Part 3&4: Walnut Oil as a Medium

by David Molesky

When I began using walnut oil, I found it to be very thin and pale; it didn’t seem to need to be diluted with a solvent. It had a different feel from thicker, stickier linseed oil. However, as a medium, straight walnut oil can start to bead-up when you begin to paint into multiple layers.

I started to search for ways that I could extend the range of from “lean” to “fat” without having to use solvents. Most materials and technique books discuss at length the use of linseed oil. It was hard to find much on walnut oil and it wasn’t easy to find it in stores either.

HISTORY OF USE

Researching in my small library of material and technique books, I became perplexed as to why walnut oil had not been mentioned to me by any previous instructors. Many of the texts mention that walnut oil was much more widely used by old masters than we realize. Many preferred it over linseed oil, including Da Vinci, Durer, and Vasari to name a few. Apparently, much of the Italian Renaissance was done with walnut oil as the vehicle, not linseed oil.

Da Vinci wrote that he preferred walnut oil over linseed because it tends to dry throughout the layer rather than drying with a skin on the surface. In Munich, I saw one of Da Vinci’s earliest experimentations with linseed oil. And I can see why he switched to walnut shortly after. The entire face of the madonna was covered with the wrinkly wavy surface you would expect to see from a pool of drying linseed oil. When he switched to walnut oil as did many of his contemporaries of the Renaissance, you no longer see this issue. Clearly walnut oil holds up exceedingly well, because these paintings are some to the best preserved from posterity. 

CHEMISTRY

In general, oils are composed of huge molecules called triglycerides, which consist of a glycerol with fatty acids attached. When an oil “dries” these fatty acids go through a chemical reaction with oxygen and bind together to create lipid chains. The type of fatty acid on the glycerol determines how aggressively the oil reacts with oxygen which translates to how much it yellows, its drying time, as well as strength of the lipid chain/paint film.

Of the drying oils, linseed oil contains the highest average—about 55%— of the most aggressively reactive fatty acid: linolenic acid (C18:3). The “3” stands for the three double bonded carbons, which are the parts of the molecule that have a chemical reaction with oxygen. Walnut oil only contains ~ 11% of this type of fatty acid. However, walnut oil contains much more (73%) of a less reactive fatty acid that has only two double bonded carbons — linolenic acid (C18:2). Linseed oil by comparison has only 17% of that kind — instead it’s composed of the most reactive fatty acid. When looking at the combined presence of both types of fatty acids, walnut oil has the highest percentage: 84% compared to 72% for linseed.

What does all this gobbly gook mean? Well it means several things. Linseed oil dries so aggressively that if you paint with any kind of thickness you will likely end up with a situation where a skin dries on the outside while the paint inside stays wet, sealed off from oxygen. The paint beneath the skin could stay wet a long time and the skin could then wrinkle. Walnut oil with its higher percentage of reactive fatty acids (mostly of the less reactive type) tends to dry more evenly throughout the paint film, without producing a surface skin. This allows for a longer “open” time, when you can work into the wet paint later. As the film cures you will notice increasing viscosity and grab over several sessions. This is how the Italian Renaissance painters were able to blend lean colour so seamlessly.

Many historical painters praised walnut oil for its ability to flow due to its lower general viscosity. This quality also allows the painter to incorporate more pigment into a paint mixture when it is used as the binder. Although the film of dried walnut oil is softer than linseed oil, and therefore possible to clean up easier, it has been known to hold up better to UV and exposure to climate. This is probably because it contains a higher density of the reactive fatty acids. The only real downside is that walnut oil has a shorter shelf life than other oils and can spoil, which Da Vinci also notes in his texts.

NEARLY SOLVENTLESS OIL PAINTING METHOD

Walnut oil offers a number of qualities that make it superior to other drying oils for use in oil painting. One of the greatest benefits is that it makes it easier to avoid solvents in your process. However, if you are not using solvents as part of your medium, you cannot just use straight walnut oil continuously in the same way throughout each layer. You will soon come to a point when the paint layer doesn’t have sufficient tooth and begins to bead up on the surface.

