There are several current painting methods whose practitioners claim that they are working just like early Flemish painters did. In the early 1400's, Flemish and Netherlandish painters (Jan van Eyck, Robert Campin, Rogier van der Weyden, and so on) invented oil painting as it is practiced today. In the 19th and early 20th centuries, art historians spent a lot of time speculating about how the first oil painters practiced their art. Unfortunately, they could only look at paintings and read old manuscripts. As it turns out, looking at paintings doesn't tell you a whole lot about the methods by which they were made. And the early oil painters didn't, so far as we know, write down their valuable trade secrets for scholars hundereds of years later to study.
So most of what you will see about "Flemish" painting methods, although sometimes used by very skilled artists, is basically what some 19th century German academics erroneously thought Northern European painting was. There is one school that alternates layers of egg-oil emulsion white with layers of oil glazes in primary colors. Another school calls for each painting to be made in seven layers, each of which dries for seven weeks (are we artists or numerologists?).
The reason why people are so fascinated by early Flemish paintings is that they are so remarkable. First, although they are very, very old, they are often in much better shape than oil paintings that are hundreds of years younger (many early 20th century paintings have deteriorated more than paintings from the 15th century). Second, the paintings glow with a sense of vitality that is seldom seen in more recent paintings. They have a quality that is often described as "jewel-like." This is especially evident if you see them in person; it's easy to pick out the Flemish paintings from those done in other periods, or those from the same period done in other places. The precise detail and gorgeous colors are marvelous to look at.
The thing is, we now know a lot more about early Flemish painting methods than we used to, because modern technical analysis provides a lot more information. So I can, with reasonable authority, describe actual "Flemish" painting methods, which were generally much simpler than those taught in special classes where the "Secrets of the Old Masters" are revealed.
The Northern European paintings we have are almost entirely done on panel, not because canvas wasn't used, but because panel paintings are the ones that have lasted. The panels were made of local hardwoods such as oak. They are often quite small; the large ones were made with several planks fastened together. The panels were planed to a smooth finish and then seasoned (usually for several years). They were then coated in hide glue, which is a gelatin that becomes liquid when heated. Then an initial priming was applied that consisted of warm hide glue mixed with chalk (calcium carbonate). This priming was applied in several layers, allowed to dry, then sanded or scraped smooth. The modern word for a priming layer like this is "gesso," although technically gesso was the Italian version, made from hide glue mixed with gypsum (calcium sulfate). Modern acrylic primer is often labelled "gesso," but that's totally incorrect. Anyone caught referring to acrylic primer as gesso will be required to stay after school and write "gesso is not made with plastic" 1000 times on the chalkboard.
There were very few pigments available at the time to make paint with, although that limitation is not obvious from looking at old master paintings. The available colors included ultramarine blue (very expensive as it had to be imported from Afghanistan and then laboriously refined), azurite blue (sort of like a cobalt blue), red lake (similar to alizarin crimson), lead tin yellow (similar to Naples yellow), vermillion (similar to cadmium red), bone black (similar to what is now, incorrectly, called "ivory" black), flake white, copper green (similar to viridian), and various earth colors (similar to modern ochres, siennas, and green earths). Paint was made by hand, usually that morning or the day before, by apprentices and studio assistants. If you haven't tried oil paint made fresh by hand, it is more fluid than the stuff you get today in tubes.
In the early 15th century, oil painters probably didn't use solvents such as spirits of turpentine or oil of spike. This demostrates that it is quite possible to apply oil paint with very fine precision without having to thin it down. It's easier to do this kind of work with handmade paint than with tube oil colors.
This method involves multiple layers that are allowed to dry in between applications. In some cases, lower layers were done in egg tempera, switching to oil later on in the sequence. Flesh tones were painted very thinly, in one or two layers. The modern oil painting approach (mostly developed in Florence in the early 16th century) emphasizes thin, transparent darks and thick, opaque lights. By comparison, early Northern European painting was primarily a glazing method in which darks were established with thick layers of transparent paint. This allows the darks to have a sense of depth and color that is not possible in direct (single layer) painting. Glazing is typically done over a layer of opaque paint of similar color; red lake over vermillion, for example. Black, since it tends to produce dull mixtures, is not used except to darken earth colors (most artists think that the idea of avoiding black was invented by the impressionists, but no). Instead, darks are created by thick glazes and by what we would today recognize as complimentary color mixtures (red lake could be darkened with ultramarine, for example).
Here's one possible reconstruction of the painting sequence:
- Make a detailed drawing on paper and transfer it to a gessoed panel. Reinforce and elaborate the drawing with ink or dark paint. Northern European artists usually made very detailed underdrawings.
- The initial layer of paint, called the primuersel, is used to define the dark areas of the painting. The primuersel color is made by mixing black with earth tones such as red or yellow ochre; it is therefore similar to the verdaccio used for flesh tones in the traditional Italian egg tempera method (but without a green earth underpainting and not just in flesh tones). This layer was probably often done in egg tempera or tempera grassa. Either way, this layer should be used to define form, edges, shadows, and other darks throughout the painting.
- Now apply the basic colors to each area of the painting, starting to work up modeling of forms with opaque colors but avoiding fine detail. This stage is called “dead coloring.” As with primuersel, some artists used egg tempera or tempera grassa for this layer. Flesh tones can be begun with mixed tones using appropriate brown or pink mixtures of white, vermillion, and earth colors. In light areas, keep the paint as thin as you can; you want the white of the gesso to show through. X-ray scans show that lead white was often used only to emphasize the brightest highlights of flesh tones, with a very thin toning of the gesso used for most light areas of flesh. This approach keeps light areas bright and avoids later yellowing by minimizing the amount of oil. In shadow areas, don’t worry so much about paint thickness, but keep the surface of the paint smooth. In areas that will later be glazed, keep light areas lighter than the intended final effect, since glazes will darken what they cover. When you have taken this stage as far as you can, let the paint dry once again.
- If you began the painting in egg tempera or tempera grassa, you will switch to pure oil paint no later than when dead coloring is completed. Use paints ground in linseed oil. When switching from tempera to oil paint, you may choose to apply a very thin layer of oil to prevent excessive absorption by the tempera underlayers. From now on, you will let each layer dry thoroughly before further painting. If the layers are thin, this will take up to several days each time, depending on the pigments you use and whether the paint contains siccatives. It may be helpful to wet sand in between layers as needed to maintain a surface that is thin and smooth.
- You will now work each area of the painting toward the intended final finish. Where desired, you can either thin the paint slightly with medium (oil mixed with a varnish or balsam) or put a very thin layer of medium onto the surface and paint onto that. Apply fine detail to light areas. Areas of dark or bright colors, especially those around the main areas of interest, can be glazed. So, for example, a red robe, after a primuersel of black mixed with red ochre and then initial opaque modeling with vermillion and white, could then be glazed with a transparent red lake, using fingers, a rag, or a soft brush to make the glaze very thin over the lights and thicker over the darks. A little ultramarine or indigo could be glazed in to neutralize and reinforce the darker shadows. Multiple glazes can be applied (allow the paint to dry and wet sand in between layers) until a clear sense of three-dimensional depth has been achieved, generally with inky darks, intense midtones, and bright lights. This process can be very time consuming, especially if you are not using siccatives, but it produces a striking effect that is immediately noticeable when comparing Flemish oil paintings to other work of the period.
- The final stages would be used primarily for detail work and to apply thin scumbles of opaque color mixed with white where needed over lighter areas of glazed color. Again, sand between coats. It is possible that some fine details were applied with egg tempera, worked into wet oil paint. This development of detail would continue until a very high degree of finish was obtained.