Notes from The Painter’s Handbook 3: Solvents
(This is the third in a series of blog posts sharing my notes on the fundamentals of building a durable oil painting, based on information in the book, The Painter’s Handbook by Mark David Gottsegen. Please understand that I am not the expert here; I’m putting these notes out there for me just as much as I am for you, the reader. Start with part one, Notes from The Painter’s Handbook 1: Oil painting supports.)
So, what’s the difference between a solvent and a thinner? In a word, nothing. Whatever material you use, it’s technically referred to as a ‘thinner’ when you use it to ‘thin’ or dilute another liquid such as a painting medium, and as ‘solvent’ when using it to dissolve something – flakes of varnish resin, for example. The author uses the term “solvent” when referring to both uses, and I will too.
Turns out there’s a quite a lengthy list of substances that artists can use as solvents, including nasty stuff like kerosene, gasoline, toluene, and many more, but I’ll only be including the two most commonly used by oil painters: turpentine and mineral spirits.
First, some general information on solvents:
- They are usually “volatile”, meaning they evaporate readily, and should leave little or no residue behind.
- Ideally, a solvent’s vapors should be as non-toxic as possible, though nearly all are at least somewhat hazardous in actual use. These are potentially dangerous materials and must be used carefully.
- The solvent should not dissolve already-dry paint layers
- The solvent should not react chemically with other materials or ingredients, or if it does, the reaction should be non-toxic to the artist or harmless to the painting.
- The solvent should mix completely with other ingredients or dissolve substances completely, leaving behind no residue.
- Most solvents are flammable and can be extremely dangerous. “Flashpoint” is the lowest temperature at which a given solvent’s vapors can be ignited by an open flame or spark. A solvent with a high flashpoint temperature is the safest. For example, a solvent labeled “Explosive” can flash at 10ºF (-12ºC) while one that’s “Combustible” has a much higher flashpoint of 100ºF – 150ºF (38ºC – 65ºC). Products labeled “Flammable” can, in fact, be dangerous at normal room temperatures.
- Aside from fire risk, solvents come with considerable health hazards. They must be used knowledgeably. Depending on the product, exposure can occur through absorption through the skin (especially broken skin), by vapor inhalation, or by ingestion. Of these, inhalation is considered the most likely because it can happen unintentionally. Effects can be immediate (acute) or come on gradually, over long periods of exposure (chronic).
- Always follow recommended safety procedures when using solvents, including – but not limited to – proper ventilation, disposal of soaked rags and storage. (See pp 96 – 99 for detailed precautions).
Common Solvents Used by Oil Painters
- Flashpoint is around 95ºF (35ºC): flammable
- Turpentine is made from the sap of pine trees, either the longleaf yellow pine or loblolly pine.
- Gum turpentine is more properly referred to as “pure gum spirits of turpentine”
- For artists’ use, it should contain no water
- Fresh turps are colorless or perhaps a bit yellow and have a very distinct odor
- Turns yellower with age and the odor becomes more pronounced
- Likely to be fresher when purchased from a hardware store where there is more rapid turnover than in an art supply store.
- Leaves behind a gummy residue as it evaporates, but this isn’t considered harmful to the mixtures in which it’s used. Less of a problem when the turpentine is fresh.
- Classified as moderately toxic, though this can vary depending on the species of tree it’s made from
- Always store in closed, opaque containers away from heat and light
- Vapors can irritate the nose, throat, and eyes
- Some people can develop severe skin reactions
- Can cause kidney and bladder damage with overexposure
- Do not use to clean brushes or hands
- Use only for dissolving varnish resin or making an oil painting medium containing damar resin
- Do not use in classroom studios
- Flashpoint is 85°F – 105ºF (30°C – 40ºC): flammable
- Mineral spirits are distilled from petroleum
- Low aromatic content
- Generic term: “paint thinner”
- Unlike gum turpentine, mineral spirits leave no gummy residue after evaporating
- Does not deteriorate with age
- Less likely to cause an allergic reaction than gum turpentine
- Cheaper than gum turpentine
- Mineral spirits “…can replace gum turps for studio operations that do not involve damar or other hard resins or varnishes.” (p. 100)
- Odorless mineral spirits (OMS) are marketed as a safer alternative to the regular stuff, since you can be exposed to the vapors much longer before being negatively affected by them. The author makes an interesting and valid point in this regard, writing that he “consider[s] the loss of the odor warning in these products to be a detriment to the user: We might not know we are becoming overexposed until it’s too late.” (p.100)
- Classified as moderately toxic; highly toxic if ingested
- The above refers to mineral spirits or paint thinners made for the industrial paint industry. Highly refined mineral spirits intended for artist use – such as Gamblin’s Gamsol, which is what I use – are much safer with higher flashpoints and lower toxicity. It is considerably more expensive, however.
When I was doing oil paintings back in high school and college, I always used gum turpentine with no apparent problems. Then I switched to mixed media techniques (no oils) for about 20 years. When I came back to oils, I found that turps gave me a headache so I changed to odorless mineral spirits (OMS). For what it’s worth, I tend to have multiple brushes on the go (for different values or colors) while working, so I don’t have to rinse them as often. My OMS bucket is a one gallon empty paint can (you can buy these in the paint section of any big box home improvement place) with a lid. While I don’t pound it down tight to seal it, the lid sits on the can at all times to keep evaporation to a minimum. I only remove the lid to clean my brushes. If I need small amounts of OMS to “loosen” my paint a tiny bit, I have a paint cup with no more than a teaspoon or so available.
In part 1 of these “Notes from the Painter’s Handbook”, I mentioned I’d been working on a painting 0f a great blue heron that “suddenly” seemed to have weak paint film problems. This issue is what prompted this whole review of my painting procedures; obviously, I’d made a mistake somewhere. Well, now I know what I did. The author writes that when starting a painting, “The usual procedure is to begin by making a loose drawing on the ground…lay out the composition in thin, fluid paint. The thinner can be odorless mineral spirits, but do not use so much thinner that the paint becomes like a watercolor glaze; this could cause later adhesion problems.” (p. 211, bold emphasis mine)
So that’s what caused the problem: I used too much OMS. I do not know what possessed me to even start this painting with thinner only; normally I use a mix of medium and OMS recommended by the manufacturer (Gamblin). For some reason I decided to “experiment” this time. In the area in question, the underpainting was very thin (yep, like a watercolor glaze) so I could see my drawing of the heron through it. Lesson learned. And yeah, I’ll have to start the painting over. Sigh.