Oil Painting Technique and Conservation

Mark Schofield

Return to articles

A basic reference guide to the preparation, creation and conservation of oil paintings.

Introduction

While oil painting is not an exact science, there are some basic principles that need to be respected in order to produce art of a reasonable quality that stands the test of time. This article will attempt to summarize the principles I have adopted and applied in my work.

Support

I prefer a rigid support for oil painting. There are three reasons for this. Firstly, I find a non rigid support such as a canvas mounted on a frame to be ill suited for drawing on. As I frequently make a fairly detailed drawing before painting, non rigid supports do not lend themselves easily to my way of working. Secondly, a canvas is extremely fragile. It is especially at risk of puncture either in transport or when in storage, for example when resting against another more rigid object. Thirdly, a rigid support provides an inflexible support for the paint layer which reduces the risk that the paint will crack. This is an important consideration when painting in oils, since the oil hardens and becomes increasingly inflexible over time (see Painting in layers below).

Daler Board

I often use Daler Board for my support. Daler Board consists of regular artist canvas stretched over a semi-rigid board. It has the advantage of being primed, so it is ready to work on straight away. It is also guaranteed to be acid-free, which ensures better conservation of the work.

Plywood

I also make frequent use of plywood supports, which I cut to size using a saw. Plywood, being composed of thin sheets of wood glued together at right angles to one other, is better than regular wood panels at resisting warping.

It is important for plywood supports to be primed before use, both to make a surface suitable to receive the oil paint but also to create a white ground that will reflect light, making the final painting more luminous. Typically I use a vinyl undercoat of the kind used for interior decoration. This needs to be applied quickly and evenly with a large brush or roller. After a couple of days when the paint is completely dry, I remove any small imperfections by rubbing the surface over with fine emery paper. Two or three coats of vinyl undercoat applied in this manner are usually enough to ensure a smooth, even surface ready to work on.

Plywood support prepared with a vinyl undercoat.

Painting materials

I always use artist quality oil paints and brushes, since lower quality ones rarely provide equally satisfying results. High quality paints are also the best guarantee that the colours will not degrade with age. Used sparingly on the palette and mixed with additional oil and solvent (see Painting in layers below), a tube of oil paint will go a very long way, providing excellent value for money. Good brushes are also worth the investment, although it should be noted that they suffer from repeated use and cleaning. In order to avoid using powerful chemical cleaners, I clean my brushes in regular linseed oil purchased from a DIY store and wipe them on a piece of dry rag.

When mixing oil paint with additional oil, I use only artist grade linseed oil. This has the advantage of being less yellow than regular linseed oil, keeping the whites pure. I also use artist grade turpentine, which is the best guarantee that it is free of other chemicals that might affect the long term conservation of the work. As I often work outside the home, I put a little of each in a small jar for ease of transport.

Low quality linseed oil for cleaning (left), artist quality linseed oil for mixing with oil paint (middle) and turpentine (right). Notice the difference in colour of the two linseed oils.

Painting

I always work from light to dark. While some visual effects require the contrary, this is the best way to develop the picture as a whole and ensure the balance of colour and tone is correct. Highlights, which involve adding light over dark are of course possible but they require a thicker layer of paint.

Painting in layers

I generally use the traditional method of painting in layers when working in oils. Essentially, this requires the colour (pigment) in the oil paint to be thinned out using additional oil (medium) and turpentine (solvent) so that it can be applied in a thin, translucent layer. Multiple tranlucent layers of oil paint superimposed one on top of another allow for the optical properties of each layer to be combined, providing a greater range of visual effects than could be provided by a single layer of oil paint alone.

Without going into too much detail, it is important for the artist to understand how a mixture of pigment, medium and solvent will react as the layer dries, as this is crucial to the long term conservation of the work. In principle, the solvent evaporates quickly and entirely from the paint surface leaving a mixture of pigment and oil on the support. Subsequently, the oil dries slowly through a process known as oxidation. This is a chemical reaction in which the oil absorbs oxygen from the air and hardens, creating an inelastic film that fixes the pigment into place.

Articles on oil painting technique frequently refer to the principle of "fat over lean". Essentially, this principle requires that each layer of an oil painting should contain a greater percentage of oil than the layer below. This is because as oil oxidises, it expands and contracts. The greater the percentage of oil in the mixture, the greater the degree of expansion and contraction in the layer. If a lower layer contains a greater percentage of oil than an upper layer, the shrinking and contracting of the lower layer could eventually cause the upper layer to crack. "Fat over lean" is therefore the key to avoiding cracks.

It is also important not to overdo the amount of solvent. If there is too much solvent in the mixture, it will leave insufficient oil on the support after evaporation. The oil will fail to form a cohesive layer and the pigment will not properly adhere to the support.

For the first layer, I generally go for a mixture of 1/2 oil paint, 1/4 linseed oil and 1/4 turpentine. In successive layers, I gradually diminish the amount of turpentine and increase the amount of oil while keeping the amount of oil paint in the mixture roughly constant.

