Flemish Painting Technique

(click here to read the original text in Portuguese)
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Detail from the polyptyc of St. Vincent, painted by Nuno Gonçalves. Museu Nacional de Arte Antiga, Lisbon, Portugal.

I would like to thank the Friends of the National Gallery of Lisbon (Museu Nacional de Arte Antiga), who organised the course I gave in that museum in 2016. I am also grateful to artist Alexandre Caetano, who compiled the notes for the course, notes that formed the basis for this text.
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16th Century Flemish Painting Technique – Mixed Egg and Oil Methods
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Flemish painting was based on the much older Byzantine technique, which was usually painted with egg tempera (egg yolk mixed with a little water and pigments of different colours). The egg tempera paint was applied to panels primed with a true gesso ground made of a mix of gypsum and animal glue.
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Today, the best way to learn the different steps and processes of this type of Byzantine painting would be to observe its execution by Orthodox monks in the mainly Russian and Greek monasteries that specialise in iconography. But this is easier said than done. The time that outsiders can spend in these monasteries is usually highly restricted. And additionally, in places like Mount Athos, women are banned from visiting the monasteries  – and this applies to female animals as well as humans! These restrictions make it difficult to learn the Byzantine technique in these places.
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We do, however, have access to a great number of written accounts of the 16th century process of Flemish painting, and using these it is possible to gain a working knowledge of the techniques involved. This is the focus of this text.
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SUPPORT
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Glues
Nowadays we can use rabbit skin glue, which is perhaps the best traditional glue available because of its resilience. In the 16th century, cheese glue (casein) or parchment glue were used. For centuries the Western technique was to paint on a highly absorbent gesso – natural glues mixed with gypsum plaster. These glues were also used for cabinet making because, after drying, they become almost indestructible; the disadvantage is their tendency for to absorb a lot of moisture.
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Rabbit skin glue should be used only on wood, not on canvas. Canvases are weaker and much harder to conserve – painted panels of the 12th and 13th centuries (executed in wood, usually of oak) are often much better preserved than 17th-century canvases. But even oak panels tend to warp, and nowadays marine plywood – usually used in boat building – is often used. It is made of birch or oak, and is theoretically more resistant to fungi, humidity, etc.
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Preparation of the panel
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Glue ingredients
10 grams of powdered rabbit skin glue
250ml of water
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Instructions for preparation
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Leave the glue in the water overnight, so it expands.
The next day, heat the glue and water in a bainmarie (max. 52 degrees) to dissolve the glue. If the mixture warms above 60 degrees, and especially if it boils, it has to be thrown out as it loses its adhesive properties.
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When this “soup” is finished, it is important to use it quickly. It lasts for a day at most. Apply two thin layers to the wood, leaving the first layer to dry before applying the second, to seal the wood and protect the paint from the wood’s acids and resins.
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You can also use animal gelatine or agar agar (vegetable gelatine), but less water should be used as gelatine is weaker than the rabbit skin glue. You can use isinglass (gelatine from the swimming bladder of the sturgeon or the codfish) or inferior fish glue. Beware of dark fish glues, which should not be used as they may be adulterated and discolour the final painting.
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The next step is to mix the previously prepared rabbit skin glue with fine gypsum (gypsum reflects light, protects against mould and is alkaline). In Italy and France they used a chalk from the regions of Bologna or Champagne and sometimes a mixture of chalk with marble stone powder. Chalk powder is smooth but stone powder has a texture similar to fine sandpaper, which paint adheres to better. I recommend a mixture of 50/50 of chalk and stone powder.
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Gesso ingredients
10 grams of rabbit skin glue
250ml of water
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Soak overnight and stir in bainmarie as described above.
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Then add
250g of marble dust (or stone powder)
250g of fine gypsum
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At this point you could also add the very mediaeval ingredient of “cow’s vomit” (ox gall). Put a few drops of synthetic ox gall in the gesso – it imparts a self-levelling quality to the plaster, and prevents the air bubbles that often form when drying.
