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Earth Floor Notes
How to Harden an Earth Floor
Finishing an Earthen Floor with Linseed Oil
By Barbara Roemer and Glenn Miller
We mixed linseed oil into our final coat for the floor because we'd had small cracks in the first two layers. Used about a gallon to 50 gals of floor mix (4:1 clay to sand, with lots of finely screened chopped straw).
The floor was down for more than a year before it developed a few hairline cracks in this dry summer. We are just ready to reoil it, and expect most of those cracks to disappear with reoiling. Used regular boiled linseed oil when we could keep the house entirely open, and Bioshield Hard Oil #9 thereafter.
The Steens have a new floor book coming out soon, and they have simplified their system such that they do not dilute the oil with solvents, putting on successively richer or leaner coats. Put it on, wipe in down as soon as the oil has had a chance to soak in. We used heated oil, but did it in a waterbath: oil in a coffee can sitting in a frying pan of water over a campstove outside. Do not leave a batch on while you are working - it takes only a few minutes to reheat it and is not worth the risk of untended oil over any kind of fire. Our floor, even with linseed oil in the final coat, soaked up three coats quickly, but it was bone dry (several months worth of drying). It could have used another coat at 9 months, and is very hard. Water pools on it rather than soaking into it.
Still, I wouldn't recommend cob with oil finish for a horizontal surface outside. The Steens' lime plastered bale or cob benches are almost always covered with a porch, and their rainfall is low and confined to one season. Don't know about alum, but the Steens have also experimented with olive oil soap rubbed into lime plasters for a beautiful sheen. Something in the soap reacts with the lime. Look for their new book - maybe it will have an explanation of the chemistry. It's a labor-intensive process suited to small decorative areas.
See also: Living Homes: Stone Masonry, Log, and Strawbale Construction.
Thomas J. Elpel
in a Day