Oil from flax seeds has a variety of applications that range from an omega-3 fatty acids-rich dietary supplement to a wood finish. Fiber from flax is used in the making of textiles, and the golden oil from the seeds is used as a preservative for ropes and concrete. With both indoor and outdoor applications on a variety of materials, linseed oil has been used as a preservative and wood beautifier for centuries.
The oil from the seeds came to be known as linseed oil. I remember my first introduction to linseed oil as the finish applied to the front door step of my home. The oil penetrates the wood and preserves it from damage caused by environmental factors. Later, I came to learn that flax seeds had nutritional value as a dietary supplement, and could be purchased at health food stores.
Flax oil has a nutty flavor and is safe to use in your diet. Even a health authority such as the Mayo Clinic recognizes the value of flax oil and seeds in your daily diet and suggests 1-2 tablespoons of flaxseed per day as beneficial.
However, linseed oil is not healthy. In that form, it contains additives that are poisonous and should never be consumed. In its raw form, though, flax oil is both safe and helpful.
Wood when dry and bare is like a sponge and will absorb linseed oil easily and deeply. The oil will protect the wood generally from liquid absorption, although it is not a waterproofing treatment. For that, some additional layer of protection will be needed, whether it be a polyurethane coat or varnish application.
The knock on linseed oil is its very, very, very slow drying and curing time. With days and days to dry and months to cure, it can be frustrating to the woodworker who has cut, joined, and assembled a lovely piece of furniture and wants to use it right away.
So, let’s talk about that and see what’s at play with those drying times.
In This Article
Raw Linseed Oil vs Boiled Linseed Oil vs Polymerized Linseed Oil
Generally speaking, raw linseed oil will take a week to dry. We say generally because the number of coats, the temperature of the room where the workpiece will dry, and the airflow in that room will impact the drying time. But, for our discussion, let’s call it a week.
It dries through the process of oxidation – exposure to air. A good air flow around the workpiece will allow this oxidation to proceed, and if the air is of low humidity, will lead to a faster drying time. High humidity, though, along with stagnant air flow, will extend the drying time.
Beyond that week’s drying time, there then will be another 2 to 10 weeks for the application to fully cure. Again, the same varying conditions will impact that cure time.
Raw linseed oil is thick and will need to be applied by hand, usually with a brush or roller. It needs to be thinned for a spray application, and spraying may be preferable for larger workpieces like a fence, for instance, or garage door panels.
Boiled linseed oil, on the other hand, is thinner. It’s not actually boiled, though. Raw linseed oil is mixed with oil that has been heated by passing hot air through it, and then further processed by adding metallic thinners to act as drying enhancements.
A boiled linseed oil finish will leave a soft color behind that will darken slightly over time unless acted upon by direct sunshine. It will provide the same degree of protection to the wood, though, and allow the beauty of its natural grain to show through.
While boiled linseed oil does not need to be thinned, it can be without detriment or diminished levels of protection to the wood.
- White spirits. Boiled linseed oil can be thinned with a little white spirits, which is a petroleum distillate. If you’ve thinned boiled linseed oil with white spirits, use it only as a first coat primer for the wood, and use undiluted boiled linseed oil for the subsequent coat(s).
- Mineral spirits. Boiled linseed oil can also be thinned with mineral spirits, also petroleum distillate. It’s a less expensive option to turpentine, which is vegetable-based, and reduces viscosity and drying time.
Boiled linseed oil’s drying time is much faster than the week of raw linseed oil, with only 2-3 days required before a second coat can be applied. Further, curing time is reduced to 30-40 days instead of up to 10 weeks with raw linseed oil.
Polymerized linseed oil also involves the use of heat. Raw linseed oil is heated to a very high temperature, much higher than in the process of “boiling” linseed oil, and this heating takes place in a vacuum. With no oxygen, the heated linseed oil will not combust.
Linseed oil is polymerized for the same reason it is “boiled” – to reduce drying and curing time. Polymerized linseed oil’s drying time is generally shorter than that of boiled linseed oil, even though it has not been treated with metallic thinners as boiled linseed oil is.
Applying Linseed Oil
As mentioned previously, raw linseed oil is quite viscous, and its application is most often by brush or roller. After allowing it to sit for a period of time, excess oil should be removed. Subsequent coats must wait for each previous coat to dry before being applied.
After the final coat (your decision on how many) has been applied, drying and curing times kick in and those 7-10 days of drying time and weeks/months of curing time can drag on before the workpiece can be used.
Thinner versions of linseed oil, whether boiled or polymerized or thinned with additives like white spirits or mineral spirits, can be applied by cloth. On larger workpieces, spraying is another option.
The drying and curing times are drastically reduced using these thinner versions, and these times can be reduced further with strong air flow, low humidity, and a warmer environment. Environmentally controlled rooms used by professionals ensure that air flow, temperature regulation, low humidity, and the drying/curing times are reduced even further, getting workpieces out and used more quickly.
Spraying thinned versions of linseed oil is a smart and time-saving option when treating larger workpieces. Wooden fencing, and even outdoor concrete, stone, or tile patios come to mind. Raw linseed oil is too thick for spraying, so thinned out linseed oil, whether by using boiled, polymerized, or additive-enhanced (white spirits or mineral spirits) are required for the spray nozzle.
While boiled linseed oil fits that bill, it’s also the customary choice for indoor applications – furniture pieces, countertops, and such. It is affordable and environmentally safe (but not food safe), and it is easily applied.
Here’s a video with a general overview of boiled linseed oil. I can watch that piece of wood in the video come to life from the oil application over and over, a very compelling and beautiful grain come to life.
And, a “how-to” video on boiled linseed oil application. Few words, just the view.
The Many Uses of Flax Derivatives
The use of linseed oil in any of its various forms is old. In woodworking, it’s been used for hundreds of years because it works. Woods are protected, preserved, and beautified by it, and ropes, concrete, and even tiles, have been enhanced in the same way.
While early woodworkers used the oil in its raw form, today we have options to make working with linseed oil so much easier and more quickly. Thinning it in “boiled’ form, or with spirits, makes it easier to work with and apply, with the results used much quicker with their shorter drying times.
Linseed oil in all its forms is as widely used as a wood finish as there is in the world today. The flax plant and its seeds continue to be used in a variety of ways.
Although flax oil is not suitable for cooking because its flash point is low compared to other cooking oils, and can produce harmful chemicals when introduced to high heat, it is used as a finisher for cooked foods and dressings. Its natural nutty flavor can be pleasing when used in this way.
I use a cast iron skillet for much of my cooking. I have come to learn that flax oil is an excellent choice for seasoning it. I use other oils to cook with (canola, peanut, vegetable) because of their higher flash point – the temperature at which they begin to smoke or combust.
The flax plant continues to provide much to our use today, just as it did centuries ago. The linseed oil that comes from its seeds extends from that front door step of my youth to furniture, kitchen shelves and counters, kitchen tile flooring, concrete and tile patios outside, and much more.
If you have the time to allow full drying and curing times, use linseed oil in its raw form. But, if you want to shorten those times and put your workpiece to use sooner, go with boiled linseed. And, if the workpiece is large, thin the boiled linseed with spirits.