To tackle the issue of creating a spectrum of lean to fat, you need to take into account the other factors that help each layer bind together. The “lean to fat” rules really are there as a guide to help ensure proper binding from one layer to the next and also to help prevent the painting from cracking during the drying process. “Fat-over-lean” does not only pertain to the oil content of the medium but it also has to do with the oil absorbency of the pigments in the paint, the absorbency of the layer below, and the stickiness of the medium. For example, colours or mediums that stay wet should not be painted over by colours or mediums that dry quickly. If you paint a high oil absorbent colour like raw umber over a slow drying colour like black, you will get cracking. Even if the oil binder is 100% the same. The difference in the pigment content changes the relative fatness or leanness of the paint film.

GROUNDS

The ground is the interface between your support and the painted surface. For rigid supports like panels, you can use certain types of grounds that would not work on flexible fabric supports. Real traditional gesso is made when chalk is sifted into hot rabbit skin glue and then mixed by pulling it through a cheesecloth not stirred. This is an incredibly smooth and absorbent surface, but it must be used on a panel because it would crack if it was on fabric.

For fabric supports, there are lots of options, and some are clearly better than others. This is the part of the process where you can make the surface become more ideal for painting. It’s really where the painting begins. How you prepare the ground and what materials you use will affect how the paint is absorbed and moves across the surface.

Most commonly used is a product labeled as “gesso” that is really white acrylic paint with a lot of filler. It makes a flexible Sure this ground is flexible, but let’s be honest, its plastic and plastic has a terrible track record for longevity even within its brief history of use. It’s cheap and you get what you pay for: plastic = garbage.

So let’s say you go to the store and you think, “aha, I’m not going to buy that plastic gesso, I’ll buy this expensive can of oil ground.” These grounds are full of solvents, chemicals, plastics, and other petroleum bi-products that will surely stink up the place and will likely give you ahead-ache if not worse. The only difference between this type of ground and acrylic is really the binder, both are in the end made from plastic and other petroleum bi-products.

HOMEMADE CHALK AND OIL GROUND

Odd Nerdrum’s ground:

While I was a student, Odd Nerdrum taught me how he makes his oil ground with powdered calcium carbonate — chalk — and a cooked linseed oil.

Using a store bought canvas as a surface, Odd would take a large pile of powdered chalk (calcium carbonate) with some cooked linseed oil.. When the correct ratios are achieved, the mixture of oil with chalk becomes a consistency similar to pancake batter. Odd does not use this mixture right away. Instead he lets it sit overnight so that the oil can seap its way into every craggy surface of the chalk particles. Just before he uses the mixture the next day, he rechecks to see if the consistency is still ideal. He’ll also add a bit of color to the mixture before applying it to the canvas.

Chalk:

The ideal chalk for the ground described above is one that is fairly fine but has some variation to the size of the granules. I think Odd uses a commercially available ground chalk that is well suited for this purpose. I buy my chalk from Kremer pigments. I mix together two different powdered chalks from Bologna to be sure there is variation on the granule size.

Having variously sized particles helps the ground refract more light.  I first encountered this concept when I was working with a painter who had trained as a restorer (which in some senses is like forgery). She mentioned that her late husband—also a conservator and painter—insisted on making his own lead white rather than using machine made paint from stores.  Apparently the homemade lead was unsurpassed in its quality to refract light and appeared identical to historic lead white. If all the granules were the same size there will be a wavelength of light perfectly suited to snaking its way around each granule suspended in the oil, which means that some light will be absorbed into the ground rather than reflected back. Having different diameters of granules helps illuminate the paint layers from within. Whiteners such as lead mixed into the ground also help refract/reflect more light. Lead has some amazing properties: it’s very flexible and stronger than other white pigments. It’s a great choice if you can use it safely (don’t snort the sanded dust or eat it) 

CHALK IN OTHER LAYERS

The inventors of oil painting conceptually based it upon glass painting. Light travels through films of varying transparency and opacity, and bounce off the ground effectively backlighting the layers. Keeping this in mind, you can imagine that it would be important to suspend the pigment in such a way that there is some space for the light to make its way to the ground. This is where chalk can come handy in the successive paint layers as part of a putty medium. As I mentioned above, chalk when mixed with oil is transparent and devoid of color. It is inert and thickens the oil into putty-like paste.

