Linseed oil is obtained from the Flax plant (Linum Usitatissimum). It’s also used for linen fiber.
However the species used for oils are different since the stems of the plant are shorter and stronger in order accomondate bigger seeds. They can press up to 40% more oil than the plant for linen fibers. The ripeness and purity determines the quality. The oil varies accordingly to the soil or fertilizer used. Some years render better quality oils than others .
Baltic Linseed oil used to provide the best quality and also the Dutch and the South American La Planta Seed. The Indian Bombay is of poor quality.
These days the flax plants are produced world wide but they are not suitable for Artistic purposes because of the special requirements. For Artistic purposes manufactures produce their own.
“The drying “ process of fatty oils is by absorption of oxygen from the air. As it oxidizes it also Polymerizes. (A polymer is composed of two different monomers chemically linked at the molecular level and repeated in chains). It’s molecular structure changes, so that once it has solidified, is quite different from it’s original form. And cannot be changed back to it’s original form. During this process all the oils increase in weight; especially Linseed by 15%.
At first the absorption is small and as the oil thickens, it becomes tacky. Since the drying comes from the outside, the oil in contact with oxygen dries first and forms a skin which slows down the drying process in the under layer. The surface becomes un-even, probably due to the increase in volume, it may form wrinkles, to more or lesser extend.
The oil finally dries, due to the amount of drying properties and forms a skin called ” Linoxyn skin”. Air in motion, light and warmth increases the drying. Quite or still air, coolness and darkness retards it.
After complete drying no more further reaction can be detected , than subsequently a gradual decrease of oxygen and hydrogen and a gradual decrease in volume occurs.
The Linoxen skin which in the beginning was elastic becomes brittle and cracks. In time the fatty oils turn yellow. Darkness and dampness increases yellowing and the yellowing begins as soon as the oil begins to dry.
Since all natural products are uneven in their composition, --thus- Linseed oil and Linseed oil are not always the same thing. Color, fluidity and drying power will vary. Mixtures with other seeds and weeds for example, insufficient care in pressing, storing in barrels or other adulterants are harmful to the oil.
The fluidity of the oil varies with temperature ,warmth makes it more fluid ,cold more viscid. The oil is not totally impermeable as we may assume.
Linseed oils are the subject of many adulterants and are hard to detect. It’s impossible to eliminate them and it is smell, appearance and drying power which will help the violin-maker, make a choice. Fatty oils are a vehicle of the preparation of varnish or pigments for a painter and often cause a problem. It’s the oils that kill the colors. (and therefore we as violin-makers are up against the same)
The different kind of processing.
Varnish linseed oils. This is the class of linseed oils produced for the making of clear varnishes. The first requirement is that it should be clear of “break”.
Steam Pressed Linseed oils renders a bigger quantity but the quality is reduced by the water vapors.
Acid refined Linseed Oil.
This is steam pressed and treated with sulfuric acid to remove mucilage(also called foots) and other impurities and the process improves it’s color. Many variations of this method are in use, using bleaching agents and other chemicals.
‘”Solvent” extraction is the most modern and leaves most impurities behind.
Alkali refined oil ,
This oil is treated with alkali in order to reduce acidity ,than washed with water to remove silts. It‘s mostly used for Artists paints.
is partially polymerized (but not un-oxidized) and made by heating the oil to 300C-575 F.
held at that temperature for many hrs, thereby an internal change takes place and the resulting product, stand oil is not the same as the raw oil. The change is molecular and polymerization .Its heavy and viscous.
It, s also called heat bodied oil, it tends to yellow less and has good leveling properties. The film gives an enamel-like surface without brush marks.
Due to it’s viscosity and low acid value it is not suitable for grinding pigments but when diluted with a thinner useful for glazing .
For satisfactory results, stand oil can only be made by large scale industrial methods from a selected varnish type oil. Formerly it was made in large open style kettles and heated for as long as 18 hrs. and the results were a dark oil, not uniform and partially oxidized.
The modern light-colored polymerized oils have great color stability and not likely to darken colors.
Some polymerized oils are in lighter grades and some in very heavy consistencies.
Sun Thickened Linseed-oil,
This process dates back to the Renaissance. .Equal parts of linseed-oil and water ,are thoroughly mixed and exposed to strong sunlight for a few weeks. The oil that results is viscous, somewhat bleached and a pretty good drier ,its partially polymerized and slightly oxidized and a good leveler.
Cold pressed Linseed -oil,
Is made by crushing the flaxseed under great pressure, and it is considered the purest and most desirable for making oil-paints. It’s color ranges from pale straw like yellow to a deeper golden yellow. Dries comparatively quickly.
Because of high cost and low yield of cold pressing, refined Linseed-oil has largely replaced it. It is used for grinding pigments and in paint films it retains it flexibility, and “embrittle’ s “ less rapidly.
Blown oil, Bodied oil and boiled oil.
Oils and Linseed oils can be thickened or bodied by an entirely different change namely oxidation in combination with oxygen.
Thus the same process as the oil dries by air.
Oxidized oil is produced by blowing air through the oil .
Heavy and viscous they should not be confused with stand oil .Boiled oil is a misnomer since the oil commonly sold under this name is not boiled but heated with driers until very slightly thickened. A great deal of boiled oil is commercially raw oil to which liquid dryers have been added.
Sun Refined or Sun Bleached Oil.
This method dates back to the 14th century or earlier and produced a more rapidly drying product. .It consists of shaking up the oil with an equal amount of water and exposing it in glass jars or tray to out door sunlight for many weeks. The first week it must be thoroughly mixed every day an its not clear how long to do this process since the required time and intended for the kind of purpose used .It depends on the power of the sun, locality and season, type of oil used and size of container.
Gelatinous or aluminous matter is removed by filtering through a cloth. By the end the water is removed by the use of a separatory funnel.
The action of the sun is 3 fold, :it partly oxidizes, partly polymerizes and bleaches the oil. After it’s optimal bleaching is achieved it will turn to a pale straw like color. This treatment increases it’s drying and leveling quality.
Raw Linseed Oil
The usual procedure is to warm the crude oil slightly and allow it to stand in barrels for up to 2 years.
During this time a considerable amount of solid mucilaginous sediment or “foots” settles out and falls to the bottom .Some seeds will purify themselves to a great degree, but some varieties will throw down foots indefinitely. After some partial purification, the steam pressed oil is fairly clear but of a rather dark yellowish brown color, sometimes with a greenish tinge. It’s mostly used in commercial paint and varnishes.
Varnish Linseed Oil
The above discussed oils are mainly used in paints or paint grinding.
Another class of refined oils is produced commercially for the use of clear varnishes.
The first requirement for varnish is that it is free from “brake”.
When linseed oil is heated rapidly to 500F ,a flocculent mass, or cloud of particles forms in the oil, it is said to “break”; oil which remains clear is acceptable as no break oil..
This break contains a large percentage of phosphorus and not to be confused with “foots”.
Another specification is that varnish should have a low acid number
The most common method of refining varnish oil is to use alkali instead of acid in the treatment. This is the type most likely to be used in Artist supply shops.
In absence of cold pressed oil, alkali refined oil is the best for all around use..
The above information I took from:
The Artist Handbook by Ralph Mayer
The Materials Of The Artist And Their Use In Painting, by Max Doerner
The Painters Handbook by Mark David Gottsegen
The Artist’s Assitant by Leslie Carlyle