By Ron Sanders
Professional Visual Artist & Instructor
Ron Sanders Art Studios
Sanders Red Easel Gallery
Many artists are unfamiliar with the basic and fundamental rule of oil painting: “Fat over Lean.” Understanding the rule and its application is necessary to avoid cracking.
Drying oils, most commonly linseed oil and walnut oil, dry by oxidation rather than evaporation. When liquids dry by evaporation, the liquid becomes a gas, leaving no trace behind. However, when a liquid dries by oxidation, the fluid combines with oxygen from the atmosphere and undergoes a molecular change, altering its form, but remaining where it was placed. Diluents, such as gum turpentine from pine trees or mineral spirits from petroleum, evaporate. And if they are pure, there is no trace of them in the paint film. Their purpose, therefore, is to thin or dilute the paint, but do little to modify the paint film. What little affect they do have will be discussed later in this article. Drying oils, such as linseed or walnut, dry by oxidation.
Evaporation= Gas Out v. Oxidation= Oxygen In
How does oxidation work?
The drying oil combines with oxygen from its surface downward, hardening as the oxygen reaches each layer. But, more importantly, in relation to the problem of cracking, as oil oxidizes it moves - expanding and contracting during the drying process. The danger arises when a dry and inflexible layer is on top of a still drying and moving lower layer. Anyone who has lived in a cold environment knows what happens to cement sidewalks when the ground beneath expands and contracts during the freeze and thaw cycles of summer and winter: it cracks! The exact same process happens in oil paintings that are incorrectly layered.
Cracking Top Layer Image
How can we avoid this troublesome problem?
There are two key components:
1. Make sure that lower layers of paint dry before upper layers; and,
2. Make sure than each successive layer has more flexibility than the previous layer. “OK, OK," you say, "but what can I do to make that happen?" The answer is the same for both and is summed up by the rule and application of "Fat over Lean."
Drying oils are fatty oils from plants and nuts. Despite what some authors have written, painting fat over lean does NOT, by definition, mean painting thick over thin. To paint fat over lean means to paint in a way that each successive layer of paint, when layering paint wet over dry, contains more fatty oil than the previous layer, allowing it to dry more slowly and to be more flexible than the previous layers. When you go to the local butcher and ask for a lean cut of meat, you are not asking the butcher to cut the meat in thin "deli-slice" fashion. You are asking him to cut off the fat. Likewise, when you buy hamburger that is 90% lean, that means that it contains 10% fat, not that it makes really skinny burgers.
Something that is lean contains less fat than something that is fatty. Simple enough.
In practice, a thin layer of paint will dry faster than a thick layer. This means that painting thin first and building toward thicker layers or areas of impasto will produce a similar desired effect. But if each layer must be thicker than the last, then that does not allow for thin layers of glazes and scumbles in top layers.
How, then, do we increase the amount of fat in our paint layers? By the addition of a medium! The purpose of a painting medium is to alter the paint's handling characteristics while also modifying the paint film's properties.
The purpose of a diluent is to dilute the paint film, affecting its handling, without modifying the properties of the paint film. However, as mentioned above, a diluent does have some effect. What effect does a diluent have on the paint? A diluent will thin the paint film, spreading the pigment particles and oils farther apart on the surface of the ground, weakening the bond and speeding the dry time. As discussed above, a diluent evaporates and is no longer a part of the paint film when dry. It therefore has no ability to create a continuous paint film or bond between particles. The danger in this process is the overuse of a diluent to thin paint for application. Too much turpentine or mineral spirits will weaken the paint film, reducing its ability to bond to the ground, causing the paint particles to fall off in a powder; however, used sparingly in an underpainting that will be overpainted with more heavy bodied paint, thinning with a diluent is acceptable and creates a generally leaner layer of paint to begin.
One can, therefore, begin a painting by sketching out the scene with a thin wash of color - simple paint and turps mix. Then, when that layer is dry, overpaint with tube oil paints alone, with no addition of the diluent.
Wash in thin colors:
Mass Background then Mass Foreground
The trick for most artists is in building up layers beyond this point. And this is where the addition of a medium comes into play. For the sake of this discussion, we will allow for the addition of four additional layers of paint, each having a greater percentage of drying oil by volume.
A common medium, used by many artists working in many styles for centuries, includes a drying oil, a resin varnish and a diluent. The most common mix used today includes Stand Oil, Damar Varnish, and Gum Turpentine. The use of real gum turpentine is important here because petroleum based odorless mineral spirits do not mix with damar varnish properly; they create a white cloudy substance that will produce a veil across your painting. You must use real turpentine from pine trees when mixing this medium.
Why, you may ask, do I use the damar varnish at all? What do these ingredients do?
