Solvents in the Shop
July 4, 2013 § 6 Comments
Oil finishes have a lot of advantages. They are easy to apply, easy to repair, safe to use, and perhaps most importantly, they add incredible beauty to wood. In most cases, however, they benefit from being thinned with an oil solvent. As such, the terms “thinner” and “solvent” are used interchangeably when discussing oils. For the woodworker, solvents come in three different flavors – turpentine, mineral spirits, and citrus solvent. While each are chemically very different, they are all soluble in oil.
Mineral spirits is a petroleum distillate, a product of the refinery process of crude oil. While colorless, it has a chemical smell to it that some find objectionable. It has less odor (and less VOCs) than turpentine, however. It is the easiest to find of any the oil solvents, and the least expensive. Turpentine was traditionally used to thin common paints (prior to latex most common household paints were oil-based), but when our petrochemical-based economy kicked into high gear, it was quickly replaced by mineral spirits. So much so that mineral spirits became commonly referred to as simply “paint thinner”. Odorless mineral spirits was developed as a less fragrant solvent to help those with chemical sensitivities, and to better comply with more stringent air quality standards. Odorless is a bit of misnomer – there is still an odor – but it is certainly less than the full strength variety. It is a weaker solvent, however. For wood finishing purposes, there is no practical difference, as odorless mineral spirits is still strong enough to thin drying oils. But if you were going to completely clean and degrease a dirty old engine, you might notice a difference. Odorless mineral spirits can also be almost twice the price of regular mineral spirits.
Turpentine is a natural product, obtained by the distillation of pine resin. Despite its natural origins, turpentine is highly toxic, has a very strong pine scent, and is high in VOCs. Turpentine is the traditional oil thinner. In the black-and-white days artists almost exclusively used turpentine to thin oil paints. Old artist studios still have a permanent pine smell to them from its abundant use. Turpentine is more expensive than mineral spirits – typically 50 – 100% more in price. It is easily found at well-stocked hardware and paint stores, but is less prevalent than plain-grade mineral spirits. As mentioned above, when petrochemicals exploded onto the commercial market, turpentine was largely replaced as the everyday paint or oil thinner.
There continues to be a search for a less toxic oil thinner. One answer has been the introduction of citrus solvent. It consists primarily of citrus peel oil (the active ingredient is d-limonene) mixed with very small amounts of water. Like turpentine, you can consider citrus solvent a natural product. Not surprisingly, it has a citrus scent that can be quite strong for some people. There are many citrus solvent-based cleaning products now in the market. Any product that you see with a bright orange label, or with orange in the name, is probably citrus solvent based. A popular cleaner you will find in the grocery store is CitraSolv, which shouldn’t be confused with pure citrus solvent (CitraSolv is 90% d-limonene and 10% water) . Compared to mineral spirits and turpentine, citrus solvent is the most expensive and the hardest to find. I’ve only ever seen it for sale online.
Outside the realm of wood finishes, an oil solvent is often used as a cleaning agent. Solvents can more effectively remove grime and grease from metal surfaces than water, with the added benefit of not contributing to future corrosion or rust. For a woodworker, however, the differences between the three are minimal. I haven’t done as much experimenting as others, but I have used all three to thin both linseed and tung oils prior to applying to wood and have found no difference in the resulting finish. Turpentine reportedly evaporates a little slower than the other two, but again, the resulting look of the finish is the same.
They also share another characteristic – they all are toxic. There is no completely non-toxic oil solvent. The Real Milk Paint Company touts their citrus solvent as a safer alternative to turpentine or mineral spirits. Personally, I think calling citrus solvent “non-toxic” is stretching it, but its MSDS is certainly more favorable than that of turpentine. But when looking at the OSHA Permissible Exposure Limit (PEL) of the three, it tells a different story. PELs are based on a time-weighted average 8-hour work day, and in this case AIHA is recommending a far lower level of exposure than both turpentine and mineral spirits. A rationale for the low PEL for citrus solvent can be found here. And though mineral spirits has a higher PEL than turpentine, if you compare the NIOSH Recommended Exposure Limit (REL) for the two, mineral spirits actually has a lower REL than turpentine.