Lastly, it is important to give each layer sufficient time to dry. While complete oxidation of an oil layer will take many years, it is more rapid in the early stages and slows with time. The oil layer may therefore be touch dry after just a few days. If the principle of "fat over lean" is respected, this is sufficient to begin work on the next layer. Otherwise, it is necessary to leave the layer to dry for as long as possible. While the layer is drying, I store the work away in a drawer to avoid it coming into contact with dust.

1st layer.
2nd layer.
3rd layer.

Varnishing

Varnishing is an important final step in the production of an oil painting. In addition to harmonizing the paint surface, it also provides a layer of protection from dirt and physical damage.

It is important not to apply the varnish too early. As we saw in the previous section, the oxidation of the oil causes the paint layer to expand and contract as it dries. The drying time will depend of course on the amount of oil used, but the overall period of time required for an oil painting to dry is a matter of years. Art conservators do not consider an oil painting completely dry until it is 60 to 80 years old. Adding a layer of varnish before the painting is reasonably dry may create a number of problems. The first is that the varnish layer has different mechanical properties to the paint, so expansion and contraction of the paint may cause the varnish to crack. The second is that the varnish puts a barrier between the paint surface and the air, reducing the rate of oxidation of the oil and further lengthening the overall drying time of the work. Thirdly, there is a risk that the paint and varnish layers will mix, with some of the pigment becoming fixed in the varnish. Should the work ever be cleaned and the varnish removed, this will result in the pigment coming off the painting.

The choice of varnish depends on the finish you want. There are essentially 3 types of varnish: matt, satin and brilliant. Matt is the least glossy, and is the best choice if you want to avoid the surface being too reflective, but it can leave the colours looking a little dull. Brilliant is the most shiny, and has the advantage of restoring the saturated appearance of the colours but reflections, especially when hung facing a window, can be an issue. Satin is essentially a compromise between matt and brilliant.

I generally wait for a year or more before varnishing my oil paintings with a single layer of satin varnish. This sometimes means I exhibit my work unvarnished, but I consider this to be a risk worth taking. I use a large, wide, nylon brush that I have reserved specifically for varnishing to ensure it is not polluted with any other painting materials. The varnish needs to be applied fairly quickly using broad, regular brush strokes both up and down and left to right to avoid any drips or brush marks. By working near a window with plenty of natural light and looking across the surface of the painting, it is possible to see the varnish as a wet sheen. This helps to spot any missed areas. Apply one layer of varnish only and make sure it is left flat to dry. Never try to retouch a drying varnish.

Storage

Storage of art work is something the artist should take very seriously. If you have spent time and effort to make an oil painting, it would be a shame to discover later that it had suffered through improper storage.

The best way to conserve an oil painting is to frame it and have it suspended on your wall. However, most artists have more paintings than they have wall space to hang them, so some paintings will invariably need to be stored.

A varnished oil painting on a rigid support is fairly resistant to physical damage when in storage. Unframed works are at risk of warping if stored vertically, so are best stored horizontally. If one work is stored on top of another, they need to be separated with a sheet of acid-free paper. Framed works are less at risk of warping and are best stored vertically, although care must be taken to ensure the fixations behind one work do not come into contact with the surface of another. For long term storage, it is a good idea either to remove the fixations or else wrap each work in bubble wrap.

As the mechanical properties of the support, the paint and the varnish are different, they will expand and contract differently when exposed to extreme changes in temperature or humidity. The resulting stress will, over time, degrade the work. Some varnishes may also become tacky if exposed to high temperatures. For all these reasons, a loft is a bad choice for storing works.

Direct sunlight will, over time, bleach colour from an oil painting and possibly cause the paint to become brittle and crack. Equally, it is best to avoid storing oil paintings in the dark since this will cause linseed oil to darken and yellow. Fortunately, this process is entirely reversed if the linseed oil is re-exposed to light.

A storage box with large, flat drawers is ideal both for storing unfinished works as well as paper and newly prepared supports.

Homemade storage box for storing work horizontally.

Framing

In general, framing is something best left to the experts. Get your work framed at a specialist framers, where you will also get advice from someone with experience in choosing the right frame for your subject. Framing an oil painting is not necessarily expensive, since all you need is the frame itself; a varnished oil painting does not need to be put behind glass.

An exception to the above rule can be made for plywood supports. These are rigid and thick, and will not easily fit behind a narrow frame. As such, a floater frame is an ideal, inexpensive solution that you can make yourself. An essential item for making a floater frame is the picture framing clamp. It ensures the edges of the frame are perfectly aligned and provides sufficient pressure for the glue to make a strong bond.

Floater frame behind an oil painting on a plywood support.

Now that you have read this article, why not take a look at my work?

Return to articles | Back to top

Mark Schofield
mark.schofield.free.fr
Last updated : 29 Sept 2012