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To apply the first layer of gesso, brush it roughly so it adheres to the whole surface of the board then immediately smooth the rough strokes by brushing horizontally from the edge to the edge of the surface, along the grain of the wood. The hand should be light, with a single stroke, to create the most even possible surface The following layer should be applied vertically, and then four or five more layers should be applied, alternating between horizontal and vertical strokes, with the last layer being applied in the direction in which the canvas is longest (ie top to bottom for a portrait orientation).
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These layers should be thicker than the first layer and applied while the previous layer is still slightly moist, otherwise the entire gesso might crack. The trick is to apply when the previous layer has just begun to lose its wet look, but has not dried completely. The layers must become thick enough that the gesso, once dry, can be sanded down with fine sandpaper without any danger of reaching the wood. Remember also that if the gesso is applied too thickly, it can crack.
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Polishing
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When the gesso application is finished and dry (after a day or two), the entire white gesso surface should be smeared by hand with a dark pigment until the surface is completely darkened. Always use a non-toxic black or earth-coloured pigment.
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An alternative is to spread a small amount of pigment on the surface of the gesso with an old hog brush. Bristle brushes are extremely resilient and therefore the hairs do not break when dry brushing against the gesso.
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After the whole surface has been covered (it should look ‘dirty’), it should be sanded with fine sandpaper (around 200 grit) followed by a finer sandpaper (around 400 grit). Sanding in circular motion to avoid directionality, be careful not to apply too much pressure as the sandpaper could cut through the gesso and reach the wood.
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You will know that the surface is completely smooth when the dark pigment is all removed and the gesso is white again.
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At this point, gently dust off the surface and apply another coat of rabbit glue, using the first formula above. Some painters prefer to apply two coats because the gesso can be too absorbent and needs to be made a little more water-resistant.
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The rabbit skin glue solution creates a slightly different texture on the gesso, as if there were a slight grain. Allow it to dry before sanding it again very lightly to smooth the surface without removing too much glue.
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There are divergences of opinion about long-term durability of traditional formulas. The Canadian Conservation Institute reports that rabbit skin glue causes long-term embrittlement and suggests the use of acid-free PVA glue to seal the boards and make the gesso. The National Gallery of Washington conducted some tests and came to the conclusion that rabbit skin glue cracks oil paintings on canvas. However, the National Gallery of London cleaves more to tradition, noting that even the best acid-free PVA glues may present unforeseen problems in the long run.
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DRAWING
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Once the surface is dry, there are two options. Those with a lot of drawing experience and want to take a freer approach can draw directly on to the surface. Those who are less confident drawing freehand can make a trace of the drawing. In the past, small holes were made in the contours of the original drawing and, after the outlines were all punctured, the drawing was transferred to the board by rubbing it with coal dust. Another option is to copy the drawing onto tracing paper, so as not to damage the original drawing, then rub the other side of the tracing paper with burnt sienna. Place the drawing on top of the panel, tape it to hold it in place, then go over the outlines with a sharp stylus. The contours will be transferred to the panel.
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This Byzantine Pantocrator represents an oriental and older way of iconography that influenced Flemish painting that was born afterwards in western Europe. Monastery of Chelandari, Mount Athos, Greece. Egg tempera on wood, 13th century.

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PAINTING I – EGG TEMPERA
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Earths of various shades were used as pigment for the egg temperas. In the painting of Byzantine icons, Isinglass (gelatine from the swimming bladder of the sturgeon or codfish) was used to make the gesso and the painting was done with egg tempera (a mix of egg yolk with pigments), which makes the act of painting more difficult and demands a more accurate application technique. Nevertheless, these ancient icons remain in good condition, often better than some of the later icons.
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The Flemish painting style is based on this much older Byzantine tradition, with some modifications. Paintings made with egg take an extremely long time to dry – it’s often months before a final oil varnish can be applied. And western painters often found the egg-tempera colours insufficiently realistic. So the Flemish modified the tradition, by painting with egg tempera as a base, then superimposing oil paint in order to create a more realistic effect.
