Paintings

Paintings are often one of the first types of art that comes to mind when one thinks about visiting a museum.They encompass a wide range of different techniques and materials. In a simple definition, a painting is anything in which a paint is applied to some support material to create an image, however loosely defined. We generally think of paintings as two-dimensional pictures, but in ancient Greece and Egypt, sculptures were often painted, and pottery such as that from ancient Greece is well-known for intricate images created by painting glazes onto ceramics which were then fired to create a lasting glass-like image. Furniture was also painted, but may have simply been coated with a drying oil or another coating such as a varnish,shellac or laquer without the addition of pigment. The technology of paints and various other types of coatings are highly related, regardless of whether the substance was intended to create an image or simply used as a coating.

Paintings can be done on any available support material, and a variety of objects over the past several millenia have been painted, even if the intent was not to create a pictorial, or even abstract, image. Understanding what the nature of the support material is crucial for the art conservators to be able to treat a painting.

The oldest known paintings were done on the rock walls of caves such as those at Lascaux, France and Altamira Spain. Some cave paintings date back over 40,000 years old. These first paintings were quite simple and sometimes abstract, sometimes simply with the artist creating pictures by blowing a mineral pigment powder through a hollow bone tube over his her hands when they were placed against the cave wall. Minerals such as chalk, iron oxides or charcoal were the first pigments used to produce these images. The pigments could simply be rubbed onto the surface of the stone, but some were mixed with animal fats to act as a binder.

Egyptian heiroglyphic carvings in tombs and temples were also decorated with bright colors, as were Greek statues that show traces of pigments indicating that they were once painted with bright colors, in contrast to our impression of them as being pristine white marble.

Many early Renaissance paintings were made on wood panels. The wood used was generally native to the area where the painting was made, so Italian paintings of the Renaissance were done on poplar, while those in northern Europe often employed oak. The backs of the wood panels used for paintings were generally not painted and so it is possible to examine them directly to determine the type of wood employed. A common problem with wood panels is that they are susceptible to expansion and contraction with changes in the humidity of the environment in which they are kept. This constant seasonal movement can lead to cracks in the wood, and it is not unusual to see paintings with cracks in them. Similarly, the paint itself cracks under the constant expansion and contraction, producing the characteristic craquelure that is often taken as a sign of the age of the work, although forgers have methods by which they introduce craquelure artificially into a painting. Wood also is limited by weight and size. As the works become larger, they can be extremely heavy, which made creating them on wood impractical. Shrinkage and cracking are not the only problems with a wood support. Wood is also susceptible to attack by insects that eat into the wood leaving noticeable holes. Beetles bore into wood on the back side of a painting and deposit eggs, which develop into larvae that then bore their way out of the wood, even through the paint layers.

Renaissance artists also painted on plastered walls producing frescoes for their wealthy patrons. These were commissioned for both private palaces as well as public spaces, including churches. The buon fresco technique involved painting water-dispersed pigments directly onto a thin later of wet plaster. Frescoes could also be made on dry plaster, but then the pigments require a binder to hold them to the surface. One of the most famous fresco paintings is the Last Supper by Leonardo da Vinci (ca. 1495-7). Unfortunately, Leonardo employed an untested technique for the painting. He emplyed oil paints on a complex set of underlayers, but it began to disintegrate shortly after it was completed and is now in poor condition. Fortunately, a full scale oil on canvas copy was made by Giampietrino in 1520 that now resides in the Royal Academy of Arts in London. Michelangelo used the traditional wet plaster method for his painting of the ceiling of the Sistine Chapel.

Metals were also used as support materials. Copper is easily rolled into smooth sheets and was routinely used for oil paintings, as in this example from Rembrandt van Rijn. Rembrandt was known to have painted five portraits on copper supports. As European paintings became larger, there was a need to find alternate support materials that were light weight and also less expensive. The choice for painters in Europe was to use canvas cloth, where oldest paintings on canvas date from the 15th century and canvas rapidly replaced wood and metal as the support material of choice. The canvas was first stretched over a wooden frame and then treated the same as a wood panel. Canvas was traditionally made from hemp but could also be produced from linen or cotton. By the time of the Impressionists, pre-stretched canvases that were already primed could be purchased from local art shops.

The Chinese had been painting on textiles well before this. The oldest Chinese paintings on silk are believed to have been made as early as 476 BCE. From China, the art of silk painting traveled to other Asian countries. In contrast to canvas, which is plant based, silk derives from silk worms and is a protein-based fiber. For a discussion of the chemical composition and properties of various textiles including cotton, linen, silk, wool, and various modern synthetics, click here.

Paints are made of at least two components - a pigment to provide the desired color and a binder to hold the pigment to the support. The paint may also contain a vehicle, which helps with the application of the paint to the support. Water colors mix the pigment with gum arabic, which serves to bind the pigment to a paper support, and then the pigment is dispersed in water, which serves as the vehicle. Oil paints use a drying oil as a binder but may also contain a paint thinner such as mineral spirits or turpentine. Modern oil paints may have additional components to serve to make them dry faster or slower or to keep preserve them from degradation. Linseed oil is a common drying oil used for oil paints, but others are also employed including poppy seed and walnut oils. Oils do not 'dry' in the sense of the oil evaporating, but instead undergo a polymerization reaction to trap the pigment into a hardened layer. Please check out the page on binders for a more detailed explanation. More recently, other man-made organic polymers have been used as the binder. In the twentieth century, some artists began using house and automotive paints because they were readily available and could be purchased readily in bulk for large works. All of these new materials provide constant challenges for art conservators because, in order to conserve an painting, you must first know what it was made of.These include acrylics and alkyd paints, and their compositions are also discussed on the binders web page.

Any substance that possesses a desirable color can be used as a pigment, dye or, in some cases, a glaze. Some are more colorfast and some have lower reactivity to other pigments or binders that make them better choices for works intended to last for posterity. If you study famous painters, you will discover that they each had their favorite set of pigments, referred to as thier palette, and these choices become characteristic of that particular artist. Some pigments also had special applications, such as lapis lazuli that was used for painting the robes of important religious figures such as the Virgin Mary.

Chemists classify pigments into two broad classes based on the chemical composition of the pigments and dyes: Inorganic Pigments and Organic Pigments. Organic pigments and dyes are carbon-based molecules, which were originally obtained as plant or animal extracts. Notable examples include Tyrian purple, Carmine red, Indian yellow, Organic molecules have hydrogen directly bound to carbon, which sets them apart from inorganic carbon molecules such as carbonate minerals, metal carbides, graphite, C₆₀, etc. Some of these organic molecues are quite colorful, but many lack durability and decompose over time. In contrast, the first inorganic pigments were obtained by grinding up colored minerals and these have a tremendous lasting power. Both classes have expanded over time as people developed an understanding of chemistry, and new man-made pigments have been developed. Since art conservators know when various pigments were created and used in paintings, the analysis of pigments can help determine the time period in which a painting was created, or perhaps when a restoration touched up areas of the painting with more modern options.