How to make sun thickened linseed oil
& understanding other forms of Linseed oil:
This post is for those of you who would like to get to grips with sun thickened linseed oil and the other forms in which linseed oil is available for us to use as artists.
The Artistic Chemist:
Understanding our materials enables us as artists to experiment, within the boundaries set by artists centuries before us. An artist in the 14th Century would have had to create his own paints and mediums from scratch. Many artists were as much a chemist as they were a painter, with a deep knowledge of the materials at hand. Since suppliers now make paints and mediums widely available and ready to use it does mean a lot of this knowledge that can is now too easily neglected. It is this knowledge that can be the difference between a cracked painting and a successful one.
In understanding our materials we can be sure our paintings will stand the test of time. I would also like to learn more about various materials. So with this in mind I will be writing a number of blogs re-creating some differnent production methods at home in order to better understand them. I’ll write them in a way that you should be able to read or watch the video whilst having a go yourselves.
This first post is primarily about making your own sun thickened linseed oil, with some extra information on the different properties and uses of linseed oil in the various states you might see it in at the shops and how they will effect your painting process and finish.
Linseed oil is pressed from the ripe seeds of a flax plant grown in all temperate or cold climates. The seed is crushed and oil extracted from it by pressing in an expeller. To get the most economical results lots of heat and pressure must be applied. This kind of linseed oil is however inferior to that which is best for painting. Cold pressed linseed oil is superior as it does not undergo the extreme heat and subsequent refinement. It therefore remains more resistant to embrittlement during the drying and ageing.
Cold pressed linseed oil:
Cold pressed linseed oil is essentially made using the same process as linseed oil, but with a lesser pressure generating little or no heat.
Properties: Cold pressed linseed oil needs much less refinement, can be filtered and in most cases is the oil used for grinding paints.
Stand oil is linseed oil which has been heated to 525- 575 degrees Fahrenheit or 300degrees Celsius. The molecular change during heating is polymerization, turning the oil heavy and giving it a honey-like consistency. Due to its viscosity it is not suitable for grinding paints. However, when added with various amounts of turpentine it becomes a very useful glazing or painting medium which will almost NOT yellow at all over time.
Properties: A key note for this medium is that it is has levelling property. This means brush marks will be smoothed out. It is slower drying and less yellowing than other linseed oils, creating a smooth and durable & flexible film.
Alkali Refined Linseed oil:
The production of refined linseed oil is a lengthy one. Toasted & dried seeds are rolled, cracking the outer shell and producing a course ‘meal’. This is then steamed before going into the press. The remaining oil is then removed from the meal through solvent extraction using Hexane.
The oil is washed with acid. A process known as ‘degumming’ done by heating the oil and acid in a vacuum at 80degrees Celsius. This hydrates free fatty acids called phospholipids contained within the mucilage linking them to the water and dragging them out of the oil. The acid is neutralised by adding Alkali refinement to the tank. Soaps are created around the mucilage which are then removed through centrifuging. The last process is cleaning by adding a clay to the oil which absorbs impurities. The clay is then filtered out of the oil before the oil undergoes a last advanced filtering system to remove any impurities.
You can use Refined linseed oil to make oil paints and mediums. Although traditionally cold pressed linseed oil coats the pigments better. However depending on the product and brand (which do vary) due to the low number of fatty acids, select refined linseed oils may be less prone to yellowing over time.
Properties: Increases transparency , gloss and flow of paint. can be used in mediums as well as paint grinding
Sun thickened linseed oil:
Sun thickening linseed oil is a method dating from the 14th Century. Having used this for some time I feel I should try and make it myself. It is also becoming less available in shops and my previous suppliers have now stopped selling it. So here I am will share the process of making some lead trays at home. These will then be used to thicken the refined linseed oil under sunlight. Feel free to leave any comments and questions below.
Making the lead trays: Stage 1:
Time needed – roughly 1hr
Stage 2: Leave in the Sunshine
Once you have the trays ready you will need a sheet of glass(slightly larger than the trays). Pour the refined linseed oil into the trays to cover the bottom with a depth of about 1 inch. (correction from video! About 1 inch thick should be plenty to allow the sun to take full effect). Leave in direct sunlight, covered with the glass to stop any debris falling in. 4 sticks on each corner of the tray will be enough to let air flow.
Once a day or every 2 days just whisk the linseed oil like you would some eggs. This allows the air to enter the oil partly oxidising & partly polymerising it effectively bleaching the oil slightly. Depending on where you live and your desired thickness for the oil will depend on how long you leave it. It should take 2- 3 months on average as I understand. You can stop when you have your preferred thickness. So feel free to go ahead and experiment to see which you prefer.
Properties: Sun thickened linseed oil helps create a medium with which will stretch and blend your colours whilst painting without them breaking. It makes it easier to soften edges until they fade away. Sun thickened linseed oil speeds up the drying of all paint and mediums. It will also leave more visible brush strokes is more flexible and may be slightly more prone to yellowing. (Dries faster than Stand Oil)