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There are some misconceptions about the discovery of oil paint. Most people think it was first used in the Europe, especially during the Renaissance period. Some attribute the invention of oil painting to the Van Eyck brothers. It’s more probable that these artists learned it from an older Flemish painter, probably Robert Campin. The Van Eyck brothers’ paintings had a special and mysterious fragrance, which was particularly admired and earned them more fame than any painter who had gone before. It is thought that they used essence of turpentine to dilute the paint, something unheard of at the time. But oil paint existed well before this – it has been around since the 7th century and was first used by the Buddhist artists in Afghanistan, in murals in more than 50 caves in the valley of Bamiyan. Only later, in the 12th century, was it developed in the West for minor decorations and weatherproof paint for wooden doors. But the claim of oil paint to being the preferred medium for artists was only confirmed in the 15th century, as its visual possibilities were finally fully explored by artists like the Van Eyck brothers.
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In the Flemish mixed technique an initial egg tempera layer should be used, especially in light colours in order to create archival results (as egg tempera does not change colour over time). The first layer would often be monochromatic and used to be called verdaccio (a green earth found in Italy and the Czech Republic, or a mixture of earths mimicking the colour of olive green) or Grisaille (a brownish grey earth most commonly encountered in southern France).
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There is a reason to use this greenish verdaccio shade. We often make the error of thinking that human skin colour is only composed of yellowish and pink tones, creams, etc. But in fact, the melanin of our skin is only the surface layer of a body whose inner colours are very different. We have bluish or greenish looking veins, and tendons and muscles that can show through the skin as cool or greyish.
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For this reason, the Flemish would use greenish colours, as is the case of verdaccio, which would create the subtle undertones of the skin, and on top then paint a more orange or rosy flesh tone with greater or lesser degree of transparency according to necessity. Veins could be subtly evoked by leaving these shades of green with only very thin veils and glazes of skin tones – a highly sophisticated process that is evident in a later phase of Mediaeval-Renaissance oil painting.
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Verdaccio was more commonly used in paintings from Northern Europe, Flanders, Belgium or Northern France, at least in part because people were paler, with more transparent skin tones revealing more of the greenish or bluish colour of the veins. If we compare the chromatic choice of the 16th century in the representation of a person with the expressionist approach of the 20th century, we can find some common characteristics, including a similar diversity of skin colours ranging from orange all the way to blue. The great difference is the subtlety of the technique in the Flemish art, as opposed to the bolder style of the 20th century.
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How to make egg tempera
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Break an egg. Keep the sack of the yolk in one hand while washing the other hand to remove the egg white. Change the sack to the washed hand and repeat this process, always washing the free hand, until all the glair is removed and the sack is clean. It will develop a rubbery texture and start to slightly stick to the hands. Then carefully hold the sack in both hands and dip it in water to wash away all final traces of glair. Remove the yolk sack, hold it over a bowl, puncture it with a needle or knife and pour just the yolk into the bowl, making sure the membrane of the sack does not fall in the bowl. This yolk can be divided in small quantities, depending on the amount you want to use to prepare different colours.
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The yolk should then be mixed with equal volume of distilled water. You can also add approximately one tablespoon of white wine (or two drops of white wine vinegar) per yolk (except in the case of ultramarine blue, which must be prepared without wine or vinegar). Nowadays, one can also use a Japanese rice wine, called mirín, for the same effect. Wine also becomes vinegar, which, being acidic, kills some bacteria and fungi. This egg binder is not very resistant at first, but after drying for a few months it becomes stronger than oil paint.
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When mixing pigments with the diluted yolk, avoid those that are sensitive to acids, as they will react with the vinegar and fade. Nowadays we can use Cerulean and Prussian blues (instead of ultramarine) or earth colours. The ancient lead white tempera, which is now banned because it is highly poisonous, would oxidise and turn black over the years – the best modern white pigments for tempera are zinc or titanium. Almost all earth pigments are perfect, ranging from yellows, oranges, reds, greens (the aforementioned verdaccio) to almost black. It’s very difficult to get hold of good shades of Terre Verte, but you can achieve similar tones by mixing yellow ochre with black.