Stand Oil is a specific type of linseed oil that has been left to "stand," literally, in open containers. The oil is thus allowed to partially oxydize or polymerize prior to bottling. This causes a thickening of the oil that makes it very viscous and somewhat difficult to spread on its own. But, this oil is best suited for use in mediums and is essential to the fat over lean process. The addition of damar varnish increases the flow of the medium, reducing its viscosity, while also adding sheen. The turpentine is used to alter the percentage of oil within the medium through dilution. Therefore, by adjusting the amount of turpentine in each medium we create, we can produce mediums for each layer we paint, each with it's own level of fattiness.
Medium Mixtures
The ratio of stand oil to damar varnish remains the same in each of our mediums - 1:1. However, the amount of turpentine will be at a ratio of 7 parts in our first mix, then 5, then 3 and finally 1. The first medium that we use will be the 1:1:7 mix, with the most turpentine and, as a percentage of the total mix, the least amount of linseed oil. This is the most "lean" mixture. Once that layer is dry, if we wish to continue painting, we would use the medium marked 1:1:5. Always make sure you mark the outside of your jars so that you can easily identify the correct mixture. Baby food jars work exceptionally well for this and the ingredients can be measured with a simple spoon. For our next layer we will use a 1:1:3 mixture which continues to increase the amount of oil by reducing the diluent. And lastly, we will use the 1:1:1 mix, but only if we had the need for this many layers.
If we begin with a thinned wash of tube color, followed by a layer of pure pigment before we even begin the use of a medium, then we can produce a painting of six layers with these four mediums.
So how do we add the medium to our paint?
"I know" what you're thinking: "we get those little metal cups that attach to our palette and we pour some out and then dip into it as we mix our paint." WRONG. Why? Because, if you use the medium as a thinner, constantly dipping into it haphazardly, you will end up with varying amounts of medium in your paint film that will destroy the calculated percentage of oil within each layer. For example: if I dish out four times as much medium in my first layer as in my second, I will actually end up with three times as much fatty oil in my first layer than in the second layer. This will result in your painting being lean over fat, the opposite of what you desire.
Always remember: A diluent is made to thin the paint, a medium is made to modify the paint film.
Therefore, there are two controlled ways to introduce the medium into the paint layer.
Option 1: Before you begin to paint, take a clean brush and spread a thin layer of the medium onto the area of the painting that you will be working on. Then paint straight tube oils directly into this wet layer of medium. The paint will mix with the oil and create a single paint film. This method has multiple benefits. First, it re-wets the previously painted areas, renewing the color and sheen of dry paint. Second, it produces a wet surface that allows for better control of edges. (This is also good for lying in glazes). And third, it produces the desired effect of fattening up the new paint layer. But be careful not to wet too large an area or it will begin to oxidize, creating a sticky surface that is difficult to paint into. Also, don't paint on the medium too thickly or you will find it difficult to control the paint.
Option 2: After laying out your palette for the day's painting, take your palette knife and drop one or two drops of medium into each pile of paint. I usually have paint piles about the size of my thumbnail. If you paint larger works and tend to put more paint out on your palette, then you may need to use more medium. However, overuse of the medium will create a pigment that is too fluid for most desired painting methods. After you have dropped the medium onto your piles of paint, stir the medium into the paint thoroughly with your knife, cleaning the knife between colors. After that, cap up your medium and don't add any more unless you put more oil paint onto your palette. This method also controls the total amount of medium in all colors and assures that the medium in thoroughly mixed throughout the paint film. This is the method I most commonly use and, to date, have never had a painting crack.
Other Questions:
Q: What if I want to paint very wet, fluid paintings? Can I add more medium or should I use a mineral spirits?
A: Larger amounts of medium can be added to create fluid (yet continuous) paint films. But the same laws apply. Therefore, if you like to paint with more medium, you must still attempt to control the amount of medium in each layer, adding increasing amounts of fatty oil to each successive layer. But beware, addition of too much oil to the paint film can lead to wrinkling and sagging.
Q: Do I have to use Turpentine throughout the painting process? I hate the smell and am concerned about the odors in my studio.
A: It has been my experience that I am still able to use odorless mineral spirits such as Turpenoid or Gamsol to rinse my brushes and even to thin the paint further, without any noticeable clouding of the paint film. In this way, the small amount of gum turpentine in the medium and smaller amount placed into the paint during a days work creates little if any noticeable odors beyond the normal oil paint smells. I would, however, recommend that you mix your mediums outdoors or in a well ventilated area, as the smell of gum turpentine being poured out during this process can be quite strong.
Q: What if I am allergic to Turpentine? Are there any other medium mixes that I can use that do not contain turpentine?
A: For those who are allergic to or suffer from breathing problems that cause them to stay away from gum turpentine, you may wish to create a medium with only linseed oil and mineral spirits or perhaps an alkyd resin such as Liquin, thinned to varying degrees with an odorless mineral spirit.
