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Oil Colour Mediums Explained

Date: 01-04-2019
News, Tips & Info

Oil colour is a very different paint to acrylic. Acrylic paints and mediums lose water content as they dry, shrinking up to 35-40%. Almost all acrylic mediums can be used on their own, as they are made from acrylic resin and solids and dry to a durable film. Oil colour dries by taking in oxygen and in doing so can increase in volume by up to 15%. Oil painting mediums cannot be used alone, and so are correctly termed additives. These two factors can cause issues such as cracking, sweating, and poor adhesion if ignored. 

Oil colour is made from pigment and binder. Pigment, being the colour component, is the most important aspect of paint. The binder is necessary to maintain the colour on the surface in a stable film. Good quality oil colours, such as the Schmincke ranges I use, provide an optimum combination of pigment and binder, producing brilliant colour in a very durable film.

Using colour straight from the tube results in a very stable paint film with little risk of cracking and with excellent adhesion. It takes Schmincke three to four months to produce their oil colour, so this is the optimum form for oilpaint. Oil paint is meant to be a paste. There are very few examples from the Masters of the past of painting with runny colour. Sir Joshua Reynolds was known for painting with a “sauce”, but he was also known by his contemporaries as “Sir Sloshua Slosh” and many of his paintings have not lasted the test of time.

Sometimes, however, it’s good to alter the characteristics of your oil colour – to speed or slow the drying rate, loosen the paint up for blending, decrease the viscosity for a nice drag, and so on. This is done with a range of additives referred to as oil painting mediums.

It’s important to remember that the key part of your paint is the pigment – the binder is already present in an optimal proportion, and unbalancing this too much can lead to problems with the paint film. Schmincke solve this problem by producing highly concentrated mediums that add certain characteristics to the paint with just a few drops. They recommend an addition of no more than 20% to your paint. It is interesting to note that in Renaissance times, addition of medium was measured in nuts & seeds – very small amounts indeed!

Just a few drops of Schmincke medium are enough to alter your paint’s characteristics. Use your palette knife as a dropper. Above right is what 4 parts colour, 1 part medium looks like – the optimum ratio for most mediums.

Using mediums can greatly expand your painting possibilities, and to fully utilise them you need to understand the simple process by which oilcolour dries.

In many paint brands, different colours dry at different rates. This can lead to cracking if a fast drying colour is applied over a slow drying colour. The top layer dries, but the underlying layer continues to expand, shattering the top layer as it does so. Schmincke address this with unique formulations that allow oilcolours that dry uniformly: Mussini Resin-oilcolour, where the absorption of oxygen is balanced by contraction of the resin, and the more modern Norma Professional, where drying is controlled with the balanced use of various plant oils. 

Colours that traditionally dry quickly include: umbers, sienna, Phthalo blues & greens.

Colours that traditionally dry very slowly include: whites, reds, ultramarine, black.

This danger of cracking has been avoided traditionally by either underpainting in fast-drying colours like the umbers; adding a little solvent to your first paint layer; or adding more oil to successive paint layers (described as “fat over lean”), as a more oily layer will dry slower than a lean layer.  Nowadays, it is also easy to add driers to slow-drying base colours to speed them up. 

The most important medium in oil painting is primer. While acrylics will stick to any grease-free surface, oilpaints require a well-prepared surface. Choose a high quality gesso and apply several layers, even over “pre-primed” canvas. Schmincke Primer 1 is a concentrated half-chalk primer that provides a smooth, sandable surface like polished plaster. On a well primed surface and using a bristlehair brush, you can easily apply colour directly from the tube.

It can be very useful to mix a medium into your first paint layer, however, either for a thinned effect or to speed the drying so you can layer quickly. Always use a palette knife to mix your medium thoroughly into the paint.

 

 

Different mediums for different stages: just a small addition of Mussini Medium 1 thins the paint for underpainting (left); Rapid Medium speeds drying for quickly working up layers (middle); while Transparent Painting Medium adds incredible lustre to final glazes (right).  

If you want a thin, lean first layer, then a solvent-based (lean) medium works well.  Solvent by itself is not so good for the paint layer – solvent breaks down the oil binder, and weakens the paint film. Too much solvent will result in a chalky, unstable paint film which subsequent layers won’t bond well to. Follow the good rule-of-thumb maximum 4 parts paint, 1 part medium. 

