Oil Painting Mediums

July 6, 2003: Update on Oil Painting Mediums
(Published in The Classical Realism Journal, Volume VIII, Issue 1. )
One of the most frequently discussed topics among oil painters is oil painting mediums. There exists a wide diversity of opinion and preferences on the subject, both in books and from artist to artist, with certain mediums enjoying almost cult-like advocacy. Much information has been published on the subject, some written by persuasive authors, many of whom disagree with one another on many points, resulting in a great deal of confusion over which ingredients are best for a given painting style. As in most things, the issue is not as simple as it might seem. It must go beyond focusing on the mediums alone, and begin with the reasons painters feel they need them in the first place.

Some commercially available oil paints, as they come from the tube, are of a stiff consistency that does not facilitate the utmost in control, and thus require the addition of something to improve their brushing qualities. Enter painting mediums. It is important to note that in the days of the Old Masters, the paints were ground in the artists' studios, by the artists and/or their apprentices, in all likelihood to the desired consistency for optimum control under the brush. In such an instance, there would be little or no need to add any medium at all. This hypothesis seems to be borne out by scientific testing conducted by the National Gallery, in London, on many of the paintings in their possession. The preponderance of the 16th and 17th century paintings tested showed no detectable resin content in the paint layers, to the surprise and disbelief of so many who had read 19th and 20th century accounts alleging that specific artists of the earlier times used mediums containing natural resins like mastic, copal or amber. What was found, in most cases, was pigment bound in linseed oil, and in some cases, walnut oil. The indication is that the addition of resin varnishes to oil paints as a common practice dates back to perhaps the mid-1700s, and no farther, with certain individual exceptions. Of course there are many unanswered questions, and the possibility remains, at least theoretically, that the testing methods used were simply unable to detect the resins that might actually have been there. Thus it is not necessarily a closed issue, at least in the minds of those mistrustful of science. But if the current prevailing opinion in the conservation science community is correct, as the best evidence strongly suggests, there may well be a correlation between the widespread use of resinous mediums and the introduction of pre-prepared oil paints on the scene. To artists making their own paint, the only concern would have been the quality of the paint, but to those in business to make a profit selling paint, the temptation to add less expensive ingredients to keep production costs down was very strong. It is known that many adulterants and fillers were added to paints sold ready-made by artists' colormen, and this undoubtedly affected the brushing qualities of the paints in question. The need for mediums to improve the consistency of the paints seems to have grown from this. In modern times, manufacturers add aluminum stearate, a soap, to mitigate the tendency for oil and pigment to separate in the tubes as the paints sit on store shelves or in artists' paintboxes. Aluminum stearate changes the oil from a fluid to a more colloidal consistency, and changes the way the paint handles. However, some manufacturers exercise greater restraint in the use of such substances than others, and artists have choices. With this in mind, the painter of today can get by with less medium simply by choosing less stiff paints to begin with, or by grinding his or her own, as was done in the days of Rembrandt and earlier. There are a number of quality paints currently on the market that exhibit a fluid consistency as they come from the tube, and require little or nothing to be added to them in order to be controllable. Should they require a bit more softening, a drop of linseed or walnut oil may be all that is needed.

From a structural standpoint, the strongest paint layer is one with the optimum ratio of binder to pigment. The best commercially available oil paints are already very close to this ratio. Adding too much of any medium is likely to produce weak spots in the resulting paint film for lack of sufficient solid matter, just as a wall made with poorly fitting stones and too much mortar will be weaker than one in which the stones fit more closely together and a minimum of mortar is used.

Natural resins were long thought to impart desirable properties to oil paint films, but this belief may well be in error, at least as far as permanence is concerned, as each introduces its defects into the paints to which it is added. Discoloring and embrittlement over time are chief among these defects, which are well documented in art conservation circles. The prevailing opinion there, based on the best information to date, is that the paintings composed of simple combinations of linseed oil and pigment are more durable and less problematic over the centuries than those containing significant amounts of any of the natural resins. The works of Rembrandt and others seem to bear this out. Of course, in the world of science, there is always the possibility that new information will come to light that will warrant the revising of opinions. Research is ongoing.

References:
National Gallery Technical Bulletin, Volumes 15 and 17, National Gallery, London

Rembrandt: The Painter at Work, by Ernst van de Wetering, published by Amsterdam University Press

Art in the Making: Rembrandt, by David Bomford, Christopher Brown, and Ashok Roy, with contributions from Jo Kirby and Raymond White, published by the National Gallery, London

On Picture Varnishes and Their Solvents, by Robert L. Feller, Nathan Stolow, and Elizabeth H. Jones, published by the National Gallery of Art, Washington, D.C.

©Virgil Elliott, 2002