Why oil paints separate?

Most oil paints separate and there are many reasons for that, good and not really. There are few myths around it, that I (Elena Nahum Leroy) am going to start with. I’m working with oil paints as an artist, tester, seller and manufacturer, so I have a good chance to get full oil paints experience from different sides.

Myths about oil paint separation

Myth: Oil paints separate because they are old

Truth: Separation in some oil paints can start as soon as in a few days after they are made. 

Myth: Oil paints separate because pigment is heavy and it sinks to the bottom of the tubes

Truth: It’s partially true. Some mineral pigments such as titanium dioxide, cobalts etc. are really heavy and it’s a challenge to keep them “floating” in the oil without “sinking”. However, in a modern paint technologies a lot of pigments are manufactured and in fact much lighter than oil and separation still happens there. 

And now, real reasons…

Oil paint separation from deep inside

Oil is a quite complicated medium. I’d say it’s the most sophisticated art medium that I’ve ever used and tried. Apart from most of other mediums, painting in oil requires knowledge of chemistry and painting technologies to succeed. History knows so many examples of oil paintings made even by great and famous artists that were ruined with time because of wrong application. We’ll talk about that too, in other articles. Now, back to the point.

Generally speaking, oil paints have binder (oil), pigments (natural or synthetic), most likely will have fillers, and may have gums, solvent, beeswax or other additives. And separation can be caused by reaction between any of these components. So, even at this point it’s clear that everything is much more complicated than it seemed before. We’ll cover just most common (not all) reasons for separation in oil paints.

Wrong pigment/binder proportion

If you mix pigment with oil that wouldn’t be enough to create good oil paints, because the pigment is not connected to oil yet. We need to apply pressure and physically bind pigment and oil together with a muller (hand-processed) or a roller machine (industrial method). Some pigments require 5 or even 8 cycles to bind with oil properly. And here we have the first reason of separation.

Different pigments have different oil absorption rate. For example, titanium white takes up to 30 g of oil for every 100 g of pigment, while pyrrole red absorbs up to 70 g for every 100 g of pigment. That’s their normal absorption rates. 

However, even the same pigment may have different grit for a reason (read about that in our special topics). And finer pigments require more oil to fill gaps between finer particles to create proper binding and make paints look and feel smooth and even. But absorption rate remains the same, it’s chemistry. So, in fact we have to add more oil than needed and then we have to remove this extra oil. Else, extra oil will separate from the paint with time.

Poor grinding

Another very common reason for oil separation is poor grinding. Different pigments require different number of grinding and milling cycles and even if oil-to-pigment proportion is observed well, insufficient time of grinding may cause quite fast separation.

Weight of pigments

Some pigments are much heavier than oil. These include titanium dioxide, cadmiums, cobalts and some mineral pigments extracted from semi-precious stones, such as lapis lazuli or amethyst. With no fillers added they will separate naturally and this doesn’t mean the paints are bad. However, to create a better look, these pigments will be most likely “diluted” with fillers to create smoother consistency that, when properly done, will technically prevent separation.

Fillers

However ,the leadership in separation causes belongs to fillers. Oil naturally makes colours look brighter that creates a temptation to add a paste, colour it and sell as a tube of oil paints. This temptation goes stronger, when legislation doesn’t  to disclose the full list of ingredients. As a result, some paints (student grade) have just 3-5% pigment load, for example. 

Barium Sulfate is most popular filler for oil paints due to its low cost and physical properties. First of all, it’s very heavy, which makes it the perfect adulterant for oil paints. Due to its weight Barium Sulfate make tubes feel heavier simulating a high pigment load. Also, its oil absorption rate is just 11, meaning for 100g of the powder we’ll need just 11 g of oil to create a good mixture. So, added to synthetic pigments Barium Sulfate will take extra oil and make oil paints look thicker, imitating higher pigment concentration. That’s another thing that buyers love – thick paints.

However, this trick also have a flip side. Barium Sulfate is very heavy and settles down dragging pigment with it and causes separation. 

So, there could be many reasons why oil paints separate and separation is not always a sign of old or poorly made paints. Test paints, compare them and choose the right ones that work better for your goals. Price here is not always the main indicator.