If you were to consider chalk a pigment you could say that it has a tinting strength close to zero. It’s like an empty pigment, without much color at all. When it is dry it is usually white or slightly yellow depending on the oil. When it is mixed with oil into a paste it becomes a gray phlegm-like color. It’s not the most appealing color, but it accepts tint readily. The tiniest amount of color from a tube or a bit of pigment can change the color of a rather large pile of chalk oil mixture.

UNDERPAINTING AND DRAWING 

Now comes the first layers of painting. There are many ways to tackle this, and I personally have never chosen one method to stick to. I enjoy the interplay, and experimental results of a wide vocabulary of approaches.

SANDING AND CLEANING

It is important to provide “tooth” to the surface before painting a new layer. Obviously you don’t want to obliterate your work, unless that is your intention, so it is best to choose the appropriate grit. For general tooth producing purposes, 220 grit is rough enough. As you get into the finer final layers you will want to switch to a finer grit sandpaper; in the end it should almost be as delicate as polishing. Of course you can also use a much more coarse sandpaper, if you want to scratch marks on the surface as I have seen Odd do to create wisps of hair.

After sanding its a good idea to wipe the dust off the surface. You could use the palm of your hand or a damp rag. For a more thorough job you could use some windex or even light solvent. Odorless mineral spirit is less powerful than distilled turp. Just be observant, watching what is coming off on the rag and what is changing in the painting while you are doing it. If the painting is months old you’ll have less to worry about than if it is fairly recent.

OILING OUT AND IMPRIMATURS

With the dried surface made as lean as possible, you can then make a choice of how you are going to approach the incoming layer.  Unless you are dry brushing this next layer, you might consider wetting the surface so that your brush marks can more easily glide across the surface. This process of preparing the surface is called “oiling-out.”

In the linseed oil approach, this might be just wetting the surface with solvent or a mixture of linseed and solvent, such as 1:1 linseed to solvent. With walnut oil this might be just pure refined walnut oil. But there are some variations to how you can adjust the qualities to maintain fat over lean conditions.

FAT OVER LEAN VARIATION FOR SOLVENTLESS WALNUT OIL TECHNIQUE 

The ability to layer methodically on a painting over a period of years is the one aspect of the medium of oil that is not afforded by other approaches to image making. Take the historical example of Titian who it is rumored used up to 40 glaze layers in his late pictures.

While I was working on my painting of the Icelandic horse while studying with Odd Nerdrum. He encouraged me to use all over “imprimatura” glazes to set back the horse. He had suggested a mixture of Lucas warm gray and kraplak that made what looked like a color akin to raspberry yogurt. The idea behind doing it was that it pushed the image back and so that I could attempt to give more volume to the horse by popping the highlights in some areas while allowing other parts to melt into the background.

Working into this pigmented layer certainly had a different drag and grab quality than a surface just oiled out.

If you’d like to extend the openness of the next layer, you might consider adding poppy oil or clove oil to your walnut oil medium. Poppy oil will slow the drying time down some, but poppy oil will slow it even more.

THICKENING OILS

Partially polymerized oils:

Cooked oils and sun-thickened oils and oils that are thickened with metals are all part of this vocabulary or increasing the fatness of your oil.

Oils you can get refined oils and cold pressed oils and then thicken them in a crock pot.