Q: I notice that many paint manufacturers use other oils than the ones you mention, such as poppyseed or safflower oil. Are these OK to use in the fat over lean method?
A: No. These oils are non-drying cooking oils. Paint manufacturers add artificial dryers to force them to dry. The reason that they are used is because linseed oil has been reported to yellow with age, whereas these lighter oils help maintain the purity of the color. Safflower and poppyseed oils are fine to use in true alla prima painting, when all the paint will be put down wet-in-wet, drying as a single paint film. But for wet-over-dry painting, these oils are missing vital ingredients. The very reason that linseed oil yellows (slightly) is the reason that it is best for layering: the linolenic and linoleic acids in linseed oil make it the most flexible oil and much less prone to cracking than poppyseed, sunflower, or safflower oils. The other naturally drying oil with good flexibility is walnut oil, which is less prone to yellowing and is suitable for whites and blues. M. Graham & Co. makes a whole line of walnut oil paints and mediums. For those wishing to use predominantly linseed oil paints, Grumbacher Pre-Test, Old Holland Oils, Gamblin, and a few others can be purchased.
Q: I am a Plein Air landscape painter. Sometimes, after I get a work back to the studio, I see areas that I want to repaint or touch up. How should I handle this?
A: If it is the same day and the painting is still wet, you can continue painting into it with no ill effect. But if the painting has dried, you will need to add a fatty medium to the paint for any overpainting. I am convinced that this is one of the biggest causes of cracking that I see in galleries today - from Plein Air painters who rework their field studies in the studio without following the fat-over-lean principle, combined with the fact that they are often using oil paints containing the lighter non-drying oils mentioned above. You can often tell where the repainting has occurred by the location of the cracks. If a painting is not structured properly, the cracking will usually occur within 6 months to a year of its creation.
Q: Do I need to work on any specific ground or support to paint in a layered technique? I have studied with some teachers who promote the use of true gesso (marble dust, calcium and animal hide glue) on a wood panel. Is this necessary?
A: No. The indirect method of painting has been produced on many types of supports. It is the flexibility of the linseed oil that makes it suitable for painting on a flexible support such as cotton or linen canvas. It is the use of the gesso described above, which is not flexible and is prone to cracking, that demands the use of a solid ground. But the common use of oil grounds or acrylic "gesso" grounds on flexible supports is perfectly suitable to the indirect method of painting.
Q: I have heard that damar varnish yellows and cracks with age. Is that true, and if so, shouldn't we avoid it in our medium?
A: Damar used as a solid top coat varnish will, with excessive age, darken and crack. However, the very small amount used in these mediums and the infinitesimal amount that is then present in our paint film when we apply these mediums in the prescribed manner makes these issues irrelevant. The general rule is that damar can be added to a paint film so long as it is no more than 50% of the liquid in the binder or medium. In our medium, we are mixing a 50/50 mix of stand oil and damar, but we are then diluting it with turpentine. In addition, we are only adding a couple of drops to a pre-mixed paint that already contains fatty oils and other ingredients, making the percentage of damar in the total paint film much smaller.
Q: If I paint alla prima, do I need to use a medium at all? Can I use a medium if I want to?
A: In alla prima painting – (wet-in-wet in one sitting) - the paint, even when applied in layers, fuses together to dry as a single film. Therefore, there is no need to use any medium. A common approach is to start with a sketch of the scene, often in a neutral color like burnt umber or an oxide red, to work out the composition. Then the value and general color patterning can be laid in with diluted paint thinly applied over the canvas. In addition, heavier paint can be applied to build the volume of the final painting with details, highlights, and deepest shadows adjusted at the end. The only reason to use a medium is if the artist is not happy with the flow and overall working properties of the paint from the tube and desires to modify the paint's handling. Manufacturers often work toward a consistency in their paint that is easy to work with, making modification rarely necessary for this approach.
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Ron Sanders began pursuing art as a small child, working in oils by the age of nine, studying anatomy soon after, and selling his first commissioned work by the age of eleven. After receiving his BFA in illustration at the Columbus College of Art & Design in Columbus, Ohio, where he graduated with honors, he began working as a freelance artist. Ron is represented by agents in New York and Chicago and has produced illustrations for advertising and promotional material, books, calendars, and newspapers for national clients.
Ron has also pursued publication of his work as fine art limited edition prints as well as painting original oil paintings for exhibition in prestigious national juried shows and more than a dozen art galleries throughout the country since 1993. Ron has studied intensely the craft of oil painting in order to make the technical second nature and to give himself the tools and vocabulary to express himself visually.
He currently lives in Venice, FL, and teaches a variety of painting and drawing classes at the regional art center.
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