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Linseed Oil - Its Uses and Limitations

Let the Natural Handyman give you the true "flax" about linseed oil!

Linseed oil, extracted from flax seed, is one of the most useful natural oils. It is used as a preservative for wood, concrete, and an ingredient in paints, varnishes, and stains. As if that wasn't enough, it is also used in soaps, inks, and in the production of linoleum! Note that the first three letters of linoleum are lin... for linseed! (graphic left of flax flowers courtesy Martin C. Michener, Ph.D., Faculty, Landscape Institute, Boston Architectural College).

But for home repair purposes (the reason you are probably reading this article) the preservative qualities of linseed oil are our focus. Just like preventive maintenance software is designed to prevent equipment problems, linseed oil does the same thing for wood surfaces. It preserves and prevents deterioration of woods and/or concrete surfaces in products that we rely on in all the places we live, work, shop, and entertain ourselves... ensuring their quality and longevity is something linseed oil has been doing for years. Let's take some time to talk about this "stealth" maintenance product most people will never use alone, but we all depend on (in its various forms) every day!

Linseed oil is a slow-drying liquid with good preservative properties and water resistance.

 Before the advent of modern preservatives and synthetics, it was commonly used as a stand-alone preservative for wood, natural (hemp) rope, and masonry, as a conditioner for natural boar's hair paint brushes, and as an additive for oil paints. It was also valuable inside as a furniture finish and for wood floors.

It was found that the addition of solvents such as mineral spirits, japan drier, and turpentine would speed linseed oil's drying time, making it a more useful product. After all, what good is putting a preservative on your deck in the spring if you wouldn't be able to use it till the fall?

Which is the 'right' linseed oil for the job... Raw or Boiled?

"Raw" linseed oil is just that... linseed oil mercilessly squeezed from flax seed (graphic left and above courtesy Martin C. Michener, Ph.D., both taken from his new e-book : Botany Everywhere, 4th edition.)  and packaged with no additional additives or preservatives. Raw linseed oil dries very slowly, taking weeks to fully cure. You should limit its use to the insides of wood gutters, chopping blocks, sawhorses, and other items exposed to the elements where drying time is not a consideration.

Slow drying is a mixed blessing. For oil-based paints, slow drying is a benefit, since this allows the paint to "level" itself, giving a smoother finish with fewer brush marks. The best looking paint jobs are invariably oil paint jobs, without question. However, when used as a wood preservative for items that are handled or walked on, such as tool handles, furniture, or wood decks, long drying times are undesirable.

Enter boiled linseed oil...

"Boiled" linseed oil is not boiled. The actual boiling of some oils changes their drying characteristics, true. With linseed oil, though, it is the addition of certain solvents that causes linseed oil to dry more quickly, acting as if it were boiled. This makes it a better product for preserving tool handles, decks, and furniture. I suppose they should have named it "sort-of-boiled linseed oil", or "kinda-like-boiled-but-not-really-boiled linseed oil". You know... to make it less confusing. Uh huh.

Is linseed oil useful for the homeowner?

Despite the claims of linseed oil manufacturers and salesmen, there is a commercial wood preservative that exceeds linseed oil's performance for almost every application. Perhaps the only reason to consider using linseed oil as a preservative is the price.  Linseed oil is less than half the price of a commercial preservative. If I was going to coat a half mile of wood fencing or other non-critical application, I just might choose linseed oil.

There are only two uses for linseed oil for which there is no reasonable alternative. Both involve glazing.

  • As an additive to glazing putty... If your oil-based glazing putty is getting a little stiff, or if the new can you just bought seems too dry (and the store is a half-hour away), add a very small amount of boiled linseed oil and mix thoroughly. If the putty is dried and lumpy, you will not be able to totally restore it by adding linseed oil... it will still retain some lumps no matter how long you mix. But don't use the Cuisinart... yes, I know it crossed your mind!
  • Dry wood in window sash when reglazing... When replacing a window pane, brush or rub some boiled linseed oil on the sash before applying the new putty. Wipe off any excess. You can apply glazing putty immediately. This will prevent the wood from drawing the oil from the putty too quickly, causing an overly dry contact point with the wood and possible premature failure of the putty.  Though you might think that adding extra linseed oil to the putty would give the same result, putty that is too soft will be very sloppy to work with and difficult to smooth.  (Excessive softness is one of the drawbacks of the current crop of latex-based glazing compounds.)

Why shouldn't you pick linseed oil as your first choice in a preservative?

Linseed oil has some negatives. Granted, it has been used with some success for many years. But welcome to the modern world! Today, there are scads of linseed oil-based paints and preservatives that expand on linseed oil's good qualities while overcoming (at least to some degree) its drawbacks.