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Regarding the amount of pigment to add to the diluted yolk (binder), it is advisable to do some tests as there is a risk of putting the wrong amount of yolk. There is no exact recipe – only experience will tell. Start by mixing equal volume of binder and pigment. When the paint dries, it should be matte, with a slight satin sheen. If there is too much pigment, the paint dries completely matte and the pigment powders off when rubbed with a tissue paper, because it did not agglutinate with enough egg yolk. If there is too much yolk, the paint dries shiny (the yolk is greasy, oily) and it can cause the painting to crack. Between the two extremes, it is preferable for the tempera to have too much pigment than too much yolk. A well-made egg tempera paint can be infinitely diluted (with water clouded with just a dash of yolk), without the risk of losing adhesiveness.
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This mixture does not need to be ground under a muller because the pigment mixes easily with the yolk. This paint only lasts a day and the painting must always be made with freshly prepared paint.
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Start by delineating the main lines of the composition, to fix the pigment that will remain after tracing the drawing. Before applying the tempera to the surface to be painted, you should always remove any excess of paint from the brush. Egg tempera should never be applied thickly as it could crack.
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After contouring the outlines of the composition and allowing the paint to dry, a transparent layer called imprimatura should be applied. Use the same paint that was used in the contours, but dilute it in water and apply it as a transparent wash over the whole board. This imprimatura creates a medium tone over the entire image – a more relaxing tone to the eye than the stark white of the naked canvas. This imprimatura should only be done after all outlines are well-delineated and dry. The imprimatura should be applied with a soft hair brush (I recommend Japanese goat hair brushes or synthetic soft nylon), in a single stroke along of the grain of the wood, or in thin layers – always allowing drying time between layers so as not to dissolve the previously created outlines. Always stir the paint when refilling the brush, because the pigment tends to settle on the bottom of the container.
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When you finish covering the panel with imprimatura, allow it to dry, turn it over and repeat the process, being careful to cover any accidental gap you might have left unpainted. You should wait several hours between each layer of imprimatura. It is also preferable to apply many thin layers than just a few thick ones. In this way the shading process is created while the base is sealed – and the more layers of imprimatura, the more sealed the gesso will be. But avoid getting too dark – just a pale base colour is enough.
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For shading, use the same tempera paint (so long as you kept it in the fridge overnight – otherwise prepare a new batch), without adding more pigment or egg. You can create shadows by using successive thin layers with an almost dry brush. Never insist twice in the same place, otherwise the moist brush can redissolve the imprimatura underneath, or even scrape it off and expose the white gesso. It is therefore necessary to alternate between different areas of the image, jumping from one to another and only returning to the initial areas when they are fully dry.
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Ideally, the brushstrokes should be short, quick, thin lines, jumping from area to area across the surface of the artwork while the other areas are left to dry. This way you can gradually darken the shadows but never fully cover the light gesso underneath. Shadows will look more luminous and mysterious if they are almost, but not totally, black. White tempera can also be used to paint in the highlights using the same technique, but this whiteness should never be too intense, otherwise it creates an exaggerated effect of chalkiness that can very easily become kitsch. A more translucent zinc white is preferable to a super opaque titanium white. Intermediate shades can also be created by mixing the dark tempera with various amounts of white tempera.
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(Additional note: When the orthodox monks paint with egg tempera, each aspect of the process is charged with symbolism. The egg is the symbol of life because it looks like a rounded pebble – yet it is alive. The wine (vinegar) used when mixing the tempera carries strong symbolic associations with the blood of Christ the Redeemer. In becoming vinegar it preserves the paint by killing fungi and bacteria that could harm the painting and its support, but it is also the symbol of God, because in alchemy the transmutation of a substance is associated with a spiritual symbolism.
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The whole process of painting an icon is accompanied by prayers. The first coat of paint, painted with skin tones in verdaccio, represents the death of the flesh and the first phase of humanity – the mortal sin of Adam and Eve. After this stage, they begin to paint the skin tones a little redder, lighter and sharper, representing the higher clarity of a second biblical stage – the prophet Moses and the ten commandments. In the third stage, the brightest highlights are rendered in bright yellow ochre with fine brush strokes, never hazy or blurred – representing Christ the Redeemer and the new clarity of God’s message.

