We write a lot about wood finishes, as you might expect. A well-assembled woodworking piece is only half of a project; the finish and presentation are the other half. So, finishing a project well becomes an important consideration.
The quick answer is yes, linseed oil darkens wood and will leave a soft and gently yellowing hue behind as cures, and if the project is not exposed to sunlight will darken a little bit over time.
Our past pieces have spoken of polyurethane as a common finish, as well as varnish and shellac, and we spent a lot of words on one of our strong likes, Rubio Monocoat. You can take a look at some of those pieces if the subjects interest you.
The choice of finish takes into account the workpiece, where it is going in the house, what other wooden furniture finishes will it be among, whether we’ve used lighter woods or darker woods in the project, and perhaps even what we might have lying around in the shop.
The wood grain is also considered. We like finishes that allow the natural beauty of the wood to show, as it adds character to the piece. As a woodworking enthusiast, we want the wood to be the star, with our woodworking skill taking a back chair (pun intended) to it.
We’ve also written in the past about linseed oil, the subject of today’s piece. As we proceed through this article, we’ll point you to some of them.
What Is Linseed Oil and Where Does It Come From?
The flax plant has lovely blue flowers, and a field of it is quite lovely. Its stalks are tall and are used, along with the plant’s seeds, to produce flax oil. In its raw form, flax oil was and continues to be used in diets for its nutritional value.
Fiber derived from the stalks was and continues to be used in the making of textiles, which along with cotton, produces linens. The oil was used as a preservative for wood and rope.
Flax oil, or as it has become known as linseed oil, in its rawest form, has been used for these purposes for centuries, at least. The earliest written records of such use date back to the 6th century CE.
In the United States, linseed oil has been produced commercially since 1793. While the linen produced at that time was its most important and commercially successful product, the oil from the seeds and stalks became valued for its many uses.
Among those uses found was for the meal produced in the processing of the stalks, which became a nutritious value in feed for a variety of animals. The amino acids in flax meal are valued highly by farmers even today.
Today, the uses of flax oil have expanded to include mixing with water-thinned latexes in the production of paints for both interior and exterior use and in the making of linoleum. It also continues to appear in diets for its nutrients and nutty flavor.
In its rawest form, flax oil, or linseed as it is called today, is safe, non-toxic, and can be purchased in capsule form as a dietary supplement prized for its omega-3 fatty acids. It also makes it usable and food-safe where it may come in contact with it. However, linseed oil that is commercially available is not safe and should never be consumed – common additives in its production are poisonous.
Linseed Oil’s Preservative Values
Linseed oil has become a popular and commonly used wood preservative. It fully penetrates wood and offers strong protection against all elements. We wrote of our first introduction to linseed oil when we applied it to the new front doorstep of our home.
It should be used only on raw wood or wood that has previously been oiled. If the wood has been previously treated with paint, varnish, or wax, the linseed oil will not penetrate it. They should be fully removed down to bare wood before linseed oil is applied.
It can be brushed on or rubbed on with a cloth, too, and when applied, it will bring out the natural color and grain of the wood. It does take a long time to dry and cure, though, which has been the biggest complaint against its use.
We applied a refresher application of linseed oil on that front doorstep each year, too. It will penetrate the wood in subsequent applications, and its protection will be extended as a result, as well as keep the wood looking fresh.
Again, though, the problem with raw linseed oil is its long drying and curing time. It will become mostly dry within a week, but then curing may take anywhere between 2 – 10 weeks, depending on the environment in which it is used.
It was the frustration with its long drying and curing time that led to ideas about how to shorten that time. Eventually, “boiling” linseed oil became the best idea, and you can now purchase boiled linseed oil as an alternative to raw linseed oil.
The Values of Boiled Linseed Oil
Actually, the name is a misnomer because linseed oil is not boiled. Rather, raw linseed oil is mixed with oil that has had hot air forced through it. In the course of being processed, metallic thinners are added that act as drying agents that will speed up the drying time for the finished product. So, it’s not technically boiled, but that became the shorthand name for that finished product.
Those additives make boiled linseed oil toxic, and you should never be confused about the two. Raw linseed oil is consumable, boiled linseed oil is not.
Among those additives are naphtha and mineral spirits. Among the metal, additives are cobalt and magnesium. We’re woodworkers, and to be honest, we don’t know the significance of any of this except to say it makes boiled linseed oil toxic and it should not be ingested
But the change in drying and curing time is dramatic. Where raw linseed oil dries in about a week and cures in 2 – 10 weeks, boiled linseed oil will dry and cure in 1 – 3 days, depending on the environment it’s in.
Raw wood is like a sponge and absorbs any liquid it is exposed to, including linseed oil in its raw state and boiled state. But we know that water is wood’s enemy and needs protection from it.
However, while linseed oil will offer some protection against water, it does not make wood waterproof. For that, a top coating is needed, whether varnish or polyurethane. Wax can also be a good choice as a top coat, and the purest in us like this choice. It’s applied by hand, of course, and that tactile pleasure just feels more genuine and authentic – producing a piece well, finishing it well, all by hand.
