Sunsickened oil is made by placing raw oil on top of water and exposing it to sunlight for a long period of time with air passing through it. It is characterized by strong drying properties, high viscosity, and little yellowing. It has good affinity with water and is suitable as a medium for emulsion techniques such as tempera grassa. Commercially available products are dark brown because they are boiled, but if you skip that process with your own products, you can get clear sunsicundo oil.
Fill a glass container, such as an aquarium, with water and pour a drying oil (such as linseed oil, poppy oil, or walnut oil) so that it forms a thin layer on top. I think it is best for the oil layer to be about 2 to 3 cm thick.
Stir as often as possible to prevent the oil from drying out and forming a film (in the summer, if you don’t stir it at least once a day, a film will form quickly). It is also important that oil and water mix. I’ve also heard of the idea of installing an aquarium pump to constantly circulate water and oil.
Cover with a transparent glass plate to prevent dirt and dust from entering, but to allow sunlight to pass through. At that time, leave a certain amount of space so as not to block the air. Then, place it in a location where it is protected from rain, such as under the eaves, but receives plenty of sunlight. It would be best if you had a sunroom, but it is difficult to do it indoors in a private home because the smell of oxidizing oil fills the room. By the way, the photo below is an example of a square aquarium covered with a pot lid (made of glass). The lid of the pot is curved, so there is a gap for air to pass through, and it also allows for rain to drain. It has a handle, which makes it convenient for later work. It is exposed to the sun for about 2 to 3 months, depending on the season. The longer you expose it, the higher the viscosity becomes, but one of the advantages of making your own is that you can adjust the exposure period yourself.
Be careful not to cause a fire due to the so-called “convergence phenomenon” caused by concentration of light rays. Glass bottles, plastic bottles, etc. filled with water act as lenses, concentrating sunlight and potentially causing a fire if there is anything flammable nearby. It is best to avoid cylindrical containers just in case. After sufficient exposure, collect only the oil. Once the water and oil are separated, you can transfer only the oil on top to a container for storage. Small insects and dirt often get mixed in during exposure, but most of them sink to the bottom of the oil, so you can collect the supernatant by scooping it up with a ladle, but just to be sure, it’s a good idea to filter it.
At first, you can scoop out just the oil with a ladle, but as the oil gets low, it becomes difficult to remove just the oil and leave the water behind. In that case, it may be a little easier to transfer the oil and water to a longer container (such as a plastic bottle cut in half) and scoop it with a small ladle. If scooping becomes difficult, you can put the plastic bottle in the freezer and transfer the oil once the water has frozen. An even simpler method is to scoop out the oil along with the water and pour it into a funnel with a coffee filter on top, allowing all the water to drain off first, then collecting the oil that slowly drips out.
Sunsickened oil reacts inside the container, creating pressure that can break the container, such as a bottle. Boiling can avoid this, but it causes various changes in its properties, such as turning it brown. If you make it yourself and use it yourself, you don’t have to worry about complaints when shipping it, so you can keep it as is. Homemade sun-thickened oil that is not boiled seems to dry even better than commercially available oil. When using unrefined cold-pressed oil, water-soluble viscous substances precipitate and get in the way, so impurities must be sufficiently removed.
