Oct
Moisture within Cob Walling
Excavated, clayey soils, depending on season and/or location will already contain a certain amount of moisture. To create a successful consistency [to build with], sufficient moisture content is crucial. Water may need to be added if the [newly excavated] material is too dry and/or the climate is hot, sunny or windy. If, on the other hand, the excavated material is too wet, mixing can still be achieved, however the newly mixed cob must be left to ‘steep’ or mature’. Good practice by a cob specialist is to allow for steeping regardless. Introducing this basic step ensures for a matured [cob] mixture to work with, rather than a sloppy, over-sticky and not so compliant building medium. Newly built cob walling (@ 2′ thick) can take up to 5 years to be considered ‘cured’.
But how much moisture remains in the cob thereafter?
The cob specialist took a 1-kilogram sample of cob from an old cob wall in Cornwall and set about finding out. The cob wall in question was in a poor state of repair ie erosions, failings etc, however this sample was taken from an internal elevation that was sufficiently protected and had not been subjected to water ingression/s. Our 1-kg sample was then ‘broken down’ into its individual components by way of sluicing with graduated sieves.
Each component was then dried in a low temperature oven. Together these components were again weighed only this time they weighed 870g. Inherent moisture content therefore = 130g (per kilo). So, there we have it, cob walling must a have moisture content to enable molecular bonding. If we crunch some more numbers; 1 tonne of cob wall (approx 1.1 sq/m @ 2′ thick) is bonded together with 1 litre of water (give or take an ounce or two).