While I was working with straight walnut oil, one of the adjunct professors at San Francisco Art Institute recommended I try adding larch turpentine to make the final layers more “fatty” and to allow me to more easily glaze on top of the layered paintings. It is true that as an additive into the final layers, a balsam such as larch or venetian turpentine (venetian is just larch diluted with turpentine) can make for a luxurious stroke where the color seems to be suspended in honey. The only problem is that the vapors are pretty noxious. I rarely use it these days, but have a little jar in my arsenal for when I need a super transparent glaze.

Luckily, through a serendipitous studio visit, I came across another method to fatten my walnut oils, to serve as a step in between the straight walnut oil and the glazing medium that contains larch/venetian turpentine.

During Bushwick open studios I was visited by a paint maker who intended to sell me his products but unknowingly taught me a useful trick that has informed my painting practice. He told me how his line of handmade walnut oil-based paints were special because they had a wonderfully viscous body to them and were not runny like some other oil colors made using walnut oil. He said the secret was that his company thickens the walnut oil for two weeks in crock pots before using it to make paint.

I usually purchase my walnut oil at my favorite supply shop in New York – Kremer Pigments. Located on 29th street on the edge of the garment district and Chelsea gallery district, Kremer is like a bulk food supplier for painters. Not only is it a cost effective way to get supplies but you can avoid the tacky packaging, product pushing culture, and made in China garbage one might usually encounter in most chain art stores.

I found the sun-thickened walnut oil to be quite expensive, about $40 for 100 ml compared to 1L of refined walnut oil for $25. I remembered I had this tiny crockpot designed for heating dipping sauces like fondu.

I poured the cold pressed oil into the small crock pot up to the level of the lid, closed it and turned it on. The idea when thickening oil is that you don’t want to allow the oil to react too much with oxygen because then you’ll have a bunch of gooey bits of oxidized oil floating in the liquid oil.

I was quite satisfied with the results of my first trial. The thickened oil was fatter and thicker than the refined walnut oil. I now use this home brew as a key ingredient in my strategy to work up layers, lean to fat. For example, I might start with no medium on an absorbent ground and then begin adding a bit of refined walnut oil, followed by a greater proportion of thickened walnut oil. This approach has worked well for almost a decade.

CHALK IN OTHER LAYERS: PUTTY MEDIUM

After having used the oil-chalk ground for a number of years, I wondered if it might be useful as an economical way to extend the volume of my paint in other layers. While working on a painting called Octopussian Waystation, a large oil on herringbone linen composition (54×78 inches), I thought “what if I use this ground medium to build up the areas where I want there to be textured form?” It seemed to work well. As you know, oil paint can be very expensive. It’s possible you could easily brush an entire $60 tube of paint in one stroke. If you were to use chalk medium with this color you could get the same impasto effect for a small fraction of the cost. I thought that during the baroque-era this must have been an invaluable insight, when pigments were absurdly expensive and were only obtained in extremely laborious ways.

During various studio visits with painter friends, I mentioned that I was beginning to experiment with chalk as an additive beyond my using it in my oil ground. It was first in a conversation with the Italian painter, Agostino Arrivabene, that I realized that this practice was indeed a secret of the old masters, one that he personally unearthed in his reading of many obscure and ancient treatises on techniques of oil painting. He mentioned that in his research he found that chalk was the secret component to the luscious butteriness found in the paintings of Velazquez, Rubens, and Rembrandt.

In fact he had personally perfected his own secret medium containing chalk and had a sample jar that he gave me as a gift. He let me know the contents and process of making his secret medium and asked me to swear to not share it with anyone else. It was way too complicated for me (and perhaps over the top approaching the absurd). It felt similar to the methods Salvador Dali outlines in his book Secrets of Magic Craftsmanship. Agostino’s mixture had many qualities that were specifically tailored to the qualities he wanted his paint to achieve. He wanted his mixture to have enamel qualities (egg), self-leveling (tree resin), and to not be too shiny (beeswax). Although it seems like I might have just given away his secrets, I myself could not replicate it even with the other information I’ve withheld. The thing is, knowing the microscopic behavior of your materials, knowing the language of them, if you will, will help make you a master chef in a well-stocked kitchen.