Some of the problems with straight linseed oil, boiled or raw, are:

  • Sometimes linseed oil can take forever to dry... or stays sticky or doesn't dry at all!!  This is a nightmare situation that happens too often when linseed oil is applied either (1) too thickly, (2) onto damp materials or (3) when the temperature is too cold.  Thinning linseed oil with turpentine can help somewhat, but even with thinning it is important to apply thin, multiple coats but allow each coat to dry before applying the next! 
  • No UV (ultraviolet) light resistance... UV causes more damage to exposed wood than any other factor, destroying wood fibers and setting it up for attack by mildew, fungus, and insects.
  • Linseed oil is mildew food... Many vegetable oils are food products for humans... all vegetable oils are food products for mildew! Linseed oil is not completely denatured, so it can encourage rather than discourage mildew growth.
  • Linseed oil does not harden sufficiently to offer enough resistance to abrasion to be a suitable deck floor preservative... at least by today's standards. Linseed oil has been used for interior wood floors, but it must be waxed for durability! Waxing an outside deck would be dangerous, even if you hang a "Slippery When Wet" sign!
  • Difficult to remove from wood... Multiple coats of linseed oil are gummy and difficult to remove fully for refinishing.

Is linseed oil a "green" alternative to typical wood preservative products?

I have received many inquiries from people who are interested in using "environmentally friendly" products to seal their decks, and want to use linseed oil instead of branded products. Raw linseed oil does not, to my knowledge, contain any dangerous chemicals.  However, any specific brand may or may not be suitable for food surfaces such as cutting boards, wood bowls, etc.  (There are other drawbacks, as mentioned above, to using raw linseed oil.)

Then, there are those dang additives.  Let me quote the warning label from a typical hardware-store can of boiled linseed oil I read the other day...

"Use of this product will expose you to arsenic, beryllium, chromium, cadmium and nickel, which are known to cause cancer; and lead which is known to cause birth defects and other reproductive harm."

These metals are used in conjunction with solvents to improve the drying time of linseed oil... the "boiled" linseed oil I mentioned earlier in this article.  Typical hardware store boiled linseed oil, unlike the linseed oil available in health food stores as an omega-3 supplement, is not suitable for human consumption.  Does this make it "unnatural" or not "green"?  It depends on your point of view. I see no reason not to use this on a fence, but I wouldn't coat a child's playpen or wooden eating surfaces with it!

A more natural alternative is heat-treated linseed oil.  Is it better?  Because heat-treatment is more expensive than adding chemicals, expect the cost to be greater but it's questionable whether heat-treated linseed oil performs better.  So is it worth the extra money?  It again depends on your point of view.

By the time you add a mildewcide... well, you get the picture. So much for all linseed oil being environmentally friendly. Face it... sometimes the most environmentally unfriendly person around is Mother Nature.

I have found one company that offers a linseed oil-based product that is purportedly food safe, as well as a few furniture-finish quality products.  (I'm sure a web search would find more.) None of these are 100% linseed oil, but have additives to improve the product:

Tried and True Wood Finishes

Alright, so you can't eat it.  But what about spontaneous combustion?

You know, like in the Sci-Fi movies when people suddenly burst into flames. Linseed oil dries through oxidation, and the process releases heat.  The faster the oxidation, the more the greater the heat.  A pile of rags or paper towels soaked with linseed oil can actually start burning without warning, leading to the manufacturer's warning that all oil-soaked rags should be stored under water in a covered, metal container, or washed before storage or disposal.

I confess that I have never had a pile of linseed soaked rags ignite, but I have received a number of reader comments that indicate that the problem is real and dangerous.  Click here for my article on Spontaneous Combustion with reader comments. 

A final note about "natural" products...

You would be amazed at the amount of negative and even downright nasty correspondence I've received about this article.  The main sources are companies who sell so-called purified or "natural" linseed oil.  They claim their products resist mildew, have none of the toxic chemicals I cited above, and dry quickly.  I cannot verify or deny their claims since I have not tested their products.  However, I can tell you definitively that without a mildewcide additive any exterior application of linseed oil will mildew, provided the conditions for mildew growth are present... warm, moist, dimly lit... you know, like in a bathroom!

Of course, I shouldn't have to remind you that this is a home repair website, not a nutritional, artistic or antique restoration website.  Our concern is with practical, around-the-home uses.  These other applications use linseed oil in unique ways that differ both practically and aesthetically from simply treating a mile of raw wooden fence.

Though I risk sounding like a broken record... errrr... scratched CD, I must restate that commercial sealants (some of which contain linseed or other drying oils) outperform "natural" linseed oil.  And since these products tend to be relatively pricey, the buyer must weight cost against durability and long useful life.  Let the buyer beware! (For me, I always choose the most durable and reliable, cost be damned!)

As the "Natural" Handyman, I've often been mislabeled to be a proponent of natural solutions to home repair problems.  Wrong.  I am a proud proponent of the best solution(s).  Sometimes, the best solutions aren't totally natural, at least as has been defined by the "mankind is unnatural" crowd.  The fact is that most products with solvents become safe when dry as the solvents have evaporated.  As long as care is taken to provide ventilation during the drying phase, health and safety concerns are minimized.

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