Will Boiled Linseed Oil Darken Over Time?
Boiled linseed will leave a soft and gently yellowing hue behind as cures, and if the project is not exposed to sunlight will darken a little bit over time. It will protect against light scratches and from environmental changes.
Wood will be less susceptible to temperature swings that can cause expansion and contraction in the wood. Cracks will be less likely as a result, and changes in humidity will have little effect on the wood. The boiled linseed oil will have penetrated deeply enough in the wood to add that degree of strength.
If scratches do occur or you notice a crack, the fix is pretty simple. The wood can be sanded down, and boiled linseed oil can be applied once again.
It will protect wood both inside and outdoors, offering that protection against the sun and rain. A yearly maintenance application is advised, as well, to extend that protection as well as preserve the wood’s appearance and strength.
Boiled linseed oil also has restorative properties and will bring old wood back to life. You do want to make sure that old wood is fully dry, though, because any moisture will be trapped inside the wood and lead to decay and rot.
Boiled linseed oil can also be thinned, even with either mineral spirits or white spirits. The difference between the two:
- White spirits are a petroleum distillate often used as a paint thinner for oil-based paints and to clean paint brushes; and,
- Mineral spirits are a cheap petroleum-based alternative to plant-based turpentine.
For the most part, they can and are often used interchangeably.
Boiled linseed oil that has been thinned will need to be applied with a cloth. After 15 minutes, any excess should be removed, or it will become a sticky surface layer and lengthen the drying time.
If you do use boiled linseed oil that has been thinned, though, use it only as a first coat. Subsequent coats should be undiluted and be sure to allow adequate drying time before adding subsequent coats.
In a recent piece, we wrote about polyurethane, we discussed disposing of rags and cloths used to apply or clean it up. It’s worth revisiting the advice we offered since safety first is an important rule in our woodworking shop.
Spontaneous combustion is the fear, and it has to do with the drying and oxidation process. Cloths and rags used to apply and/or clean up after using boiled linseed oil should be fully dried before you dispose of them. Hang them outside, or as we suggested, lay them out on your driveway, and be sure they are fully dry before you finally dispose of them.
DO NOT JUST THROW THEM IN A CORNER PILE.
What Else Is Boiled Linseed Oil Used For Besides a Wood Finish?
We’ve mentioned a few, like the manufacture of linens, as a dietary supplement, and as a meal for farm animals. Beyond that, though, there are others:
- It’s added to paint to provide color and application enhancement;
- It’s used to clean paint brushes;
- It’s used to polish metals and protect them from rust and corrosion;
- It’s also used to clean leather and act as a softener for leather products (boots, gloves, belts, for instance)
As you can see, it’s a pretty versatile product with value well beyond wood finish and protector.
Polymerized Linseed Oil
Another form of linseed oil beyond boiled is polymerized linseed oil. It’s produced by heating the oil in a vacuum (without oxygen) to about 575 F over the course of a few days.
While it’s being heated, a reaction takes place that increases the linseed oil’s viscosity while at the same time reducing its drying time. This was another of the ideas that came out of frustration with raw linseed oil’s extended drying and curing time.
There are no VOCs (volatile organic compounds) in either. VOCs will create strong odors and can cause irritation to the nose, eyes, and throat, and in some people, will cause an allergic skin reaction, rashes, and hives. When using products that do contain VOCs, plenty of ventilation is required, and respirators should be worn.
Fortunately, raw and polymerized linseed oil contains none, and no such precautions are necessary during its use.
How Does Boiled Linseed Oil Stand Up To Alternatives?
Among the alternatives, linseed oil is compared to both tung oil and Danish oil
Tung oil also comes from a plant – the seeds of the tung tree in southeast Asia. It has been used as a wood preservative for thousands of years, and it has the advantage of drying faster than raw linseed oil. It will not turn to a soft yellow color over time as linseed oil will, and this makes it a good alternative to linseed oil on lighter-colored woods like maple.
Like raw linseed oil, tung oil is friendly to the environment, is non-toxic, and is also food-safe. It does take more coats than linseed oil to fully penetrate wood and should always be used in its 100% purity rather than some tung oil compounds that are mixed with other oils.
Danish oil is a hard product to define as there is no generally agreed-upon composition that identifies it. The name came into use when Scandinavian furniture became popular in the mid-20th century, and “Danish” was used more as a marketing feature than as a specific formulation. It can contain both tung oil and linseed oil, and also varnish, as well as drying additives and thinners. It does provide protection for the wood and gives a nice finish appearance.
Linseed Oil Rag Safety
Just to put an exclamation point on rag safety mentioned earlier, here’s a video on boiled linseed oil rag disposal.
Please be careful disposing your rags.
Linseed oil, whether in its raw state or boiled state, is an inexpensive wood finish that is environmentally friendly and easy to apply. It will give your project a beautiful, well-protected, and professional finish. Yes, it will softly yellow over time and perhaps darken a little, which speaks more to choosing the right wood to use it with (not light-colored woods like maple) than choosing not to use it at all. Don’t let that dissuade you from the beautiful finish it can be for your project.