Mussini Medium 1, made from resin and safflower oil dissolved in mineral spirits, will retain the film integrity and lustre of your oilcolour. If using over several layers, reduce the amount of Mussini Medium 1 in each successive layer so you’re painting fat over lean.

If you want to produce a “wash” or a dribbly paint effect that demands a drastic dilution of the paint with solvent, you could consider using a much healthier option: 

Schmincke Medium W is an exciting oil painting medium that can be added to any oilcolour to make it water-soluble. No harmful fumes: mixing 1 part Medium W into 2 parts oil colour and you can now thin your paint with water, either a lot or a little, and rinse your brushes out with water!

Medium W is perfect for artists who currently use turpentine or other solvents in their painting, or to achieve thin passages of paint or dribble effects in layered painting. It can also be used in place of linseed oil to loosen the paint for blending, and instead of fast-drying alkyd resins, as Medium W dries quickly too. Oil colour diluted with Medium W and water can also be used over standard, dry oil paint, and vice versa.

If you really insist on using solvent, do be aware that vegetable turpentine and mineral spirits are cumulative poisons. Always have proper ventilation when using solvents and avoid skin contact. It is better for you and your paint to use low or non-aromatic mineral spirits, though these should still be regarded as highly toxic.

Mediums called alkyd resins are fast-drying, adding gloss and altering consistency, excellent for quick underpainting and for quick layering of colour.

Medium L is an alkyd resin with a thixotropic character for improved flow and drying. Thixotropic mediums become thick and gel-like when they’re left to stand for a time, yet when agitated turn more liquid. This allows a fine degree of control over the viscosity of the oil colour. Medium L allows the painter to either gently fold the medium into their oil paint to preserve some structure, while increasing the drying speed, or to mix with some speed and pressure using a palette knife to achieve a thinner, more spreadable consistency. It is a lot lighter in colour than other thixotropic alkyd resins.

 

You can use thixotropic alkyd mediums such as Medium L either as a gel or a fluid, depending how briskly you mix it.

Rapid Medium is an alkyd resin that is good for speeding the drying and reducing the consistency of oil colours. Alkyd resin’s flexibility makes it great for quick layering techniques, and remember to avoid using more than 20% to maintain durable successive layers.

Alkyd resins don’t just thin your paint. Transparent Gel can be mixed into your paint to increase the gloss and transparency without affecting the consistency of your paint, so is good for heavier glazing effects. Megilp is used for thicker oilpainting techniques to stabilize thick brush strokes, and for knife-painting, thickening the consistency and drying the colour faster.

Being synthetic, overuse of alkyd resins can make your paint film look slightly plastic. Mussini Medium 3 speeds drying and reduces the consistency for easy brushing, while its rich resin content enhances the lustre and strengthens the paint layer.

Plant oils are great for loosening up your oil paint, for blending or glazing, without the fast-drying effects of alkyds. The best oil to use is a high quality Linseed. Although yellowish in colour, Linseed has the best drying characteristics of all the plant oils. Clearer oils such as Safflower and Poppy are not good driers, and should only be used sparingly or in conjuction with Linseed. Remember “nuts and seeds” and only add a maximum of 20% oil into your paint or you can have problems with drying, adhesion, sweating & wrinkling. Oil should not be added to your initial paint layer.

 

Mediums in action: Medium W allows you to thin your oilpaint with water (left); Cold Press Linseed Oil loosens paint for easy blending (middle); while Megilp adds a stiff impasto quality for sharp brushlines (right).

Cold Press Linseed Oil is the precious first pressing of the lin-seeds, which produces pure oil of exceptional silkiness, which will not yellow as much as other linseed oils. Stand Linseed Oil is a thicker oil that is less yellowing and slightly faster drying than Cold Pressed Linseed Oil. Stand Oil is good for glazing and in later layers. The thicker viscosity provides for distinct brush marks and subtle blending through its enamelising affect.

Schmincke Transparent Painting Medium is an “Old Masters” recipe of stand oil, dammar resin, and siccatives, which is like painting with liquid honey – a lovely drag with the brush lines closing over. The Stand Oil increases transparency, Dammar provides film hardness and increases the lustre, while the siccatives help drying. Transparent Painting Medium is best used on final layers, and can be added up to 50%. It can also be thinned with a little turpentine, if needed.

So oil painting mediums can be divided into three categories for simplification: Solvents (or lean medium), Alkyd Resins, and Plant Oils. These alter the viscosity and drying times of you paint. Choose which you need depending on your painting style – or you may wish to use colour as it comes from the tube.

 

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