle 50 - 60 - 50 - 40 - 50 - 60. The traces are displaced on the graph, for clarity.

There is a nice division into cellulosics with high sorption, natural clays with medium absorption (because they are always mixed with inert silt) and fired bricks with very low absorption. (Moler is a unique Danish geological stratum of mixed diatomite, clay and volcanic ash. Unfired, it is widely used for cat litter.)

However, the distinction between the cellulosics and the mineral absorbers diminishes if one plots the sorption against volume rather than weight. Seasoned softwood has a density of about 500 kg/m³ while an unfired brick has density around 2000 kg/m³. For a given exposed surface area, unfired brick has comparable buffer capacity to cellulosics.

This graph shows the same information as the graph above, but simplified and supplemented by the curves recalculated as kg/m³ which are the paler lines, to be read against the right hand y-axis. Notice that the performance of brick clay matches that of wood when the materials are compared by volume, which is equivalent to a comparison by surface area of an effectively infinite thickness of material. Note that the unfired bricks do not show the same increase in exchangeable water as the brick clay. This is because the bricks are perforated so their density is lower.

The sorption curve, even after conversion to volume units, gives only a partial indication of the merit of a particular material. This sorbed water has to be available for evaporation or absorption over a period which is small compared with the air exchange rate of the room. Wood has a notably slow diffusion rate of water through the cell walls. Diffusion through unfired brick is faster. See the experiment to measure the rate of sorption/desorption.