Few things divide oil painters as much as the use of mediums. Are they the key to the ‘secrets’ of the Old Masters or merely unnecessary complications to painting? Tutor Martin Kinnear experiments
Oil paint is simple stuff – just coloured powder (pigment) and binder (oil). Before 1851 and the introduction of the first collapsible paint tube, artists would mix each colour from scratch,adding whatever adulterants were necessary for the task in hand. Less opacity could be created by adding resins, faster drying by adding lead, extra gloss by varnish, dullness by waxes, and so forth.
Modifying the paint was part of the craft of painting. Combinations that worked were handed down from master to apprentice, new combinations jealously guarded, new materials eagerly sought and tested. By the start of the 19th century it had become not simply impractical but unthinkable to paint without a medium of some kind, typically a general purpose medium to be added at a fixed ratio to all freshly-made paint.
A medium, then, is simply an additive to paint formulated to enhance a desirable characteristic (such as lustre or translucency) or offset an undesirable one (such as a long drying time). In doing this, a medium will also often allow you to effect a technique – such as scumbling, glazing or impasto – more easily, subtly, forcefully or quickly, depending on its formulation.
If you are happy to live with variable drying rates, sinking in and all the vagaries of ‘raw’ oil paint, then there is no reason whatsoever to encumber yourself with mediums. However, if you aspire to recreate some of the classic effects of the past – such as those in the works of Turner or Rembrandt – you will need to modify your paint, as they did, with mediums. Although here I concentrate on oils, everything is generally true of mediums in other types of paint.
Most mediums are made from a very limited number of ingredients, and once you understand these you will be able to predict how any combination of them will work.
Oils with solvents
Drying oils such as linseed, poppy and walnut will oxidise into a translucent glossy film over time, giving oil paint its characteristic richness, lustre and depth. Unlike solvents, oils increase the drying time of paints, and improve the overall adhesion and appearance. However, excess use of oils will not only substantially increase the drying time, but can cause the paint film to wrinkle. A compromise is to mix oils with solvents – often called ‘Van Eyck’ medium; a simple 1:1 oil/solvent mixture.
Wax is commonly added in small amounts to paint as a stabiliser, but used in larger amounts it creates a wonderful impasto effect that dries quickly to a matt finish. Wax has been used for centuries to create softly luminous effects when painted thinly. It can be applied either as a cold paste mixed with oils or heated using the encaustic method. A thin final coat of cold wax makes a wonderful matting agent, allowing paintings to be hung in difficult light. Wax is both brittle and easy to melt, limiting its use to rigid panels unless it is combined with another ingredient.
Painters have long used tree resins dissolved in turpentine (a varnish) to add both gloss to the paint and decrease drying time. Historically, all manner of resins were used – amber, dammar, gum arabic or mastic, for example – but most have fallen by the wayside as they have proved unstable in the long term, causing the paint film to yellow, peel or crack. Despite these known defects, resins add such amazing and subtle effects to oil paints that they continue to be used. A modern synthetic resin known as alkyd promises to solve these defects, allowing painters to recreate effects similar to Turner, without the fading and cracking that afflicts his work.
A solvent is a thinner such as turpentine or white spirit. Solvents significantly decrease drying time, but weaken and dull the paint film, making them ideal for an imprimatura (stained ground) or other initial painting stages. Excess use of solvents will cause lack of adhesion or ‘chalking’, however, and they often have unpleasant fumes. Solvents are also a key ingredient in resinous mediums. They can be used to rapidly underpaint oils and then be worked over in a matter of minutes. Solvents applied to upper layers of oils can create interesting effects. A low-odour solvent, such as Gamsol or Sansodor, will make this safer and more pleasant but extend the drying time.
Martin Kinnear is a professional painter and owner of the Norfolk Painting School (www.norfolkpaintingschool.com) where he teaches traditional oil painting and the use of mediums.