Artcare - Auckland Art Gallery

Paper: Works of Art / Books /Archival CollectionsPaper is thought to have been first made inChina around AD 105, and since that timehas been commonly used as the support forimages in a variety of media. Until the 19thcentury most Western papers were madefrom linen rags. However, by the end of the19th century the vast majority of paper wasderived from plant sources such as wood. Inthe East plant fibres from the inner bark ofnative shrubs have, for many centuries,formed the basis of papermaking. As a general rule, duringthe 20th century, woodpulp is found as the stockfibre of papers worldwide.When paper deteriorates it may become discolouredand develop brown spot stains, known as foxing. Foxingrequires metallic impurities in the paper and microorganismsfor its development. In advanced stages ofdeterioration the paper can become increasingly brittle.Deterioration of paper is promoted through the combinationof a number of internal and external factors.Internal Sources of DeteriorationAcidity—the correct pH level is vital to the stabilityof any paper. The pH scale measures the concentrationof hydrogen ions in a material, thus giving an indicationof the level of acidity or alkalinity. The scale is measuredfrom 0–14, with pH 7 recording the neutral point. BelowpH 7 a substance is regarded as being acidic and abovepH 7 as being alkaline.A high level of acidity is generallyregarded as the single most damaging factorin the deterioration of works of art onpaper. Chemically, paper is composed ofcellulose, a long-chain polymer of carbon,hydrogen and oxygen. Pure forms of celluloseare derived from linen and cotton.Other fibre sources, such as unrefinedwoodpulp, have a high lignin content andproduce less stable papers which have atendency to become inherently acidic. Some manufacturingprocesses compound the problem by utilisingchemicals such as bleaches and acidic sizes which resultin acidic residues being left in the paper.Sometimes the paper becomes acidic as a result ofthe media which have been applied to it. Media such asiron gall inks and pigments such as those green and bluepigments which are based on copper can catalyse thedegradation of cellulose, causing localised areas ofimages to fragment through embrittlement.External Sources of DeteriorationAcidity—paper is also subject to degradation subsequentto the manufacturing process. Acidity is high onthe list of external sources of damage, but in the form ofmigratory acidity. When works of art on paper aremounted with poor quality boards and adhesives, acidiccomponents will migrate into the paper support causingdiscoloration, staining and embrittlement. Pigments are8