The gold leaf in the icons’ haloes is applied on a size made with a red clay (red bole mixed with animal glue), common in Armenia. This gilding is made using a soft brush to paint this size over the areas that are to be gilded. After applying successive layers and allowing it to dry, the size is polished until totally smooth. Then the monks breathe three times on the size, representing the Holy Trinity: the Father, the Son and the Holy Spirit. This size becomes sticky with the humidity of the breath and swiftly the gold leaf is laid over it, sticking immediately. It has been discovered over the years that if the monk first wets his mouth in vodka before giving the three breaths, the gilding will be more effective, and it has become a tradition to use vodka to assist the gilding process in Eastern Europe. Gold over red clay size also has a special meaning for orthodox painting. Gold is a pure, incorruptible, and eternally shinning material, representing the immortality of the soul, and transmuting the mortality of the body represented by the red clay.)
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PAINTING II – OIL
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The next step is to allow the egg tempera to dry thoroughly, ideally for a few days. A sealer can then be applied (you can use varnish of shellac dissolved in alcohol) before painting with oils. Shellac resin is purified faeces of an insect (the female of the lac bug), which lives on trees in India and Thailand. This wood is heated next to a fire and the faeces melt and drip, before being collected and rendered into varnish. Some artists used pure yolk to seal the egg tempera instead of shellac (as pure yolk is oily in its pure state), but this is not advisable. Other methods can also be used but I found shellac consistently effective.
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Apply several thin layers of shellac varnish (highly diluted in alcohol), allowing each layer to dry. Keep applying until the painting becomes satin or glossy. Let the varnish dry for a few minutes more and lightly polish it with superfine sandpaper. The dried and varnished egg tempera should then be oiled with a painting medium such as Sun Thickened Linseed Oil (always buy the cold pressed type). This oil can be pre-mixed with a bit of Juniper Resin or Larch Turpentine so it dries with an increased gloss – use a ratio of 10% resin to 90% linseed oil. A more modern resin like Canada Balsam also does the job if you cannot find the aforementioned resins.
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Some artists also add a droplet of a drier to this medium, to speed up the drying of the paint, but this can be risky as the oil paint will age badly if too much drier is added. Avoid black driers – the more clear and transparent the drier is, the less harmful it should be (driers based on cobalt salts are the best). The black dryers, although they dry quickly, do not dry out in depth and cause the paint to form a superficial skin which can crack. Better to do without them and wait for the oil paint to dry naturally. Another alternative medium is to use Stand Oil or a special medium called Liquid Glass. This special medium has a secret formula known only to its maker, Dr Cranswick. You can buy it in the UK at Cornelissen or order it over the internet and I highly recommend it, but a mix of Stand Oil and some resin also works well.
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The medium of your choice is applied by rubbing a few drops of it over the area to be painted that day. Then remove the excess with the softest side of the palm of your hand, always wiping your hand on a lint free cloth to clean it first.
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After applying the aforementioned ‘oiling out’ of medium, you can then colour the surface with thin oil paint layers, which can go from transparent to opaque. The dark colours should always be painted first, in the shadows with transparent glazes. The paint should be spread pure and in very small quantities, without any thinner added to it. Spread it so thinly that the egg tempera verdaccio is still visible underneath it.This technique of painting on top of an ‘oiled out’ surface is called couching.
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The light zones can then be painted with opaque oil paints, to give greater intensity and to create a greater brilliance for the highlights. This is how you can achieve the beautiful 3D effect that is so characteristic of Flemish painting.
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You should not overload the brush with excessive paint as this makes the painting appear less delicate. There should also be no excessive outlining, or the painting will look cartoonish. You can disguise any accidentally overstated outlines with delicate shading, but really they must be practically non-existent.
