[Cob] Oils for finishing cob/plaster/wood

[Shannon C. Dealy]

In terms of finishing work, there are two types of oils, drying and 
non-drying.  The drying oils will ultimately dry (though some take a 
long time) to form hard surface, the non-drying oils will not dry and the 
surface will retain at least a slight oily-ness, and touching it you 
would get a bit of oil on your hands.  If you were to use a non-drying oil 
on a wooden bench, it would stain your clothes when you sat on it, even 
months or possibly years after the oil was applied (in time the wood may
soak it up to the point it's not noticable on your clothes.  Most (I 
believe nearly all of them) vegetable oils are naturally non-drying 
(including linseed/flax oil).  To make it a drying oil, linseed oil must 
be boiled which alters it chemically, in addition for most commercial 
preparations a number of chemicals are added to improve it's 
characteristics so it will dry much faster, a couple of days versus some 
much longer period (I don't know how long it naturally takes).  There are 
some commercial boiled linseed oils today which use less chemical 
additives and/or safer additives for people concerned with the toxicity of 
the product.  To the best of my knowledge, there is no commercially 
available drying oil that is cheaper than linseed oil (tung oil is 
probably the most widely used alternative but it costs more).

It is important to understand this difference before you start 
substituting non-drying for drying oils, not only do you have the issue of 
the oil getting onto things (though if it is used in small quantities in a 
plaster mix it may not be noticable), but on some surfaces such a wood, 
since the oil doesn't dry, if you wash it down periodically the oil will 
wash out, and far worse, on any surface that you put it on, a non-drying 
oil can go rancid and/or become a growth medium for mold (though in a 
lime based plaster, they lime may be sufficient to prevent this).  Even 
drying oils can support mold growth when dried, but the hard surface and 
nasty chemical additives make it less likely.

One additional point, putting oils in/on plasters on cob, straw bale, etc. 
should be restricted to only one side of a wall and only in areas that 
need serious protection from water (like certain limited parts of a 
bathroom) since if your walls can't breathe you are inviting all sorts of 
problems.

Shannon C. Dealy      |               DeaTech Research Inc.
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