Solvents

Solvents and their hazards is a subject that I've seen discussed almost endlessly online, in workshops, and in my own studio. At first it seems to be a boring discussion of an obvious topic. When you read these conversations, you can become convinced that everyone knows that odorless mineral spirits is a good, non-toxic solvent for oil paint, for example. Turpentine, by contrast, is commonly considered "toxic" and as a result is often banned from public studios. Received wisdom is that mineral spirits is a safe solvent and turpentine is not. In the face of a majority who seem to believe that, I've asked myself  "is that really true?" Further, is there a difference among solvents, especially with regard to making special mediums? And also,what about some of the other solvents out there? There are unusual ones like citrus-based "Turpenoid Natural" (a brand name), oil of spike lavender, and others. Some are touted as natural, implying safety. Of course, crude oil extracted from the ground is natural, too. 

I decided to try and figure out the truth about solvents, mostly using online research, even into special collections such as the National Library of Medicine. This is what I found.

Mineral Spirits

A commonly-used brand of odorless mineral spirits

Mineral spirits is a mix of hydrocarbon molecules--molecules made up only of hydrogen and carbon--generically called alkanes (molecules in straight-chains). A familiar alkane to most of us is "octane," an straight molecule comprising eight carbon atoms that is found in gasoline. The "octane rating" of gasoline is one way of stating its power. Mineral spirits in various forms is a mix of hydrocarbon molecules, some straight chains and some in exotic rings of carbon and hydrogen. Mineral spirits that smell generally include multiple ring structures while odorless ones are almost exclusively the straight chain kind. Mineral spirits is usually clear as water.

Odorless mineral spirits have had the ring molecules extracted, leaving behind straight-chain carbon molecules. A simple ringed molecule has six carbon atoms (a "benzene ring") arranged in a circle. In various combinations of rings and chains, these "aromatic" molecules can be toxic and/or carcinogenic to humans. Odorless mineral spirits (OMS) have had most or all of these extracted, resulting in a clear, odorless liquid comprising straight-chain hydrocarbon molecules of seven to twelve carbons that is commonly used by oil painters and in other situations. The liquid has a low toxicity, mainly as an irritant to skin and mucous membranes in small quantities, but in larger exposures whether inhalation or ingestion can be significantly toxic or fatal.

Other names for mineral spirits include "paint thinner," or "white spirit" (in Britain), and there are various brand names of OMS on the market. Further, a slightly different form of mineral spirits is widely employed in dry cleaning. And jet fuel that's commonly used today is very similar too--kerosene is one name for it--and comprises similarly-sized carbon chains to those of mineral spirits. All of these are products of crude oil. To be clear, mineral spirits are a product derived from petroleum. The molecules come from fractional distillation of crude oil, extracted from the ground.

Many times I've seen comments online that odorless mineral spirits is safer than turpentine, perhaps in part because OMS is indeed odorless. But the safety of mineral spirits implied by many, including manufacturers and re-sellers, is false. In online materials, and in other advertising, some sellers flatly state that their brand of mineral spirits is "less toxic" than other brands of odorless mineral spirits, mineral spirits (the smelly kind), or turpentine since any "harmful aromatic solvent component" is removed. It's true that the aromatics (ring molecules) are well-known to be harmful to humans and animals, but equating no smell to no toxicity is wrong. Hydrocarbons in straight chains can be harmful to humans as well, particularly in high concentrations. The truth is that mineral spirits is not "non-toxic." Mineral spirits are well-known to cause both an acute and a chronic toxicity.  Particularly troubling is a condition called chronic toxic encephalopathy, in other words chronic brain poisoning, that can occur.

Mineral spirits, in my experience, is a reliable solvent. I've used either kind--odorless or not--to dissolve and remove oil paint from brushes; to thin linseed oil; even to make painting mediums. In my hands odorless mineral spirits is fine for cleaning brushes but simply not as good as turpentine for altering paint or making mediums. It's tempting to say that this is because one is a petroleum product and the others come from trees.

In general, I now use odorless mineral spirits to clean my brushes of paint residue when the painting session is over. Sometimes I only use soap. I don't use mineral spirits in mediums or to thin my paint unless forced to do so by location. Quite a number of public studios, including my own local art center, have restrictions on the use of turpentine in unventilated spaces. (Yet none on mineral spirits.)
  
