Linseed and Tung Oil: A Comparison of Chemical Additives and Offgassing

This article is a comparison of linseed and tung oils: are they non-toxic, what types of chemical additives are used in the different oil types, what compounds do they naturally offgas, and when do they cure and become safe for the chemically sensitive.

I review the following oils: Tried and True polymerised linseed (pure and with a stain), 100% Tung oil, Rubio Monocoat (plain and a blue colour), and Odies Oil.

This post contains affiliate links. Upon purchase, I earn a small commission though affiliate links at no extra cost to you.

The video compliments the post:

Linseed and Tung Oil - Chemicals and Offgassing 

Linseed and Tung Oil are the traditional natural finishing oils for wood - some forms can also work well as stone and concrete sealers. Both of these oils are drying oils which polymerise in the presence of oxygen forming a durable and elastic finish. They do not go rancid like semi drying or non drying oils.

While all of these oils, the pure ones and ones with additives, claim 0-VOC (a legal term), they do give off natural VOCs, whether these are enough to be considered toxic depends on you and your tolerance. The term "non-toxic" means that the toxins are not at high enough levels to do harm to the average person. During oxidation (curing) of these oils, aldehydes and hydrocarbons are produced. Peroxides, alcohols, ketones and acids may also offgas during oxidation (1). I could pick up this odour very acutely. In low concentrations these compounds are not toxic for most people. For those who are comprised, aldehydes especially, can be harmful or cause symptoms.

If you are wondering if linseed and tung are suitable for you and your sensitives be sure to test both in the 100% pure versions, as well as Rubio Monocoat and see how you do. For those who like pure natural finishes and who are not bothered by the natural aromatics of plants and oils, both 100% linseed and 100% tung would be great options. I wasn't expecting to like Rubio Monocoat, the modified linseed, because of the lack of transparency of ingredients, but I explain why it turned out to be my top choice in many ways.

The finishes produce slightly different looks and you should also check the application procedure for the oils you are considering, as well as upkeep

I'm going to compare tung and linseed (including Rubio Monocoat and Odies Oil) in terms of the chemicals added as well as what compounds they naturally give off when curing.

100% Pure Tung 

When choosing a tung oil you are looking for 100% pure tung oil. Not the danish oil mixes from the hardware store or anything premixed with a thinner. My top brands are Real Milk PaintThe Hope Company and Heritage. We want to start with something pure that does not have additional chemicals added.

Tung oil can be applied without a thinner to weathered wood, new wood floors as well as some old wood floors and concrete. Check to see if your application requires a thinner before starting, because the thinners are chemical compounds that may not be tolerable or desirable.

Common thinners for tung are citrus solvent, mineral spirits, or odorless mineral spirits. The thinners can be very hard to tolerate. I describe the citrus solvent as “brutal” - it wipes me out and makes me very sick, it does not dissipate quickly. Just because it's natural does not mean it is safe. If I had to pick one I would use odourless mineral spirits which should dissipate (offgas) quickly, in theory. However, whenever you are applying something yourself things get more tricky. If a non sensitive person is applying the oil you may opt to use an additive that will dissipate quickly and wear protective gear.

100% tung oil is food safe on butcher block, cutting boards, wooden counter tops, concrete counter tops and wooden toys once cured. (To err on the safe side, wait 30 days for it to be cured).

Apart from wood, tung oil can be used on raw slate (slabs or tiles), it should not be used on marble, whether you should use it on granite is up for debate - do a test first on raw granite. It can be used on concrete counter tops as well as concrete floors.

You can use it on metal, in fact cans for food used to be lined in tung. A coat on some types of steel helps prevent rust.

The companies say it normally takes 30 days to a full cure. This means you may be able to pick up the offgassing odour for 30 days if you are sensitive. However in my results at the end of this post I compare all the oils at 30 days to see if I can still pick up the offgassing. Companies claim 95-98% cured within 7-14 days and depending on conditions, it can take up to 90 days for a full cure. There is a statement on wikipedia that tung comes to a full cure whereas linseed continues to polymerise for years. I was not able to confirm that statement about tung after looking through all the scientific literature on tung that I could find, and talking to many companies. In fact my results after 30 days does not confirm this (comparison at the end of the post).

