When using oil colors for painting, it is common to thin the paint for more precise application, glazing, or improved blending. While linseed oil (or another drying oil) can be used for this purpose, the addition of oils and gums in painting can extend the drying time, which may be inconvenient for many artists. Personally, I do not mind waiting, but not everyone are like that.
Typically, artists use turpentine oil or solvent as thinners for their paints. Both substances have a strong odor and can be toxic if used without adequate ventilation. Even if the odor is masked, the toxins remain present due to their intended purpose of quick evaporation to expedite the drying time of the painting. However, for the purpose of this discussion, I will focus on the physical effects on oil painting rather than the toxicity aspect (assuming you are already aware of and accept the risks).
What is turpentine oil?
Turpentine, known by various names such as spirit of turpentine, oil of turpentine, or wood turpentine, is a liquid derived through the distillation of resin extracted from live trees, primarily pines. Technically, it’s a natural product.
When reviewing product labels, it’s important not to confuse turpentine oil with turpentine gum. Turpentine oil is a liquid with a strong (toxic) odor and is used as a thinner, whereas turpentine gum is a solid pine sap. The gum can be transformed into a honey-like consistency by heating and mixing it with oil (such as linseed oil) or by dissolving it in solvents.
In oil painting, turpentine oil serves as a thinner without acting as a solvent that disrupts the bond between the oil and pigment. Turpentine oil, classified as a volatile oil, evaporates quickly, hastening the drying process and facilitating paint blending without compromising the bond. However, overuse of this thinner can weaken the underpainting layer and elevate the risk of peeling, as it diminishes the adhesion between the paint and canvas. It is advisable not to exceed a 50% ratio of turpentine oil to your chosen painting oil (such as linseed or walnut oil) for optimal results.
Pure turpentine oil, without added solvents, should be stored in a dark place (such as a light-protected container) and used within one year from the date of manufacture. Such restrictions and also costs of production led manufacturers to substitute turpentine oil with solvents.
What is solvent?
Mineral spirits are derived from petroleum. They have gained popularity among artists due to their lower cost (compared to turpentine oil), reduced odour, and easier storage requirements. With the introduction of odourless solvents, manufacturers even eliminated the “skull and bones” label to indicate non-toxicity. However, subsequent research revealed that even in the absence of odor, the product remains highly toxic. Anyway, I promised, not to dig here.
Apart from turpentine oil, mineral spirits, or solvents, break the bond between the oil and pigment. This was the original purpose of these products – to effectively clean grease, oil, and other substances from surfaces.
When used with care, solvents appear to function similarly to thinners by accelerating drying time and thinning the paint. However, if solvents are applied in the same manner as turpentine oil, the paint application may become uneven, the painting layer can become excessively fragile, leading to potential cracking, and the adhesion between the paint and the canvas may decrease. Additionally, solvents can impact previously applied layers by removing and slightly damaging them.
So, turpentine oil and solvents are not the same or even similar products. Turpentine oil is a thinner, while mineral spirit is a cleanser.