Turpentine

A brand of artist-grade gum spirits of turpentine

The proper name for this solvent is "spirit of turpentine." You will also see it marketed as "gum spirits" of turpentine, meaning that the source is tree sap, or gum, and the traditional term artists use is "turps." Specifically though, the sticky gum from trees is turpentine. Sometimes the thick, resinous gum is used without alteration (eg Venice turpentine) but artist-grade turpentine is distilled from the sap. Spirits of turpentine is therefore a plant product of course. Many other materials used in oil painting are plant-based too. Linseed oil is extracted from flax, for example. And we sometimes use other resins--Canada balsam, dammar, etc. in making specific mediums for painting. When plant products are used together, the resulting mediums seem smoother and work better with oil paints, in my experience.

When I began oil painting, I was taught that turpentine ("turps") is just a paint thinner. My first teachers had me put out some linseed oil in a cup and some spirit of  turpentine in another. The idea was that you started painting by thinning the colors with turpentine to a kind of washiness, gradually moving into thinning the colors bit by bit with the oil in subsequent layers, thus obeying the "fat over lean" principle." As a way of constructing an oil painting, it's not a bad sequence.

Tuprentine pines, tapped for their sap

It took a number of years before I found out that turpentine is actually tree sap, primarily pine sap--and that the solvent is distilled from the sap or from ground stumps and other tree refuse. A variety of pines has been used for many years as the source of  turpentine. The trees are tapped and the sap that is extruded is collected. The collected sap is then distilled, resulting in the clear spirit of turpentine and a resinous residue that is further treated to obtain more of the distilled spirits. You sometimes see "rectified" turpentine in artist grade material, which is another word for distilled.

Spirit of turpentine has many industrial uses including everything from paint thinner to cosmetics. You can easily find turps in home improvement centers and hardware stores, though now it's used less often by builders and paint contractors, who prefer mineral spirits. Cheaper spirits of turpentine can be easily found in home centers and building supply stores, but should be avoided by oil painters for a number of reasons. In particular, this sort of spirit of turpentine made by pressure-steam distillation of wood pulp from stumps and all manner of wood refuse rather than pure tree sap. The quality of the resulting turps varies widely and the mixture of chemicals it contains is considerably different that artist-grade turps. Using this sort of spirit of turpentine in oil painting could be a problem because of the contaminating, compounds. And it usually smells terrible too. Good quality gum spirits of turpentine has a pleasant, piney smell. If you've used pine oil cleansers at home, you probably remember that similar pine scent. Artist quality turps is nearly water-clear with a slight yellow tinge. It feels slightly oily to touch, but in general dries with very little residue. You can check that property by dropping a bit on a piece of clean glass and waiting for it to dry.

As to toxicity, artist-grade spirits of turpentine isn't particularly harmful. Turpentine can be an irritant to skin and mucous membranes, so folks with sensitive skin should either wear gloves or if necessary avoid turps. It is a known respiratory irritant as well, and like all such should be avoided by people with asthma and other respiratory ailments. Some, including me, have gotten a temporary headache when exposed to the vapors in a poorly ventilated space. The headache clears with removal of the exposure. Of course, you can poison yourself by drinking turps in large quantities, but it can even be taken orally (not that I would!) in small doses and used to be employed a century or more ago as a "spring tonic" intended to route out intestinal parasites.

In summary, my experience with turps is very positive. In artist grades, gum spirits of turpentine is superior for making mediums, in my hands, and preferred. Good turps smells good and is best obtained via artist supply companies. Finally, turps is generally safe, especially if your studio is well-ventilated, as it should be.

Oil of Spike Lavender
Oil of spike lavender or "oil of spike" is also a plant-based product, and has a relatively long history of use. According to at least some histories, oil of spike predates the use of turpentine but was supplanted by it long ago owing to expense. This is an exceptionally pleasant-smelling and useful solvent. The source is the spike lavender plant Lavandula latifolia. The oil is a distillate only of that plant but there are other lavender oils, extracted from other Lavandula species. While it's still on the market, oil of spike is significantly expensive even in small quantities, making it impractical for high volume use like brush cleaning. Nevertheless, oil of spike is worth the expense for me, in making certain special painting mediums. I still use oil of spike occasionally, but much less frequently than turpentine.

