Materials and Techniques of Old Master Painting

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MATERIALS AND TECHNIQUES OF OLD MASTER PAINTING

The following guide is divided into four sections, according to the four basic layers comprising most paintings: Support, Ground, Paint, and Varnish.

SUPPORT

WOOD (called "Panel Painting")

Primary Period of Use

This was the most common support for paintings until the 16th century and was still common in the 17th century. After that, canvas was preferred.

Evidence

Look for wood grain, cracks and splits, joined panels of wood comprising a larger panel, and warping. The latter is most evident when looking along the edges where the painting meets the frame. Crack patterns in the paint layer of panel painting tend to be more rectalinear. Because panel surfaces tend to be smoother than canvas, the color is darker and more luminous. With the more irregular surface of canvas, more light is scattered across the paint surface, giving a slightly hazy effect.

Deterioration

1. As humidity levels rise and fall, the wood expands and contracts. Since the paint layer doesn't respond to water or move in conjunction with the wooden panel, it can only crack. This is the primary reason for cracks in the paint layer. This humidity-induced movement also makes wood crack and warp. (In the past, people tried to control the warping by gluing a rectalinear grid of wooden strips to the rear of the panel, but this only increased the stress on the wood. This technique, called cradling, has been largely abandoned today. I mention it because many panel paintings still have cradles on them.

2. Rotting and insect damage also cause panels to deteriorate so they no longer offer a stable support to the ground, paint, and varnish layers. A transfer was often performed, where the panel was turned upside down, the wood scraped away, and a new piece of wood (or canvas) glued on to the back of the ground layer. Because this radical procedure often damaged the paint layers, this once common treatment is now a last resort, and performed with new safeguards.

SLATE AND COPPER

Since water is not absorbed, slate or copper paintings don't expand and contract or warp, nor do insects eat them. These materials were used occasionally in the 16th and 17th centuries, perhaps because they didn't warp like wood or scatter light like the rougher canvas surface. Slate and copper offer smooth, glassy surfaces on which an uncracked paint layer sits with deep luminous tones. Unfortunately, the ground layer, and thus the paint and varnish layers, do not adhere well to such non-porous materials, thus their rarity. Slate and copper can be recognized by the absolute smoothness of a crack-free paint surface, the absence of signs of wood grain and cracks and canvas weave, and, if copper, the occasional presence of dents.

CANVAS

Period of Use

Though paintings on unstretched linen were fairly common in the 15th century North, only a handful have survived. Surviving, stretched, framed canvas paintings are common first in 16th century Venice, where high humidity made wood warp and frescoes deteriorate. (Venetian painters often covered walls with large canvases rather than frescos.) The trafficking of paintings in Renaissance Venice may have also helped make canvas popular since canvas was light, easily transportable, and could be rolled up.

Evidence

Look for a wavy or saggy surface and for the canvas weave, especially in the dark areas where the paint layers tend to be thinner. The color of canvas paintings is less rich and dark compared to panel paintings because the irregular canvas surface scatters more light.

Deterioration

Like wood, canvas dimensions change with changes in humidity, putting stress on the canvas and other layers. As it ages, canvas rots and becomes brittle and weak. It may no longer "support" the other layers well. In the past, a transfer or relining was performed. As with wood panels, a canvas transfer involved turning the canvas upside down, scraping off the old canvas, and attaching a new canvas to the ground layer, either with glue or, in more recent years, wax. Transferring canvases also tended to result in damage and is a last resort today. "Relining" means turning the old canvas upside down, putting a new canvas on top, and sinking some adhesive through the new and old canvas and into the ground layer. Most old master paintings have been relined at least once, especially those in major museums. If only the tacking margin along the edges of the canvas has deteriorated, a "strip lining" is performed whereby only these ragged edges are given a new canvas backing. Relining in the 19th and early 20th century was performed with a very hot, heavy press, roller, or iron, thereby crushing the delicately brushed ridges of paint and destroying the artist's brushwork, his or her "signature". This was particularly disastrous with those 17th-19th century artists who used thick, heavy brushstrokes (impasto). Unfortunately, the life of countless old master paintings has literally been crushed out of them. Recent relining technique avoids harsh pressure on the painting through the use of a slightly heated heated vacuum table. The most recent relining theory avoids the wax adhesive favored in the 1960's and 1970's since this tends to sink into the ground, paint, and even varnish layers, darkening them and disturbing the painting's original tonal balance.

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GROUND

Grounds serve three purposes. They allow a smoother surface than bare wood or canvas and minimize thereby the scattering of light while maximizing the richness of tones. Grounds also give the artist a good surface to make an underdrawing on. Finally, they minimize the sponge effect that bare wood would have on the paint layer. Bare wood would suck up all the medium (oil, egg, etc.) in which the pigment (colored powder) is mixed. From the 17th century on, colored grounds were popular with some artists to warm up (or cool down) the colors of the paint layers above. With a ground of medium tone such as grey or tan, painters could work more quickly, adding lights and darks and leaving the ground to work as the middle tone.

Grounds for wood or canvas were made from layers of "gesso" brushed onto the wood or canvas and sanded to a smooth surface when dry. Gesso is a watery mixture of glue and gypsum (in Southern Europe) or chalk (Northern Europe). On canvas, the gesso layers are much thinner because of the flexible support. Since the 19th century, many artists have used commercially prepared canvases with grounds made from the pigment, lead white, mixed with oil. Unlike inert gesso, these oil grounds have quickened the deterioration of the canvas' cellulose fibers so that most of these commercial canvases have needed relining. Some current, commercially prepared canvases use a less harmful acrylic ground mixed with titanium white. "Color field" painters like Rothko, Frankenthaler, and Louis don't use grounds but paint directly onto bare canvas with turpentine as a medium. The turpentine evaporates leaving a paint layer with no protective varnish layer added. Such works are easily and permanently damaged whenever anything rubs against them.

