Sunday, September 18, 2022

Refining Linseed Oil


Linseed oil in mid-wash. The stuff in the middle is mucilage, and you don't really want it in your oil.


Note: This post originally had a section on making varnish. I decided to pull the information because I felt that it was leading people in the wrong direction, and the varnishes I recommended weren't ideal. If you're interested in making your own varnish, please see my post "Making Copal varnish."

To make varnish, you need good oil. The procedure below describes how I prepare oil for varnish making. But this oil doesn't have to be used only for varnish--you can use it as a standalone finish, or combine it with wax for a "soft wax" finish. It will dry roughly twice as fast as raw oil, and will be significantly less yellowing over the long term.

 Refining Oil

The following is a pretty simple method of washing oil. It takes a relatively long time, but requires very little effort. For a comprehensive look at oil washing, see Tad Spurgeon’s book Living Craft, available as a download in the previous post.

 Stuff you’ll need:

  • Cold pressed linseed oil. Avoid most oil sold as flax oil in health food stores, and avoid all bargain basement oils available online. The Swedish cold pressed oils (Ottosson, Allbäck) are very good. I especially like Ottosson, but it has to be raw; do not use boiled linseed oil. 
  • Water. Tap water is usually fine for this method, especially if it’s relatively soft, but if it’s hard it may be too aggressive. If you have difficulty getting the oil and mucilage layers to separate, try spring or distilled water.
  • Canning or kosher salt. Look at the label and make sure it’s just salt, with no extra ingredients like anti-caking additives. Diamond Crystal brand is good and cheap.
  • Screw top jars (Ball jars work great) and a bulb baster.

Washing: Fill a jar three-quarters full with two parts water, one part oil, and a cup of salt for every 16 ounces of oil. Screw the lid on tight and shake vigorously for one to two minutes, then let the jar sit undisturbed, on a sunny windowsill if possible. Perform the shaking procedure three times per day.

After the first shaking, the oil and water will separate within a few minutes. But after several rounds of shaking, a mucilage layer will form in the middle. With more rounds of shaking, the mixture will separate more slowly, and the mucilage layer will grow larger. This may take a few days, or it may take a week: It depends on the oil and the water. If you reach a point where the mucilage takes a long time to separate, change the water. If not, change the water after a week.

To change the water, use a bulb baster to transfer most of the oil to a clean jar. To remove the last couple ounces (you can’t do it with the baster), gently pour water into the jar until it’s completely full. Wait a few minutes for the remaining oil to separate,, then use a spoon to remove as much of the remaining oil as you can. Try to avoid picking up mucilage, but don’t worry if you get a little–you don’t need to be really careful until the final wash.

An alternative method of removing the oil is to simply freeze the jar, then pour off the oil. There are pros and cons to each method: Freezing takes longer and not all the mucilage may freeze, but it’s much less fuss. Jars may crack after repeated freezing cycles, so you need to inspect them carefully.

After you’ve removed the oil, repeat the whole process. The second wash may require fewer rounds of shaking to reach the point when the water needs to be changed. Then do a third and fourth wash, but do not use salt for these last two washes. After the final wash, transfer the oil to a clean jar, being careful to avoid picking up any mucilage. About a third of the oil will be lost during the washing process.

Optional Pre-wash: Before the first wash, you can do a pre-wash with water and vinegar. The vinegar helps the mucilage separate more easily in subsequent washes. To 16 ounces of oil, add 3 ounces water and 1 ounce apple cider vinegar. Shake together for two minutes, allow a few hours or overnight to separate, then remove the oil. 

Safety note: Always dispose of rags soaked with linseed oil safely--there is a small but real chance they could catch fire as they dry. I like to spread them out, weighted down with pebbles, until they dry thoroughly.