A favorite painting medium I've used is made of stand oil and oil of spike, mixed about one-to-one by volume. Of course you can vary those proportions, using more or less solvent. Using too little oil of spike means a thicker, stickier and (in my hands anyway) less useful medium. This medium, used sparingly with oil paint, provides a relatively glossy, leveled paint surface. These days most of my work is intended to be more matte, so I've used this medium less and less over the past several years. (I do sometimes sketch using a bit of it to thin my tube paints, just for the wonderful smell!)

Miscellaneous
Citrus oil solvents.
These are sometimes used to clean brushes, as a substitute for turps or OMS. The idea some promote is that these pleasant-smelling liquids are safer because they are "organic" or "natural." These products often contain orange oil, an extract of orange peel, containing a high concentration of a molecule named d-limonene.  Although it's a fair solvent with a pleasant odor, d-limonene is more commonly used in "green insecticide" and in particular to kill ants. Research into human effects is scanty, so far as I know, but the chemical is a known carcinogen in rats. So far as I can determine, citrus oil solvents have no place in oil painting mediums and are less effective in dissolving oils than OMS or turps. In short, these compounds and mixtures may be useful if you need to avoid mineral spirits, but provide no advantages in working with oil paint and making mediums.

Kerosene
Mentioned above, kerosene is still used by some--particularly commercial sign painters and house painters--though probably less and less. For one thing, kerosene is really just a kind of mineral spirits. It may be a bit "hotter" in the sense of cutting through grease, but otherwise the chemical mixture is similar to mineral spirits. Kerosene is also sold as "lamp oil." In my grandmother's day it was called "coal oil." Although it's an excellent solvent, it's also inflammable and full of nasty aromatics. Today there are better substitutes for oil painters.

Denatured alcohol
Not a paint solvent strictly speaking, denatured alcohol is quite useful in the studio. It's chemical name is ethanol, basically the same kind of alcohol found in beer, wine, and spirits, but made poisonous by adding other chemicals. It's clearly labelled a poison and can't be made non-poisonous. In other words, you can't drink it safely. But it's a wonderful way to dissolve oils. In my studio, I use a piece of tempered glass as a palette and clean it with denatured alcohol. The alcohol will strip even dried paint from glass, leaving virtually no residue. Alcohol dries by evaporation. I keep a squeeze bottle of denatured alcohol near the palette, and as paint become used up or dry, I squirt a bit onto the glass, wait a bit and wipe it off. I also sometimes use a bit in a rag or paper towel to clean palette knives and the like. And of course, ethanol is also plant-based.

Using Solvents 
Solvents should be used sparingly.

Over the years, I've seen people use enormous volumes of various solvents when a small amount would have sufficed. In some studios, brush washing cans set out with OMS often are left uncovered. That's a bad idea since OMS evaporates quickly--quicker than turps--and so can contaminate the air in an unventilated space. Granted, the exposure you get even in an unventilated studio is probably not enough to cause injury, but long-term exposure in high concentrations could be harmful, so why tempt fate?
Another way I've seen solvent overused is constant rinsing of brushes. Some painters rinse brushes thoroughly between each stroke, or between color changes, so the brushes are exposed to considerably more mineral spirits or turps. The result, I suspect, is of such exposure is damage to the hairs or fibers and shortened brush life. In my experience, using a separate brush for each color, wiping off excess between strokes rather than rinsing, and cleaning only with small amounts of solvent before washing with soap is a better sequence of use.

Summary
For me, the best all-around solvent for oil painting is gum spirit of turpentine. My reasons are first that turps melds with oils and resins to make wonderful mediums while mineral spirits don't seem to make the same lovely mixes. Second, turps is a renewable, plant-based resource unlike OMS and other petroleum-based materials. Oil of spike and denatured alcohol have the same positive advantage.

As to "toxicity," solvents can all have certain negative effects. Mineral spirits can cause chronic brain poisoning besides being a respiratory and skin irritant and poisonous when taken internally. Turps can also be a skin and respiratory irritant and can be poisonous taken internally in anything more than tiny quantities. Turps also can cause significant headache in unventilated spaces.

Finally, consider that studios should be well-ventilated, which will significantly reduce exposure to vapors of any of the solvents mentioned. Overusing solvents will very likely produce more trouble, both to brushes and materials and to one's health.