Drying oils are natural oils that undergo a chemical transformation when exposed to air, changing from a liquid to a solid, durable film. Unlike other oils, they chemically react rather than merely evaporating, forming a lasting, protective finish. This makes them valuable where a hardening, resilient layer is desired.
The Science Behind “Drying”
The “drying” process of these oils is a chemical reaction involving oxygen, not evaporation. This transformation, more accurately described as curing, begins with autoxidation, where oxygen molecules from the air interact with the oil’s unsaturated fatty acids. This initial interaction forms unstable hydroperoxides.
These hydroperoxides then undergo further reactions, leading to cross-linking between neighboring fatty acid chains. This cross-linking process is known as polymerization, where individual oil molecules link together to form a larger, three-dimensional polymer network. This network is what creates the solid, tough, and elastic film that characterizes a dried oil. The more unsaturated fatty acids an oil contains, indicated by a higher iodine number, the faster and more completely it will dry.
The hardening of the oil is an exothermic process, meaning it releases heat. As the oil absorbs oxygen, its weight can increase significantly, with linseed oil, for instance, gaining up to 17 percent in weight. Metal catalysts, often called driers, can accelerate this chemical reaction, leading to faster cross-linking.
Common Varieties of Drying Oils
Linseed oil, derived from flax seeds, is a widely recognized drying oil. It has been a primary component in artists’ paints and industrial applications for centuries. Tung oil, extracted from the nuts of the tung tree, is known for producing a highly water-resistant and durable finish. Pure tung oil is considered one of the fastest natural drying oils.
Other common drying oils include:
Walnut oil, valued for its lighter color and less tendency to yellow compared to linseed oil, though it dries more slowly.
Poppyseed oil, among the slowest-drying oils, often chosen for fine art painting due to its very pale color and minimal yellowing.
Safflower oil, another light-colored oil that dries slowly and is noted for its non-yellowing properties, suitable for lighter pigments.
Oiticica oil, recognized for its quick-drying and water-resistant qualities, serving as a substitute for tung oil in some industrial applications.
Where Drying Oils Are Used
Drying oils find extensive use in various applications. They are a primary component in oil paints, acting as a binder for pigments and forming a resilient layer as they cure. Varnishes also frequently incorporate drying oils, often combined with resins, to create protective coatings for various surfaces.
In wood finishing, these oils penetrate the wood grain and harden, offering protection and enhancing the wood’s natural appearance. Their drying property allows them to form a flexible film that accommodates the natural movement of wood due to temperature and humidity changes. Linoleum, a durable floor covering, is traditionally made using solidified linseed oil, which hardens through oxidation to create a robust material. Similarly, oilcloth, a fabric treated with oil for waterproofing, utilizes the film-forming capabilities of drying oils.
Important Safety and Storage Information
Drying oils, especially when soaked into rags, pose a risk of spontaneous combustion. The oxidation process that hardens these oils releases heat, and if this heat cannot dissipate, it can build up to ignite the material. This risk is present with products like linseed oil and finishes containing metallic driers.
To prevent spontaneous combustion, oil-soaked rags should never be left in a pile. Instead, spread them out flat to air dry in a safe, well-ventilated area, preferably outdoors, ensuring they are weighted down. Once completely dry, dispose of them safely, ideally in an outdoor trash can.
For regular users, professional disposal or storing rags in an airtight, non-combustible container filled with water is recommended before hazardous waste collection. Proper storage of drying oils involves keeping them in airtight containers to prevent premature oxidation. Good ventilation is also advisable when working with these oils.