Works of art on paper

By Bettina Ebert, Paintings Conservator and Researcher, Asiarta Foundation

Works of art on paper may include drawings, watercolours, preparatory sketches, prints, posters, engravings and other techniques, all of which share the use of paper as a support. In addition, works on paper also include archival material, such as books, diaries, newsprint, or any other form of writing.

Paper has been in use as a writing and drawing support for thousands of years, with the earliest paper being manufactured in Egypt from the papyrus plant. The use of animal skin, or parchment, as a support is also rooted in history. However, what we commonly refer to as paper was invented in China, probably around 100 BC. Subsequently, paper and the papermaking process spread throughout Asia and into the Middle East. By the 10th century, paper was being supplied to Europe, though it was not until the 12th century that the papermaking process was also established in Europe.

The papermaking process

The manufacture of paper developed significantly throughout the subsequent centuries, although the main steps involved are essentially the same. While mulberry, bamboo and other plant-derived raw materials may be used directly in the production of pulp for paper, especially in Asia, European papermaking favoured rags from hemp, flax and cotton.

The rags would be sorted, torn into small pieces and washed before being beaten to a pulp. Raw materials from plants in the form of bast fibres, grass or bark are usually washed, cooked and beaten, which helps break down the fibres. Chemical pulping involves the addition of acids or alkalis to break down the fibres and remove unwanted material such as lignin and hemi-cellulose. Pulp may also be bleached if a whiter paper is desired.

Traditional papermaking in Vietnam

Large vats contain the stock pulp, essentially a slurry of fibres in water. Paper moulds usually consist of a frame with a wire mesh or parallel lengths of wire or bamboo tied together to form a flexible screen. The paper mould is dipped into the vat and lifted out such that the fibres are trapped against the screen while excess water drains out. This mould is the basis of a sheet of paper, which is then removed and placed between felts. A stack of paper and felt is pressed under weights or in a screw press, squeezing out excess water. The individual sheets are then dried.

Size may be added into this slurry or applied subsequently. Materials used for size may include rosin, gelatin, animal glue, starch or modified celluloses and wax emulsions. The purpose of sizing is to increase the paper’s wetting resistance, thus reducing absorbency. Surface sizing involves dipping the dried sheets into tubs containing the sizing agent or rolling them through a press.

Other additives such as fillers, pigments, dyes or coatings may be applied. Fillers such as calcium carbonate, clay or titanium dioxide are added to the slurry prior to sheet formation to alter the appearance of the paper. Sheets of paper may also be coated with these chemicals in order to further reduce moisture absorption and make the paper more receptive to printing ink. Optical brighteners in the form of fluorescent dyes may be added for very white paper, while coloured papers are made by the addition of pigments or dyes. Furthermore, paper may also be impregnated with resins, plasticisers, gums or starches, all of which alter the properties of the resulting paper.

Degradation processes

The main component in works of art on paper is the cellulose that makes up the paper. Cellulose is an organic compound of the formula (C₆H₁₀O₅)_n. Cellulose is a straight-chain polymer consisting of glucose units linked together. Cellulose deteriorates by means of hydrolysis and oxidation, leading to the breakdown of long chains of molecules into smaller ones. This results in loss of flexibility – paper becomes brittle and may break easily or even crumble.

Foxing on a watercolour

Light has a significant impact on the deterioration of works of art on paper, as it causes photo-oxidation of cellulose. Photo-oxidation of lignin results in the formation of free radicals which react to form conjugated chromophores, or yellow deterioration products, hence leading to yellowing and darkening of paper. Brown spots often seen in aged paper are called foxing, and may be caused by various factors, such as the degradation of metallic contaminants in the paper, or fungal growth.

The acidity of paper increases over time for various reasons. When alum which is added during sizing degrades, it produces sulfuric acid. Other bleaching and sizing agents also increase the acidity of paper. Lignin is acidic, while acidic compounds may also be formed as part of the degradation processes in cellulose. Another source of acids may be from atmospheric pollutants, or from contact with acidic materials used in mounting.

Metallic ions may contaminate paper during the manufacturing process. These metal ions may act as catalysts for the various degradation processes. Additives such as resins, gums and starches may cross-link and darken, causing stains and discolouration. Paper strength also relies to a large extent on the length of the fibres. For example, wood pulp paper has much shorter fibres than paper derived from bast fibres. Repeated folding of paper weakens it and may cause breaking of fibres.

Watercolours consist of pigments and a gum binder applied to the paper as a suspension in water. They dry by evaporation of water. Other water-based paints applied to paper may incorporate animal glue or other proteins as the binding medium. Animal glue is a proteinaceous material derived from collagen, a fibrous structural protein of the connective tissue in animals and fish. When boiled in water, collagen is hydrolyzed irreversibly and turns into gelatin. In water, gelatin forms a high viscosity solution which sets into a gel on cooling.

Pigments used for gouache painting in Vietnam

Pigments may derive from mineral, vegetable, animal, or synthetic sources. Earth pigments include the ochres, umbers and sienas. Some natural pigments are ultramarine, malachite, azurite and orpiment. Examples of manufactured inorganic or mineral pigments are lead white, vermillion and zinc white. Organic pigments that are of vegetable origin include gamboge, indigo, madder lake and saffron, while examples of organic pigments of animal origin are Indian yellow and cochineal. Modern synthetic organic pigments have been manufactured, and include phthalocyanine blue, alizarin and Hansa yellow among others. Lakes are pigments that are made by precipitating a dye upon an almost colourless pigment such as alumina hydrate, clay or barytes.

Light has an effect on many pigments, initiating chemical changes or causing fading as a result of photo-oxidation, which reduces the extensive conjugation in organic molecules, leading to the formation of smaller colourless molecules. Pollutants such as sulphur dioxide can also affect pigments. Dyes used in modern inks may fade within a very short period of time.

Mould growth

Other factors that may contribute to the deterioration of works of art on paper are the application of adhesives and tapes. These become less soluble as a result of cross-linking and discolour, often staining the paper. The removal of tape is a common procedure in paper conservation. The agents of deterioration described in the introduction to conservation also apply to works of art on paper. In particular, the thin nature of a sheet of paper means that insect attack often leaves holes in the paper support. Mould growth in damp conditions will stain and weaken the paper substrate. Tears are a common form of damage associated with works on paper that are handled frequently.

Paper conservation is a specialist field requiring extensive knowledge. Here is a brief summary of the different techniques available to paper conservators.

The following articles provide more information on conservation of oil paintings, acrylic paintings and lacquer paintings.