Architects memo no. 114: January 2014
the good oil
The treatment and finishing of wood with oils, particularly
vegetable oils, goes back into the mists of time and satisfies
the conception that this ‘living’ material (i.e. wood) needs
‘nourishing’ with natural oils.
Of course, wood ceased being a ‘living’ thing the moment
the forester’s chainsaw went through it, and the only
nourishment that the natural oil provides is to the host of
fungi and mould spores that alight upon it.
A significant and varying proportion of wood is comprised
of hemicelluloses and sugars. These hygroscopic materials
will absorb and release water as the relative humidity
fluctuates, causing changes in the dimensions of the wood
- leading to the erroneous ‘living’ tag.
Oils typically have low viscosity and are able to penetrate
into porous substrates, which can bring some improved
stability and water resistance to the bulk of the substrate -
oil tempered hardboard is a good example of how (linseed)
oil can improve bulk properties.
Vegetable oils contain more or less unsaturation and the
most highly (poly) unsaturated oils have the ability to use
atmospheric oxygen as a catalyst to react with themselves
and dry (polymerise) into intractable solid materials. This
property made them highly desirable as paint and caulk
(linseed oil putty) binders. This drying reaction is naturally
very slow and somewhat unpredictable.
The fastest drying oil is Tung oil which is derived from the
nuts of the China Wood Oil tree. The difficulty of sourcing
this product led Western artisans and artists to rely on
linseed oil, which is derived from flaxseeds.
As noted above, raw linseed oil is very slow to dry but,
by heating it in the presence of certain metal soaps, this
property could be dramatically improved. Boiled oil became
the ‘go to’ product for oil-based paints and stains.
One very famous linseed oil based stain was known as the
‘Madison Formula’ which was developed by the U.S Forest
Products Laboratories. It was successfully marketed in NZ
under several names, the most famous of which was Goldex
NF 11. The product was based on pale boiled linseed oil; turps; paraffin wax to improve initial appearance and watershedding
properties, and lashings of pentachlorophenol to
keep the bugs at bay.
Contrary to popular belief, linseed oil based stains are film
forming. Their durability was poor, however, and the film failed
by powdering away, which led to easy preparation for re-coating.
The fact that they didn’t ‘flake’, as more durable films can do,
has probably lead to the ‘non film forming’ misconception.
Around this time some entrepreneurs realised that by adding
pigments to used sump oil, a fair looking ‘timberstain’ could
be produced. The ‘mineral’ oils on which they were based
are chemically very different in nature to ‘vegetable oils’. The
major requirement for lubricating oils is that they remain very
stable under heat and in the presence of metal contaminates
- therefore there must be no tendency to polymerise.
They are truly non-film forming and will penetrate readily
into open pores in wood, thereby increasing the water
resistant properties of it. Penetration is required as any
excess material on the surface (unless it has been modified
with wax) will remain liquid and sticky. Wiping off excess is
Where the oil is ‘visible’ to U.V. light, it will degrade quite
rapidly and protection of the timber is achieved by the pigment
content which is loosely held on the surface. The oil which is
‘invisible’ to U.V. (either deep in the wood or protected by the
surface layer of pigment) will remain liquid and mobile.
This style of product offers benefits in the areas of safety,
mould resistance and overcoating and formulations have
become refined with the use of more highly specified ‘white’
The ease of overcoating, however, comes with a major caveat
- they can only safely be overcoated with similar products;
there is a danger of the residual ‘free’ oils interfering with
the curing chemistry of the new coating or causing adhesion
and/or blistering problems.
It is therefore important to ensure that the original product
specified is well documented to all interested parties so that the
subsequent maintenance work can also be properly specified.
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