A medium can be defined as a pre-mixed blend of a drying oil, possibly a resin, and possibly a turpentine vehicle, which is commonly added to the later layers of paint in a work, so as to facilitate handling, and/or enable transparency of layers, and/or increase the gloss and colorific intensity of these layers. Most artists will already know this, and some reliable recipes for these are given under the section of this site Resources, in the pages Painting Mediums and Recipes.
The above advice constitutes more one of the Don’t’s of technique rather than the Do’s. Towards the end of the 18th century, as the gradual replacement of the workshop system by the art academies was almost complete, many painters were vaguely aware that a body of practical knowledge was being lost with this change. A corresponding notion developed that the great 16th and 17th century artists had achieved their effects by dint of some knowledge, now lost, of complex blends of oils, wax and certain hard resins, but if these could be reproduced in a medium, the “Venetian Secret”, as it was called, could be regained. To this end, through the next century, painters experimented ceaselessly with mixtures of heated oils and resins such as Mastic or Copal. Since no means of photochemical and spectroscopic analysis were really available for use on historical works until the late 1970’s, there was no evidence to contradict the claims of authenticity made for these preparations. You can still find some of them, often with picturesque names, on sale in some art shops today.
The results of modern analysis are at once more prosaic and more daunting. The binders in Renaissance and Baroque paintings are largely walnut and latterly linseed oil, with small amounts of pine resin or fir balsam being added on topmost layers of working. Copal and Mastic were not used, and, as the last 150 years have shown, are liable to yellow and darken conspicuously with time anyway. If the oils were treated at all, it was merely by being thickened by heating or sunlight alone (see the pages referred to above for directions for these procedures).
There is still, however a subtle hype surrounding some of the proprietary mixtures sold as paint media, offering artists a chance to import “old master” qualities into their work, at no extra artistic effort. As a result some painters have liberally added medium into every stage of their creations, painting fat over fat, with dire consequences in appearance and preservation.
So if you wish to select a commercially prepared medium, exercise the same degree of scepticism that you might with an ostentatiously labelled wine. Most manufacturers nowadays will to some extent list the constituents, and of these you can form some immediate judgements:
- Avoid those media containing Mastic and Copal if you wish to see your work not age into a warm “gallery tone”.
- Be circumspect with those which contain siccatives (agents to accelerate drying) and are of a dark appearance. The presence of driers can darken and embrittle dry paint films.
- Remember that, if present in large quantities, even Damar, (a reliable varnish resin discovered in the Dutch East Indies in the late 17th century) makes a dry paint film vulnerable to later applications of solvents, including turpentine and also those used by conservators.
- Be aware that Ketone, a varnish resin which yellows less than all others, including Damar, is also the most vulnerable to solvent action. This makes it a fine, easily replaceable varnish on its own, but for that reason it is not a robust constituent for a medium which should be a durable binder of paint.
- Remember that medium is there to provide possibilities of effects when the possibilities of normal tube colour have been exhausted. It would be unwise to envisage it as an addition to an underpainting or to preliminary paint layers. It has its place in the “fat over lean” order mentioned above.
- Buy a small quantity and try it out first. Does it feel right for you?
Since the choice of characteristics of medium is even more a matter of personal taste than that of priming, it might be better to make your own. As well as the recipes given in this site, one of the books cited in the Booklist pages, Formulas for Painters by Robert Massey, gives some interesting examples, although he tends to endorse the use of some of the resins about which we have reservations.
Finally, think of a medium as a facilitator of good handling; it will not guarantee it.