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Oil paints tend to yellow over time (owing to the yellowing of the linseed oil), while the egg has exactly the opposite chromatic effect because it gradually loses the yellow tone of the yolk. Flemish painting thus created another ideal symbiosis for preserving colours over time.
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Another interesting topic is the effectiveness of the verdaccio as base tone. For example, some Flemish portraits reveal parts of the verdaccio done in egg tempera. There is an ingenious explanation. Imagine two squares, one painted muted olive green and the other a brighter pink. Owing to a trick of perception, the green appears to us to be distant, as it is more neutral and cold, and the pink, being a stronger and warmer colour, looks like it is closer. So some Flemish painters would render the cold vein colours in verdaccio first (to make them look distant) and then more or less cover them with the warmer, reddish flesh tones, so that the skin colour looks closer than the veins.
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Paint drying speeds
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An ideal way of applying the paints is to use quick-drying paints for the first layer. After drying this layer, couch it again with very little medium and add another layer that dries more slowly. After drying, couch it again and paint a third layer with paints that dry even more slowly. A painting made using slow-drying paints first is much more likely to crack over time.
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It is important to make drying speed tests of the paints you have, in a separate white plastic palette, as these vary from colour to colour and from manufacturer to manufacturer. Squeeze a bit of each colour on the plastic palette, with varying thicknesses, and see how many days each colour takes to dry. This way you will know the drying speed of each tube of paint and can determine which colours should be painted first or last. I’ve also noticed that the transparency square printed on the paint tube does not always correspond to reality so it is also good to visually check which colours are more transparent by smearing a little bit of paint on aluminium foil. The colours that look more vivid and metallic when spread thinly are the transparent ones and should be used for the shadows in the painting. The colours that look chalkier when spread thinly are opaque and should be used exclusively to paint the light colours in the painting.
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(A note on opacity: A good way of thinking about paint opacity is to consider this example: if you put a few drops of water in a jug of milk, you will not notice any difference in the colour or opacity of the milk. However, if you put a few drops of milk in a jug of water, the water visibly loses its transparency and becomes cloudy. Hence, in Flemish painting, the opaque colours of light should not invade and cloud the transparent colours used to paint the shadows, unless one wishes to achieve a dull and dirty effect in certain areas. Transparency is one of the finest qualities in oil painting and is the quickest to disappear if the paint is applied carelessly. A method to safeguard this transparency is to first paint the colour of the shadows with a transparent pigment, in a very thin layer, covering not only the shadow areas but also the light zones. Afterwards, on apply the light colours to the light areas with a more opaque paint. At the very end, paint the lightest highlights with a totally opaque paint, which can be thickly impastoed).
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After each thin coat of paint, the work should rest and dry thoroughly before applying the next layer of medium couch and paint on top. As usual, paint the larger shapes first, then approach the smaller details.
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Pretend that you are not painting with oils but just ‘dyeing’, leaving the verdaccio in egg tempera to show through these transparent veils of oil paint, as if a black and white photo was colorised with thin watercolour paint. Remember to always apply the darker and more transparent paints first and then the opaque and lighter tones on top. In shadow areas, a rich saturation is best achieved through thin transparent layers overlapping gradually. Never paint dark shadows with thick, opaque paint layers. Most shade areas should be more neutral and have less detail than the lights (but still be transparent). According to Leonardo da Vinci, the human eye cannot see details in shadow, which means that the brush marks in the shadow areas must be blurred with a soft brush and a very light touch, as if you were tickling the surface of the paint.
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In summary: paint dark shades in transparent glazes and with a more neutral colour; paint highlights with more intense and opaque layers.
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Beginners tend to chalk up portraits. Skin tones are almost always much darker than most people think – compare your skin colour with a white sheet of paper and appreciate its relative darkness. Another common error is to paint people too orange or yellow: most skin colour is much more grey than beginners believe it to be.
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Albrecht Dürer painted pictures with fantastic detail. According to some recent studies, Dürer used an oil paint base to paint the hair and the fine lines of his portraits. While this base was still wet, with a thin brush (apparently made with hairs from the back of his hand) the details were painted with Tempera Grassa over the still-wet oil paint. When the tempera dries the still-wet oil paint tends to repel it and, in the process, make these painted lines even thinner.
