Conservation & Heritage Issue 49 October 2025 - Flipbook - Page 41
The Science
There are two basic types of flax – oilseed flax and spinning
flax. Oilseed flax grows with shorter fibres and produces
seeds that are rich in oil. Spinning flax has long fibres, and
less oil in its seeds. Linseed oil (or flax seed oil) is referred
to as a ‘drying oil’, meaning the initial material is liquid
but after a period of exposure to air it hardens to a tough,
solid film. The ‘drying’ process (there is no evaporation)
is the result of an oxidative reaction (or oxidisation) where
oxygen attacks the hydrocarbon chain and as a result the
oil polymerizes forming long chain-like molecules. As
time passes the polymer chains cross-link resulting in a
vast polymer network. The result is an aged material that
is stable and rigid but remains somewhat elastic. Most
applications of linseed oil exploit these drying properties.
Above, Window restoration showing bare wood, linseed oil &
painted sections
durability. Natural granular solids were typically used
including clays, oxides and carbon, with these pigments a
palette of natural earth colours can be achieved including
sienna, ochre and umber. These traditional linseed oil
paints have lasted centuries and continue to protect exterior
timbers, our windows, doors and iron-mongery. Paint failure
was rare if not unheard of.
Above, Cold pressed linseed oil
Linseed oil molecules are also small and aided by an
expansion in volume of around 10% during drying, it offers
excellent penetration into wood pores, both visible and
microscopic but without expanding the wood itself. Linseed
oil is also hydrophobic; its molecules are repelled by water.
Droplets of water will form on the linseed oil film much like
morning dew on grass or on the surface of a leaf. Returning
from the allotment last weekend and one or two good
shakes of leafy vegetables and ‘miraculously’ all the water
droplets are gone.
It wasn’t long before colour pigments were ‘suspended’
in the linseed oil and its use as the preferred supporting
medium (binder) for making oil paint. These have been
used in Europe and England since the 13th century and
were widely adopted as an artistic medium during the
Renaissance period. Pigments essentially serve to
thicken the film, increase the volume of paint and provide
colour, they also affect drying time and
All was well….aside from some poisonous pigments
& solvents
Various poisonous minerals and metals have been added
to paint over the years. Emerald Green is a pigment based
on arsenic and Vermilion on mercury. These were favoured
colours by the impressionist painters. Cezanne developed
severe diabetes (a chronic form of arsenic poisoning), Monet
went blind, and Van Gogh suffered neurological disorders.
These illnesses may also in part be attributed to use of other
commonly used substances including liquor & absinthe,
solvents such as turpentine and lead pigment. The addition
of lead as a pigment did improve paint performance and
increase durability. It is a ‘dryer’, resists moisture and retains
the paint appearance. Lead was used in high levels in paint
between the 1930’s and mid-1950’s, including for domestic
use, and in significant quantities up to the early 1960’s. Lead
is however poisonous, particularly for younger children,
and can cause nervous system damage, stunted growth,
delayed development and kidney damage. European Union
legislation banned the sale of lead-based paint to the public
in the early 1990’s.
Right, Hazard Pictogram
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