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PAINT

All paint is a mixture of a medium (egg, oil, wax, water) and a colored powder or pigment. Most medieval and Italian 15th century pictures use egg as a medium. After around 1470, oil painting, in use in Northern Europe since the early 15th century, becomes common in Italy.

EGG

More opaque than oil, egg dries almost immediately upon application. Since the strokes cannot be blended once they are laid down, they are applied with a tiny brush and blend in the eye at a distance. Egg tempera, as it is usually called, has a fine network of linear brushstrokes upon close inspection, an overall linear vibrancy and sparkle, strong outlines to the various forms, and a flat, opaque, bright color. Unfortunately, many egg tempera picture will not show this delicate linear surface because of past abrasion and other damage. The absence of clear brushwork is itself clear evidence of damage in an egg tempera picture, as is the presence of greenish flesh tones. Egg tempera painters generally used a greenish underpainting for their flesh tones; if you see green, it means the flesh tones (and the upper paint layers in general) have been at least partially rubbed away. (The application of heavy varnish layers in later centuries may give egg tempera pictures a superficial, glossy appearance of an oil painting.)

OIL

Oil is translucent so that the light penetrates the colors, bounces off the white gesso, and passes back through the oil color again. No wonder oil colors are more luminous and have a glowing depth compared to egg tempera color. A more atmospheric, airy color is possible only with oil. This greater depth and harmony of space explains why oil replaced egg in late 15th century Italy as artists struggled to make a more plausible and compelling pictorial world. Oil paintings are somewhat like jewels in their translucency and depth of light. An oil layer is actually many superimposed layers of brushstrokes, each slowly built up by the artist to give the desired optical complexity, richness, and depth. Since it stays wet on the canvas, oil strokes can be blended subtly and slowly, adding to the greater unity of color, light, and space.

Generally, darks are laid down first, with lighter, half-tones put on next, and highlights (literally the uppermost paint layers) added at the end. Since the dark pigments generally "hide" or cover better than the lights, they are often put on more thinly, and, with harsh cleaning, tend to disappear more quickly. As a result, it is not uncommon to find a painting where the "lights" have been cleaned, but the more delicate "darks" have been left alone, thus throwing off the tonal balance drastically.

Deterioration

Since the ground, paint, and varnish layers do not absorb water with humidity changes, they cannot expand and contract with the wood or canvas. Instead they crack. The edges of these cracks often buckle upward where they are easily worn away with abrasion or rough cleaning. Pieces of paint may also flake off, requiring a conservator to repaint the lost area. If just the lost area is retouched, it is called "inpainting". If the whole area around the loss is retouched in an effort to blend in the inpainting with the rest of the picture, it is called "overpainting". The latter is characteristic of past restoration; modern conservators tend to "inpaint" and often inpaint in a way which is obvious to the eye on close inspection but unobtrusive at the average viewing distance. Old inpaintings and overpaintings discolor at their own rate (since the paint is comprised of different materials) and they eventually stand out as smudgy blotches of different color. Close inspection also reveals a different crack pattern or no crack pattern if the repainting is recent. Some deceptive restorers paint in fake cracks to blend in their repainted areas with the original paint. (This is visible in the bottom five inches of Rembrandt's Polish Rider in the Frick Collection, New York.)

Beside buckling, paint losses, and repaintings, there is abrasion, that is, the rubbing of the upper varnish and paint layers. The uppermost paint layers go first, especially the delicate highlights, thereby destroying the spatial effects which depends on contrasts of light and dark. The greenish faces and hands of many egg tempera paintings results from the flesh tones being partially or completely worn away, revealing the greenish underpainting used under flesh tones by egg tempera painters. Improper past cleaning often led to abrasion. The modern conservator uses solvents to clean away dirty, yellow varnish layers, with minimal rubbing and virtually no abrasion. Solvents are chosen so as to be strong enough to lift off the varnish without rubbing, but weak enough to minimize or eliminate damage to the paint layer. Most paintings have suffered from abrasion and relining damage, giving them a flat look. The fact that "darks" tend to darken more than "lights" makes dark scenes even more likely to look flat. In many portraits where black clothing appears, the black sections look completely flat. They have lost the subtle, volume-creating modulations of color and light still visible in the light areas. The original tonal balance of the picture is also permanently altered.

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VARNISH

Varnish has two jobs. It protects the paint layer, and in its smoothness and transparency, allows the light to penetrate into the paint layer to bring out the color instead of scattering across an irregular surface. Think of how the richness of color returns to a battered antique table when varnished, to a scuffed shoe when polished, or to the colored lines under a skating rink after the zamboni lays down a smooth, clear layer of ice. The same thing happens when a painting is varnished.

Most varnishes are either a mixture of "drying" oils [1] and resins (such as amber and copal) or resins dissolved in spirits of turpentine (such as mastic, rosin, and dammar). Those dissolved in turpentine are easier to remove with solvents, but they tend to crack microscopically, causing a milky effect called "bloom" which hides the color beneath. This is often treated temporarily by applying another coat of varnish which fills in all those tiny cracks, restoring transparency to the varnish and depth to the color beneath it. But bloom will return as the new layers of varnish crack microscopically, and adding more varnish only compounds another varnish problem, namely its tendency to darken and yellow with age. The solution is to remove the varnish layers completely, something which is often difficult or even impossible with tough, oil based varnishes.