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Nowadays, there are certain traditional colour mixes that are difficult to recreate as some pigments are difficult to obtain. Lead white, for example, probably caused the death of many artists (eg Caravaggio). Lead can penetrate the body through cracked skin or by eating without washing your hands, and cause poisoning. Mercury, exposure to which can lead to madness, was also widely used in paints. Perhaps the caricature of the eccentric or outright crazy artist is better explained by mercury poisoning than by any innate temperamental disposition.
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The exact composition of the paints used by the Flemish remains a mystery to this day. The artists of the 16th century had secret formulas for oil painting and some even killed others to acquire or keep them. These were times when artistic knowledge was highly coveted. One thing is almost certain: these paints were not made up simply of linseed oil and pigment. The artists used to belong to the same guild as the alchemists or the craftsmen specialised in the varnishing of musical instruments. Back then, it was well known that linseed oil dried faster if it was purified and then boiled. Nowadays, our modern Stand Oil approaches the properties of boiled linseed oil, but it dries much more slowly (in the modern process the stand oil is boiled in vacuum). With a little resin added to it, the oil becomes even more transparent, glossy and resistant to moisture. The transparency of the paint is also a function of the type of pigment and care must be taken with the proportions of, for example, the resin in the blend. If too much resin is added, the paint may crack or darken over time, so it is recommended a maximum of about 10–20% of resin mixed with the linseed oil. There is some scientific evidence indicating the occasional use of liquid amber or juniper varnishes mixed with the oil paint for certain colours.
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In some older European treatises on painting (including the Portuguese treatises ‘Art of Painting, Symmetry and Perspective’ written in 1615 by Filipe Nunes and the ‘Brief Treaty of Illumination’ written by a monk of the Order of Christ before 1640) it is mentioned that fugitive pigments, such as carmine, should be mixed with resin in order to improve their lightfastness. It appears that the resin seals the pigments and makes them more resistant to moisture and oxygen, helping to preserve the colours of these more fugitive pigments.
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The topic of the chemical composition of old masters’ paints is still controversial. The National Gallery of London, for example, stated that it did not find resins in most of the samples that they analysed. However, there are many ancient paint treatises in which the use of the resins is mentioned. This does not mean that resins were definitely not used for certain fugitive or extremely slow-drying pigments. The matter is the subject of much discussion and controversy: resins may have been used, but not as much as it is advocated by some modern painters who want to rediscover the ‘secrets’ of the old masters. On the other hand, through the process of boiling the oil with the resin, some ingredients of the resin may have been annulled or polymerised, making it difficult to chemically analyse its original structure. There is also no absolute evidence that this mixed technique of oils over egg tempera was widely used, as the tests are not conclusive.
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Technically, oil painting had its apogee in the XV–XVII centuries. The liberal revolutions that followed were much necessary for European society but catastrophic for the technique of painting, as they involved the widespread destruction of libraries and monasteries that kept an immense body of knowledge about these older processes. In the 20th century knowing how to paint even went ‘out of style’ and solid technical training stopped being taught in fine art academies. Painting technique was even attacked by several modernist and conceptualist movements, which considered painting to be an outdated reactionary type of art, and this led to an even greater loss of knowledge.
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BIBLIOGRAPHY
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Unfortunately, as far as I know, there are still no good manuals about the practical methods of Flemish painting. I find it unbelievable that almost nothing is published about one of the most important artistic styles of western culture, and the most complex and durable of all the historical techniques for oil painting. The manual I recommend, although it contains some errors, was published in 1935 by Vaclav Vytlacil: ‘Egg Tempera Painting, Tempera Underpainting, Oil Emulsion Painting – A Manual of Technique.’
(Please note this book recommends the use of Maroger and Meglip mediums or Dammar varnish, which have been proven to crack and discolour over time. These substances do not make justice to the Flemish methods. Please refer to the book for understanding the methods but ignore its formulas and use instead the recipes for oil mediums that I provided in the text above).