|Mineral Spirits (Stoddard Solvent)||500 ppm²||OSHA|
|Citrus Solvent||30 ppm||AIHA|
|¹All PELs shown as time weighted average (TWA) over an 8-hour workday|
|²NIOSH REL for mineral spirits is 60 ppm; NIOSH REL for turpentine is 100 ppm|
|PEL = Permissible Exposure Limit|
|REL = Recommended Exposure Limit|
You could summarize the solvents in this manner:
Advantages: Most readily available, least expensive, highest PEL, odorless mineral spirits has the least odor amongst the solvents discussed here
Disadvantages: Not biodegradable, must be disposed of properly with other hazardous chemicals, regular mineral spirits has chemical odor some find unpleasant, petrochemical-based (for those looking for a more natural product)
Advantages: Readily available from hardware and paint stores, less expensive than citrus solvent, a natural product with no petroleum distillates, pine scent for some more pleasing than chemical scent of mineral spirits, higher REL than mineral spirits and higher PEL than citrus solvent
Disadvantages: High in VOCs, very strong pine scent is unpleasant to some, lower PEL than mineral spirits, must be disposed of properly with other hazardous chemicals, highly toxic if accidentally ingested
Advantages: Citrus odor less harsh to some than strong pine scent of turpentine or chemical scent of mineral spirits, a “natural” product with no petroleum distillates, biodegradable – no special disposal precautions
Disadvantages: Limited availability (likely only online for most), most expensive, lowest PEL, citrus scent may be too strong for some, relatively short history in the market means not all health concerns may be known
Of course you could go without any thinner and use straight oil. Tried and True recommends their products not be thinned, so as to keep the finish non-toxic (the recommendation to not thin the oil has nothing to do with compatibility with a thinner – any oil can be thinned – only to keep the finish non-toxic). Not thinning oil makes application more difficult, however. Oils not thinned are more viscous and will not penetrate the wood as easily. Coats have to be applied very thin and all excess carefully wiped off so as to not create a sticky mess that never dries. Note that while solvents thin oil and make it less viscous, they are not driers. Solvents do nothing chemically to an oil to reduce drying time. Thinned oil will mean less oil is applied with each wipe of the rag, however, and a thinner film of oil will dry faster than a thicker one. Stephen Shepherd, in his excellent book Shellac, Linseed Oil, & Paint, suggests a routine of “fat over lean” when applying oils. Start with a very lean cut of oil, maybe 80/20 or 70/30 solvent to oil ratio, and then gradually reverse the ratios. On the fourth or fifth coat you will have mostly oil with a little bit of solvent.
As mentioned above, I’ve used all these solvents at one time or another and have largely settled on two – citrus solvent and odorless mineral spirits. Turpentine gives me an awful headache at just one whiff – even if used outside – so I no longer use it. I do keep a can of plain old paint thinner around for general cleanup of brushes and restoration work, but I’ll use the odorless variety when mixing my oil finish. I have used the Gamblin brand of odorless mineral spirits (a local company here in Portland), but when you look at the MSDS for this product, it’s actually a form of naphtha (further adding to confusion with these products). I started using citrus solvent a few years back. The citrus scent doesn’t bother me and I like the fact that it is a natural product. The odor is strong, however. If your shop is in your house, be prepared for the entire place to smell like oranges. Whenever I order tung oil or milk paint from The Real Milk Paint Company, I pick up another small batch of citrus solvent as well.
I would be remiss if I didn’t mention perhaps the most important safety precautions when using any of these solvents with oil finishes. Oil soaked rags need to be spread out and allowed to dry before disposal. Soaking them in water first can also help. A wadded up rag that is wet with oil can spontaneous combust and start a fire. This isn’t an urban myth – unfortunately it happens several times a year. Again, there is no completely non-toxic oil solvent. Even though some are lower in VOCs than others, it is strongly recommended that you use any of these products in a well-ventilated space. Preferably, mix your oil and solvent outdoors. If that isn’t possible, open the windows or maybe set up a fan near you. If you are sensitive to any of these solvents, as I am to turpentine, you will know it immediately. Don’t just accept your headaches or dizziness as part of the routine – find another solvent, or find another finish.
If you’ve just skipped down here to the end hoping for a link to someone who knows more about what they are talking about, check out this article by Bob Flexner.