Absorbent properties of materials

We have chosen to test materials which are abundant and cheap and which are either already manufactured on a large scale, or could be manufactured without extensive development.

cellular concrete is the best presently available material. It is not concrete but a calcium silicate made by reacting sand with calcium hydroxide and aluminium powder. Its buffer capacity is only about a tenth that of paper.

Cement has a high sorption capacity, but is generally used to make concrete and renders which are so dense that only a thin surface layer is available for water exchange on a daily cycle. Cement foam, made from cement with a frothing additive, is very hygroscopic but has hitherto only been used for high temperature thermal insulation. It is not available in ready made block form but can be used as a sprayed plaster.

Unfired brick is a good moisture buffer. The active ingredient is montmorillonite clay. A brick clay contains mostly sand and silt, which gives dimensional stability with changing water content but dilutes the action of the clay. It has a fairly slow water vapour diffusion but this can be enhanced by perforating the brick, provided air can circulate through the perforation.

The most water absorbent clay is Wyoming montmorillonite, known as Bentonite. It swells enormously with absorbed water, so needs to be locked into a structural matrix which maintains dimensional stability while allowing trapped clay nodules to expand and contract with exchanged water. A material that was successful in small scale operations is a mixture of clay with perlite (an expanded volcanic glass foam). However, brick making equipment would need considerable modification to deal with this mixture on a large scale.

There are several organic buffer materials. Hemcrete is hemp stem fragments embedded in a lime mortar. It is commercially available as a spray mixture which can build up to several cm thickness.

Organic materials outgas various molecules, largely acetic acid. This is dangerous to museum objects but it is possible that the lime binder for Hemcrete will absorb the predominantly acidic gases that are given off. It may even be possible to use end grain wood, which is the best of all buffers yet tested, if it is covered by a porous material which reacts with the acid gases.

[This part of the investigation will involve Morten Ryhl-Svendsen as indoor air quality consultant.]

The measured properties are density, water vapour sorption, water vapour diffusion, dimensional change.

The full sorption curve over the full RH range is not relevant, so for many materials a two point sorption line is defined: between 40% and 60% RH.

Materials

wood, end grain

Sub plot: how to stop acetic acid escaping

possible treatments: lime wash, lime wash + charcoal. Ask Morten

Maybe develop impregnated wallpaper, or just alkaline paper.

Hemcrete (hemp core residues + lime + a few other things

clay on or mixed with a support

Wyoming bentonite

Taasinge bentonite

Damolin a/s bentonite

Brick clay (needs a definite source)

Moler, unburned (needs definite source)

waterglass as binder for other stuff.

perlite as binder