In the literature I did see lots of reference to the idea that tung oil has a superior ability to polymerise due to its a-elaeostearic acid (77–82%) content. In theory this gives it a greater chemical reactivity and excellent ability to polymerise (cure) (2, 3). However, my sniff test at 17 and 30 days revealed more offgassing of tung than all of the linseeds I tested.

The offgassing odours of tung are a little different from linseed. In my assessment, tung is not as pungent when in liquid form, but they both offgas similar VOCs while curing. During oxidation aldehydes and hydrocarbons offgass. Peroxides, alcohols, ketones and acids may also offgas during curing. (1). At the end the two had reversed in my assessment: tung was the strongest at 30 days.

How to apply it instructions here.

Linseed Oil

Linseed comes in three types:
1.  Raw, which is pure, not usually used for wood, as it takes a long time to dry and is often cut with a thinner.
2. "Boiled" which has driers, usually metals like colbalt, or petroleum (which I would want to avoid), this version could be considered toxic if it has toxic metals.
3. Polymerised/stand oil which is pure, food safe, and faster to dry. This oil does not require a thinner. Polymerised is the type you want to use. I like the brand Tried and True because it's pure 100% linseed, and because of their total transparency with ingredients and additives.

The odours of these oils (and the citrus solvent) comes from their naturally occurring VOCs (4). For the chemically sensitive, these two oils have odours that are stronger than some other finishes. The solvents/thinners should be tested separately, are not always tolerable (or needed). The pungent smell of linseed comes mostly from the aldehydes (5). You should see if the pure version works for you. I prefer to use this on applications that do not require solvents/thinners.

Linseed will dry within a few days. But according to the literature, the drying reaction of linseed oil continues for many years even when the oil film seems to completely dry in a few days (7).
You can speed up the offgasing by increasing temperature and decreasing humidity. Raw linseed might be different in this regard compared to modified oils with driers.

Tung oil has milder offgassing compared to polymerised linseed in my assessment at first, but at the end of the 30 days tung had a stronger odour left. Rubio Monocoat (modified linseed with driers), was the exception, it was far less pungent than either polymerised linseed or tung.

Other areas to use linseed - it should not be used on marble or granite. Odies Oil has been used as a concrete sealer which I am quite certain is linseed based, but the ingredients are not disclosed. In the video I discuss why I think Odies Oil is linseed based.

Here are the instructions for application of polymerised linseed oil.

Is Linseed Mould-Prone?

Woodworkers having varying opinions on the two oils, each camp preferring one over the other with some noting caution to linseed as being less mould resistant in certain conditions. Outside in high humidity linseed does not do well. For indoors with regular humidity levels, linseed will work just fine, and I have no trouble recommending it. Tung oil has the advantage of holding up well to water and mildew outside

Brands of Linseed:

Rubio Monocoat

Rubio Monocoat makes linseed based finishes (natural and modified) that smell like honey or lemony incense. It contains a wax component (natural and modified) as well. They claim zero-VOCs but you should test any linseed product before using it. The accelerator which is a part B drying agent, lists hexamethylene diisocyanate as an ingredient (0.5%), but not all ingredients are listed. Because part A (without the drier) takes a very long time to dry, it sounds like part raw linseed to me (though it does not smell like raw linseed). This means Part B is unknown driers and hexamethylene diisocyanate.

It does contain non-aromatic hydrocarbons, I could not get the company to claim no metallic driers, but they do claim no solvents. I will always prefer brands that disclose all their ingredients.

Sensitive folks reporting that they really like this brand is important and caused me to seriously consider it. It turned out to be my top pick based on odours, offgassing level as well as personal tolerance. I was surprised by how mild it was from start to finish, though I did still pick up an odour at 30 days in the plain colour. I was very impressed with Sapphire which I found cured to my nose at 30 days.  Because it's easy to apply, easy to touch up and seemed to have the least offgassing, much lower than I expected, I have to say it is my top pick. Though with reservations, since we don't know everything that is in it.

