Linseed oil is the binder in most oil paint. It is pressed from flax seed, and when it is exposed to air it does not evaporate like a solvent, it oxidizes and hardens into a tough transparent film with the pigment locked inside. That reaction is what makes oil painting possible, and it is why an oil painting takes days to months to dry rather than minutes.
Artists also buy linseed oil separately, as a medium to add to paint straight from the tube. This covers what that addition actually does, which type to buy, and the two rules that stop it going wrong.
Read this first
Artist's linseed oil is not a food, a supplement or a skin treatment. It is a studio material, it is not produced to food-grade standards, and some varieties contain added metallic driers that are toxic to ingest.
Culinary flaxseed oil, sold in health food shops and refrigerated, is a different product processed differently and sold for a different purpose. The two share a source plant and nothing else that matters. Do not swallow the contents of a bottle from an art supplier or put it on your skin, and do not assume anything written about dietary flaxseed applies to the tin in your studio.
What adding linseed oil does to paint
Paint from the tube is formulated to be stiff. Adding oil changes four things at once:
- It thins the paint and improves flow, so the brush drags less and strokes level out.
- It slows drying further, extending the time you can blend.
- It increases gloss and transparency, which deepens colors and is what glazing depends on.
- It makes the film more flexible, and more prone to yellowing.
Note that oil and solvent do opposite jobs. Turpentine or odorless mineral spirits thin paint and speed the early drying up, because they evaporate. Oil thins paint and slows drying down. Which you reach for depends on where you are in the painting, which brings us to the rule.
Fat over lean
This is the one rule that matters and it is the one the internet most often leaves out. Each layer of an oil painting must contain more oil than the layer beneath it. Lean layers first, fat layers on top.
The reason is mechanical. Oil-rich paint stays flexible for a long time and moves as it cures. Lean paint sets harder and sooner. Put a lean, fast-setting layer over a fat, still-moving one and the top layer cannot follow the movement underneath, so it cracks. This can take years to appear, which is why people get away with breaking the rule long enough to recommend it to others.
In practice: thin the first layers with solvent and little or no added oil, then increase the proportion of oil in each successive layer. The final glazes carry the most oil. If you only remember one thing from this page, remember this one.
The types you will see on a shelf
Refined linseed oil
The general-purpose choice and the one to buy if you are buying one bottle. Pale, moderate viscosity, dries in roughly three to five days depending on the film thickness and the pigment. Refining removes the impurities that would otherwise darken the oil.
Cold-pressed linseed oil
Pressed without heat. It is slightly more viscous, wets pigment particularly well, and many painters consider it to give a tougher and more flexible film. It is more expensive and slightly more yellow. It is entirely compatible with ordinary oil paint, despite claims to the contrary.
Stand oil
Linseed oil heated without oxygen until it thickens to the consistency of honey. Thick, very pale, slow to dry, and it levels beautifully, so brush marks flatten out. It yellows less than the other varieties. This is the one for smooth glazes and for anywhere you do not want the brushwork to show. It is usually thinned with solvent before use because it is too thick to brush neat.
Sun-thickened oil
Similar idea, thickened in sunlight instead. Faster drying than stand oil and a little more yellow. Less common.
Drying or boiled oil
Linseed oil with metallic driers added to accelerate curing. Useful in small amounts in underlayers. Two cautions: use it sparingly, because heavily dried films become brittle with age, and do not substitute hardware-store boiled linseed oil, which is a wood finish with a different and less refined drier package.
Not a thing
There is no such product as corn linseed oil. Linseed comes from flax; corn oil is an unrelated cooking oil that is not a drying oil and will never harden. If a source lists it, that source is not reliable.
Yellowing, honestly
All linseed oil yellows over time. This is a genuine property of the material, not a defect in a particular brand, and it is most visible in whites, pale blues and grays. Two things make it worse: an oil-heavy film, and storage in the dark. Yellowing from darkness is partly reversible, and a painting that has yellowed in storage will often lighten again after some weeks in indirect daylight.
Where it matters, painters swap the oil rather than the technique. Poppy oil and safflower oil are much paler and yellow far less, so they are traditionally used for whites and light tints. The tradeoff is that both are slower to dry and give a weaker, more crack-prone film, so they are used for the light passages rather than throughout. Walnut oil sits between the two, paler than linseed and stronger than poppy.
Mixing a simple medium
The classic studio medium is linseed oil thinned with solvent, commonly one part oil to two parts turpentine or odorless mineral spirits, sometimes with a resin such as damar added for gloss.
They combine immediately. Oil and solvent are fully miscible and a few seconds of stirring is all it takes. Any instruction telling you to leave the jar for days waiting for them to blend is describing something that does not happen.
What not to do
Do not coat a whole canvas in linseed oil before you start painting. This gets recommended and it is a genuinely bad idea. It gives you a slick, non-absorbent, oil-saturated surface for the first layer to sit on, which is fat under lean, exactly backward. Paint applied over it grips poorly, can slide, and can wrinkle as it cures.
The technique this is a garbled version of is called oiling out, and it is remedial rather than preparatory. When a passage of dry paint has gone matte and sunken, you wipe the thinnest possible film of oil over that area with a lint-free cloth and buff nearly all of it off, which restores the color depth and gives the next layer something to bond with. It is applied to a dry layer, in a tiny quantity, and wiped back. It is not a primer.
Do not overdo it generally. Too much oil gives a film that stays soft, wrinkles as it skins, yellows heavily and attracts dust for weeks. Most paint needs far less than people add.
Safety: oily rags start fires
This is the part of oil painting that genuinely deserves care. Linseed oil cures by oxidation, and oxidation gives off heat. A crumpled pile of oil-soaked rags insulates that heat instead of letting it escape, the temperature climbs, and the pile can ignite on its own with no flame anywhere near it. Fire services issue warnings about this every year, mostly to decorators rather than painters, but the chemistry does not care which you are.
Two safe methods:
- Lay rags out flat and singly on a non-flammable surface in a ventilated place until they are stiff and dry. Once the oil has fully hardened the risk is over and they can go in the trash.
- Or soak them in water, seal them in a bag or an airtight metal can, and put them outside.
What you must not do is drop a wad of oily rags into a bin and close the lid on them.
Does any of this apply to a paint by numbers kit
No. Paint by numbers kits, including ours, use acrylic paint, which has a completely different chemistry: an acrylic polymer binder carried in water. There is no linseed oil in them, nothing to oil out, no fat over lean, no fire risk from the rags, and adding linseed oil to acrylic will simply stop it working. Egg tempera works to a different rule again, and what the yolk actually does is worth knowing before you try it. Water mixable oils keep most of that behaviour while dropping the solvents.
If you want to thin or extend acrylic, the tools are water in small quantities, or an acrylic flow improver, retarder or glazing medium. Our guide to the basics of acrylic painting covers those, and the glazing technique guide is the acrylic equivalent of what oil painters use linseed oil for.
Ledgebay kits are acrylic on pre-primed cotton canvas, rolled in a tube rather than folded, with vacuum-sealed paints, brushes in a range of sizes and an 11 x 17 reference print. You can see the range in the catalog, or turn your own photograph into a numbered canvas with custom paint by numbers from a photo. If you are curious about the layer underneath the paint, priming a canvas for oil paint covers grounds in detail.
