Linseed Oils - Basic Properties
Linseed Oil is extracted from the seeds of the Flax Plant, the very same plant that provides the painter with linen canvas fabric.
We have seen already that oils are fatty substances, which have in common the property of drying in presence of oxygen, i.e. they polymerise thus becoming a completely different substance after drying.
Linseed Oil is extracted in a few different ways, of which the most important are by the cold pressing method and hot pressed method. Both methods have been around for centuries, many centuries. In the cold pressed method, seeds from the plant are pressed under great pressure exuding the oil out which after some basic purification and cleansing from seed rind and other impurities, is then bottled. The hot pressing method involves the heating of the seeds which renders them softer allowing a more copious amount of oil being extracted.
Linseed Oil is treated in many ways, more than any other oil and is today the most important oil in use for oil painting. Treatment of linseed oil provides other linseed oils, of which the most important are definitely Sun-Thickened Oil, Stand Oil, Raw Cold Pressed and Refined Linseed Oil. Linseed Oil is also available as an alkyd modified. Such modifications of the oil have an important impact on the properties and drying times of each quality of oil and impart different finishes when applied to painting.
Stand Oil is processed by heating linseed oil with carbolic acid. The heating process needs to be done in a vacuum vessel, so oxygen (the chief element for the drying process) doesn't get in the oil. The main purpose is to thicken the oil, render it out-levelling and reduce yellowing. Stand oil is the least yellowing oil of all treated linseed oils and when applied to painting a picture, it is self-levelling, eliminating brushstroke marks and producing a high gloss film which will look like enamel. It is one of the favourite oils for the production of home and studio-made oil painting mediums, a characteristic it shares with Sun-Thickened Oil. The thickness of Stand Oil is measured in Poise units, the higher the Poise, the thicker the oil. Poise is a standard method to measure viscosity levels in industry, as glass industry. Standards are that normally sold stand oil will be supplied in 40 Poise thickness, but thicker oils are also available, especially in 60 Poise. For painting mediums, 40 Poise thick is fine, but 60 Poise can also be used safely. Stand Oil is a little slow to dry, therefore it is good practice to mix it with a natural resin and turpentine in order to improve its drying rate in oil painting mediums.
Raw Cold-Pressed Oil
Cold Pressed Linseed Oil is on its own a fine ingredient for painting mediums or grinding dry pigments. We have mentioned before the process of extracting the oil from the flax seeds. Some painters prefer it to stand oil. I recommend you use only raw cold pressed for grinding dry pigments.
Refined Linseed Oil
Refined Linseed Oil is widely available and is manufactured by most art materials manufacturers. Such oils are supposed to have been freed from their mucilage matter and other impurities, thus rendering the task of doing the washing in the studio completely needless. The most popular oils are the so-called Alkali-Refined. The oil is heated up to 85 degrees Celsius in a aqueous solution of one or more alkali compounds, usually sodium hydroxide or sodium carbonate. Mucilage and impurities settle to the bottom of industrial tanks or through a spinning programme are drained off. The oil is then treated with caustic soda and finally mixed and washed with just water to remove any trace of the previous treatment. It finally is separated from water. Alkali-Refined Oils are clear, of a pale colour, slow drying but very fluid and "soft". Some refined oils may become rancid, which is not usually the case with linseed oil. Alkali refined oils are also used for grinding oil colours. Pigment is mixed with the treated oil and then milled to disperse pigment particles equally. Colours ground in such oils have a different "feel", less glossy and are not as "buttery" as those ground in pure linseed raw pressed oil. However such finish may be preferred by painters seeking to create modern and contemporary pictures finished with matt synthetic varnishes and waxes, etc.