I would personally be reluctant to use this on large products like floors since it has unknown ingredients and I might not be picking up all of the offgassing with my nose. I would however feel more confident using this on cabinets or furniture, since it tested so well for me personally. I would like the company to confirm no metals, since metals are a major health problem for many of us.

This brand offers unique colours in their stains that no other natural option offers (greys and white). Be sure to sample both Part A and Part B as well as different colours, which for me tested quite different. I tested plain and sapphire.

Rubio Monocoat (Part A & B mixed) takes one week to cure, they say. You could use just part A which takes 3 weeks to dry.

Although I say in the video it is approaching offgased at 48 hours, at one week my sense of smell ramped way up (due to a reaction I had to something else), at this point I could still smell it very noticeably. So the time that it cures will very much be dependent on you and your level of sensitivity.

Tried and True - Polymerised Linseed

Tried and True Java
Polymerised Linseed
Tried and True is my top recommendation for pure linseed with a stain because it is pure linseed and because of their disclosure of ingredients. No driers are used. This company can claim that metals are not a component in their pigments other than what comes naturally from the soil. I really liked the look of the Java stain (pictured), it went on beautifully even though I did not prep my wood properly. That’s going to give you a high quality stain/finish in one. I was really impressed with how the stain looked after my disaster trying to mix my own pigments with hemp oil.

They offer versions with waxes and resins as well, check out the different formulas to see which suits your project.

Other Brands

Fusion is a similar oil and stain in one but they don't reveal all the siccatives (driers). This brand does have other products that I am a fan of, but unknown driers is not one of them.

Oils labeled as teak oil or danish oil from hardware stores are usually a mix of oils like linseed or tung, solvents, and possibly resins, UV inhibitors and mildewcides.

Odies Oil (Universal Finish)

Odies Oil (the Universal Finish) is another oil I tested in the video.

I suspect this to be a linseed based finish. The ingredients are listed as: lubricating oil, drying oil, natural waxes; essential oil. They claim solvent free, that it contains UV inhibitors, and it has the warning for spontaneous combustion (which is seen on almost all drying oils).

I made the following mix which came close to the odour of Odies: mineral oil, linseed, beeswax, carnuaba wax, lemon and orange oil. I did not get the exact match but it seemed pretty close. There is still is another essential oil I think, lavender or similar, something flowery. And there is a different mix of waxes I think, compared to my guess. But I got quite close to their mix, according on my nose.

This tested quite strong to me at first, with the essential oils being overpowering for me in the first 24 hours, but at 48 hours that had faded significantly, and what smelled to me like linseed was then dominant. Those who are reactive to essential oils will likely have trouble with this one and might not find it fades as quickly as I did.

As always, I’m not a fan of using products that I don’t know the ingredients of, we don't know if there are toxins in here, but it did cure faster than some of the others.

They claim 2-3 weeks to cure.

My Test Results

My test results at 24 and 48 hours are in the video.

My testing at 17 days 

  • Rubio 1st place 
  • Tried and True Java 2nd place
  • Odies Oil and Tried and True still quite noticeable odour, 3rd place
  • Tung actually the strongest at 17 days, last place

My testing at 30 days when they should all be "cured"

  • Rubio plain - I can pick up the Rubio plain at 30 days though it is very light. My nose is unable to pick up any offgassing from the Rubio Sapphire - this is the only one that seems totally cured to me at 30 days 1st place.
  • Rubio plain is in 2nd place
  • Odies oil - very slight odour in 3rd place for offgassing levels
  • Tried and True Java in 4th place very slight odour
  • Polymerized linseed - Tried and True plain still offgassing 5th place
  • Tung oil - strongest one. I do not agree that tung oil cures in 30 days, at 30 days I can still pick up the odour of one coat. I kept it at 30C and about 50% humidity. I cannot say it is 100% offgassed. Last place.

Other Options

If these two don't work for you, try hemp oil. I was really happy with pure refined hemp oil. If you want a totally natural purist option, hemp is a good choice. It's not as strong of an odour as the others.

Other finishing oils reviewed in my main post on finishes are coconut, walnut and rosewood. That post is also is also a comprehensive overview of water based 0-VOC and non-toxic finishes for wood as well as stone and masonry.

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