Troubleshooting: If the mucilage is separating very slowly, try gently spinning the jar for about thirty seconds. Repeat every few hours. A stubborn emulsion can sometimes take several days to fully separate. If it’s not separating at all, put the jar in the freezer overnight (make sure it’s not completely full, and don’t screw the lid on too tight). Take it out the next day and let it thaw completely; this will break the emulsion. After the emulsion breaks, change the water; if you’ve been using tap water, try distilled or spring water instead.

Clearing: After the last wash the oil is clean, but still turbid due to residual water. The safest and easiest way to clear the oil is to cover the jar with cheesecloth, and let it sit in front of a sunny window for a few days. When the oil is completely transparent, it is ready to use–just decant it to another container, leaving any residual water behind. 

You can also clear the oil with low heat, but be aware that this is potentially quite dangerous. You must heat the oil slowly to the temperature of boiling water (212°F/100°C); do not exceed this temperature by more than a few degrees. If you heat the oil too fast or too hot, the boiling water, trapped on the bottom, may erupt in jets, causing serious burns. You will need to monitor the temperature carefully, and if there is much water, it can take a long time to evaporate. Even when done correctly, this method is messy–the water makes the oil spatter, covering everything within a few feet with a fine, greasy spray. I much prefer the first method.

When the oil is cleared, you will have a raw, refined oil that greatly exceeds in quality anything you can buy. It can be used anywhere you’d normally use raw linseed oil, but it will dry considerably faster. However, there is still one more step before we can use it to make varnish.

Breaking: This step eliminates impurities that cannot be washed out, and can coagulate when the resin and oil are combined.

Stuff you’ll need: 

  • Electric hot plate. For this step, almost any cheap hot plate will work. 
  • Thermometer. Infrared thermometers are cheap, can read very hot temperatures, and don’t require immersion, which comes in handy when melting resins. Immersion thermometers are more accurate, but good ones are expensive and can’t read the high temperatures needed for some resins. Get both if you can afford it; if not, get an infrared.
  • Stirring rod (glass or stainless steel is best). 
  • Saucepan. For oil, and for rosin varnishes, a cheap, stainless steel 1 quart pan will work well. 

Heat the oil (remember, outside, on an electric hotplate) to 450°- 475°F (230° - 250°C), and hold for thirty minutes. If you did not use heat to clear the oil, watch carefully as the temperature reaches 212°F/100°C, and if you see bubbles, hold the temperature until the bubbles disappear. Monitor the temperature carefully; if you go much above 500°F, linseed oil starts to break down, so avoid getting it too hot. You may actually see the “break,” which will appear as an opaque cloud that suddenly dissipates; but you may not–it happens pretty quickly. Safety note: Don't walk away while the oil is heating. The temperature can climb pretty quickly, and if it climbs high enough, the whole thing could catch fire.  

After heating, the oil will dry faster, and it will also appear much darker than before. However, if you put a drop on a piece of glass and spread it out, it will be completely clear. If you apply it to wood, it will look no darker than before it was heated. The color of oil is fugitive; how it looks in a jar has almost no relationship to the color of a thin film.

Congratulations; you're ready to make varnish with this oil, or use it however you like!



  1. Thanks for sharing. As a chemist, I'd be inclined to use a separatory funnel do remove the water directly from the bottom but most folks won't have that. If one had a gravy strainer that may be another simple way to do. That mucilage layer isn't unique to just this oil. I've done thousands of organic solvent with salt solution extractions and almost all of them had a mucilage layer. All the tricks you mention are the same ones I've used over my career to help them separate.

    1. I have never studied organic chemistry. What is going on with the salt water and oil? What is the composition of the muscilage ? This seems like it may be a way to make my own faster drying linseed oil.
      Thanks Dan

  2. Thanks for the comment. For this post, I wanted to keep the equipment and techniques as simple as possible, since most folks don't have experience with it. The funnels are expensive, but you're right that a more efficient way of separating would be nice. I've got a couple gallon jars with metal lids, and I was thinking about trying to install some kind of valve in one of them (and then inverting the jar while it separates, obviously).