What Causes Discolouration in Paper?

Discolouration of paper, the natural process of change from a sheet’s original tone, is something that can cause disruption to the aesthetic appreciation of a work. It has multiple causes but can primarily be attributed to the chemical composition of the paper itself as well as the environment in which the item is kept; with the degradation reactions having other detrimental effects aside from the visual.

F. Bartolozzi after Guercino; engraving on handmade laid paper - “The assassination of Amnon at the feast of his half-brother, Absalom.” (1764). The support of this work shows localised foxing as well as an overall discolouration of the entire suppo… — F. Bartolozzi after Guercino; engraving on handmade laid paper - “The assassination of Amnon at the feast of his half-brother, Absalom.” (1764). The support of this work shows localised foxing as well as an overall discolouration of the entire support

Chemical Composition of Paper

Paper, from when sheets were first cast in the 2nd Century AD, has been made from cellulose (glucose molecules chemically bonded together in a straight chain) based materials; plants. Whilst present day paper is predominantly made from wood (with the exception of some high quality artist papers), this development was only bought into regular use in the middle of the 19th century. Prior to this, paper, in Europe, was made from cotton and linen in the form of recycled rags that were treated and pounded in the presence of water to create a slurry of fibres; the paper making stock, that would then be cast on a mould to form the sheet.

The difference in the source of cellulose has a profound impact on the chemical makeup of the sheet. Cotton and linen contain a considerably purer grade of cellulose compared to wood, which contains a far greater proportion of contaminants such as lignin and hemicellulose (celluosic materials that contain branched chains within their structure with reactive side groups). These 2 substances are far more easily oxidised in comparison to cellulose, and can be reduced into smaller molecules that then have a greater propensity to form acids and chromophores (molecules that appear coloured due to their structure) within the paper’s structure, resulting in a yellow/brown colour forming as well as acids that can propagate these reactions further.

“Discolouration is a natural consequence of ageing...”

— Daniels, V (1996)

Degradation Reactions in Paper

Oxidation and Acid Catalysed Hydrolysis are 2 processes that work in tandem to cause the discolouration of paper. They work by causing the relatively stable cellulose chains to sheer in 2 resulting in smaller molecules that then have the potential to form conjugated double bonded systems that will appear coloured to the eye (chromophores). These processes are also self propagating, producing a free proton that can then repeat the degradation reaction. The rate of an oxidation reaction can be sped up in the presence of light, a good example that can easily be seen is how newspaper, when left in the light, will become discoloured and brittle comparatively quickly. Increased brittleness is a direct result of these reactions, by sheering the cellulose chains in 2 it reduces the degree of polymerisation and reduces the supports structural integrity.

Environmental Factors

Whilst the internal factor of the chemical composition of the paper itself is one reason for discolouration over time, another is the external factors; the environment in which the paper is held. Acids can transfer, through gas and liquid diffusion, from acidic materials into the paper structure. This is evidenced in the figures below which show a 19th Century engraving on cotton/linen wove paper that has been impacted by the materials used to frame it. The backing board, made of wooden shingles, is inherently acidic and will emit acetic and formic acids into its surroundings; hardwoods containing 4-6% of its original weight in these acids. Knots in the wood have a higher acid concentration and will transfer more into its surrounding materials, this can bee seen through darker areas that correspond to the knots on the bottom and middle shingles. 20th Century mount boards can also become acidic over time, transferring their acidity to paper that they are in contact with, resulting in patterns of discolouration; this is often referred to as “mount burn”.

Verso (Framed). A framed work where the backing board has been constructed from wooden shingles.

Verso (Unframed). Brown lines corresponding to the gaps in the shingles otherwise known as “air burn”.

Recto. The work unframed displays “air burn” that has transferred all the way through to the front.

Preferential access to the atmosphere can also result in an increase in discolouration. The absorption of nitrogen and sulphur dioxide, present in the atmosphere, when in the presence of water and oxygen will form sulphuric and nitric acids that readily ionise, providing a proton to catalyse the degradation reaction of hydrolysis resulting in discolouration and an increased brittleness. This can often be seen in 20th Century printed books where the edges of the page will become discoloured through their exposure to the atmosphere, whilst the center of the page will be relatively protected by remaining closed. It can also be seen in the example above where gaps in the backing board have resulted in the preferential movement of atmospheric pollutants over these localised areas creating markings that mirror the backing board; this is referred to as “air burn”.

Localised Discolouration

As well as overall discolouration, works on paper can exhibit local areas of discolouration that show themselves as brown spots; marks of this type are known as foxing. The cause of foxing is not fully understood but it is generally accepted to be caused by biological infection as well as metal particulates; copper, iron and zinc, often present from the paper making or print making process. One theory is that these spots are caused by the presence of localised condensation points that act as areas that preferentially attract moisture and as a result become localised spots where the degradation reactions of hydrolysis and oxidation can take place. These condensation points could include irregularities in the paper such as; folds, tears, dirt particles, the presence of metal particulates or fungal spores/bacteria.

Foxing on an 18th Century aquatint, printed on handmade cotton/linen wove; the result of a biological outbreak.

Foxing on an 18th Century engraving, printed on handmade cotton/linen wove.

Photomicrograph of a foxing mark on an 18th Century engraving cause by a metal particulate.

Whilst it may be visually distracting and chemically detrimental to the overall health of a work on paper, there are many combinations of treatment options available for works demonstrating discolouration, whether it be overall or local, that can be implemented in order to restore the aesthetic and chemical integrity of a work.

It must however be understood that, like people, paper ages. As such an overall brown tone will often be acceptable for a work of a certain age and type, where reducing this colouration could be deemed unethical and unnatural.

If you have a work that’s suffering from discolouration or want to discuss treatment options for other works in your collection, then please get in touch using the CONTACT US menu option at the top of the page or directly by emailing us at stubbsconservation@gmail.com.