This applies whether you are going to make your own heat-treated oil or not.
Linseed oil is used in normal, household decorating as an oil that is either used on its own to seal wood or as part of paints and varnishes. It is therefore likely to get on clothes or overalls which are then washed and put into tumble driers. Linseed oil normally reacts with oxygen in the air but when it is heated, this oxidation process proceeds so quickly that the heat it gives off can build up to the point whereby it will spontaneously combust - this is called 'pyrophoric'.
The temperature that this happens at is below that used in tumble driers so a cooling cycle is added to the end of the normal heating cycle on tumble driers - this lasting around 17 minutes on driers I have seen. If clothes still contaminated with linseed oil are removed from a tumble drier without having gone through the cooling cycle, enough oxygen will be absorbed by the linseed oil for it to start a fire. Always try to make sure that clothes you put in tumble driers have not got linseed oil or other drying oils on them, that they have been washed properly, and let the drier go all of the way to the end of the cooling cycle to allow the clothes to cool properly.
Note that this is contaminated clothes were the contaminated fibers give the oil a large surface area to mass ratio - this scenario is not likely in a pot of oil, such as a saucepan.
Anybody who has been oil painting for a while will be familiar with this stuff - it is linseed oil that has started to polymerise and if, instead of throwing it out, you mix some of it in with the paint you are painting with, it will dry quite a bit sooner (a day or two instead of a week or two, depending upon a lot of other factors as well - this stuff will get your rose madder paint to dry in around two days instead of the usual two weeks, for instance).
So, if this is so useful, is there any evidence that is was used by artists years ago and how do we make it?
Analysis of paintings by renaissance painters such as Jan Van Eyck and so on shows that this was used by them and it has lasted quite well. You can get linseed oil that has been treated in the high street today - usually used for painting on outdoor wood such as fences and so on - but you don't know what is in it and there can be surface active agents (surfactants) and siccatives (chemicals, usually organometal compounds, that change the rate of polymerisation of the oil) that will mess up your painting (breaking down lower layers), might react with pigments, might cause metal soap formation and so on. In other words, you really need to be able to make your own using ingredients that you can trust.
So, we are looking at using food-grade, cold-pressed linseed oil that we can change using heat.
On the left is the starting material that I used. You can get an organic version of it if you like but the non-organic version will do fine. With it, I treated it with heat and then left it around at room temperature for a fwe months, exposed to the air.
Note that the beautifully gloopy, quick-drying stuff you end up with in your palette pots has been exposed to normal room heat and air and is produced over a month or so although this is new linseed oil put into a palette pot that has already had some of the gloopy stuff in it. It might be that a little bit of gloopy oil that is left will polymerise the new oil.
There are claims on the internet, by people who claim to be artists, that if you heat up linseed oil, it will explode although no evidence is provided that might suggest that this has ever happened. Whatever they have heated up, it is not linseed oil. It might explode if you mix it with some powerful oxidiser and heat it up to some spectacular temperature but nothing like that will happen at the sort of temperature that we are going to heat it to.
If you look at the Material Safety Data Sheet (MSDS) for linseed oil, it says that the boiling point is greater than 315.6C - it clearly doesn't explode at that temperature - and the MSDS, perhaps more importantly, also states that the 'closed cup flash point' is 206.1C. As an analytical chemist, I used to do closed cup flash points on samples several times a week.
The closed cup flash point temperature is determined by slowly heating up a sample, usually in a water bath or an oil bath so that the sample container doesn't come into contact with a flame or other heater thus avoiding localised hotspots that could lead to false results. The container consists of a small pot with a lid that seals but has a small trapdoor that when opened, points a small flame into the air/vapour mixture inside. If there is enough vapour in that air, it will flash - there is only around 5-20mls of air so it is not hazardous if you do the test normally.
The procedure is that the sample is put into the clean container, the lid put on, the small flame lit and the sample is heated gently using the water/oil bath. With the temperature of the liquid monitored continuously, a lever is pulled and the air volume is opened up and the small flame is dipped into the volume. If it flashes, the temperature is noted and the sample allowed to cool down.
This represents the lowest temperature that will support a flame and it eliminates enough variables such as draughts and so on for a consistent and meaningful result to be obtained. When we heat up the oil, we are not making a closed cup as it is open to the air so if we keep it below the closed cup flash point, it will not represent a fire hazard.
Here, you can see the oil heated up - not the the sort of confined space that the closed cup determination produces - to its flash point in a normal kitchen, non-stick saucepan (remember that this is a food product). I turned down the heat to maintain around 200C for around 20 minutes. The recommended time for this temperature is around an hour but I started to get moaned at about the smell which I would argue is no worse than a chip pan. So, after around 20 minutes, I let it cool down and then poured it into a contained and left it for a month. This had the effect of increasing the viscosity to the gloopy level.
In Il Libro dell'Arte by Cennino d'Andrea Cennini, it suggests that during summer, you can put linseed oil into a bronze or copper pan and leave it. The reactive metal surface might be what sets the polimerisation reaction going as could the metal of the inside of the artist's palette pots.
All images and original artwork Copyright ©2019-2020 Paul Alan Grosse.