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Mending a Tear in an Aboriginal Drum
Mending a Tear in an Aboriginal Drum

Distortions and Dimensional Changes
in Paper (Part 3)
Distortions and Dimensional Changes
in Paper (Part 3)

Distortions and Dimensional Changes
in Paper (Part 2)

Distortions and Dimensional Changes
in Paper (Part 1)

After treatment
Oscar Cahén: Innovative Conservation
for an Innovative Artist

Rigid Water Gels: New Treatment Options for Paper Conservators

Structural Remedies for Canvas Paintings

Organizing and Preserving Collections - Part 4: Digital-based Material

Organizing and Preserving Collections - Part 3: Photo-based Material

Organizing and Preserving Collections - Part 2: Paper-based Material

First Steps
Organizing and Preserving Collections - Part 1: The First Steps

Natural Dyes
The Use of Natural Dyes in Textile Conservation

A Relocation Project

Challenges of Preserving Contemporary Artwork

Preserve Your Investment through Art Conservation

A Project Completed: Heritage Preserved

Old and New Methods for Cleaning Paintings

I Can See Clearly Now – Or Can I? Part 2

I Can See Clearly Now – Or Can I?

The E.J. Hughes Mural: An Expanded Project

Is She or Is She Not an Emily

Treating Art with Sensitive Media

Malaspina Mural: An Update

For the Artist: Testing Your Materials

Conservator as Art Historian

Alum Sizing and the Art of W.J. Phillips

Treatment of an Elizabeth Keith Wood Block Print

Structural Treatment of an Emily Carr

The Treatment of a Monumental Wall Hanging

Changing Images

Preserving a Rare Record

Gold Leaf: Imitation and Genuine

The Case Against Canvas Backings

Heritage Colours: Research Discovers Original Colours

Lighting Your Art: Balancing Seeing and Protecting

The Double-Sided Emily Carr Painting

Choosing a Period Picture Frame

How to Identify a Picture Frame

Stretching Canvas and Restretching Artwork

Mounting Textiles

Aging Paintings:
Some Causes and Effects

Chine Collé Prints

What's Your Favourite Color?

Backing Removals

Rips, Holes and Tears

Filling in the Gaps

DIY – Preventative Care of Paintings

Frame it Right

Fire, Water and Smoke-Damaged Paintings

Inherent Vice

Saturated Problems:
A Water-Damaged Painting

Moldy Paper

Conserving Time

Conserving Paper: Dos and Don'ts

Repair of Textiles

Conserving Wood

Rescuing Endangered Murals

Repairing Acid-Matte Burn

Art Services & Materials
Exhibition Openings & Events

Conservation Corner Back September-October 2016

Why Paper Discolours (Part 1)

by Rebecca Pavitt



We’ve all seen the results of paper deterioration – book pages darkening at their exposed edges, art on paper suffering from discolouration and mat burn. Even after professional cleaning, paper has a frustrating tendency to revert to its former discoloured state, a phenomenon aptly called “colour reversion” by art conservators and those in the pulp and paper industry.

The main ingredient in paper is cellulose, and a close-up look at its chemical structure and behaviour will help us to understand the reasons for its discolouration.

Cellulose is a polymer whose building blocks are repeating sugar rings joined by oxygen links. On these sugar rings are two alcohol functional groups (-OH) where oxygen is attached to the ring by a single bond.

Oxidation. Through the natural aging process, some of these single bonds are oxidized to become double bonds. Alcohols are transformed first to aldehydes and then to acids. This new pattern of alternating single and double bonds is called a chromophore; it is these chromophores that give cellulose a yellow colour.

Acid Hydrolysis. As cellulose continues to age it becomes more and more acidic. These acids break the oxygen links between the molecular chain’s sugar groups in a reaction called acid hydrolysis. This, in turn, forms additional acids and chromophores on the molecule, initiating a self-perpetuating chain reaction of increasing discolouration and chain scission.



We should remember that pure cellulose is actually very stable. Extraneous materials – as found in unpurified wood pulp and additives such as alum rosin size and metal contaminated fillers – are more chemically active. When these materials deteriorate, they introduce a chemical stew of acids and oxidizing agents into the paper sheet, leading to their own and cellulose’s deterioration and discolouration.

Phenols. There is another reason why cellulose yellows. The alcohol groups in cellulose form strong hydrogen bonds with phenolic groups that are found, for example, in tannin. Tannin is a chemical found in many organic materials (including unpurified wood pulp) and acts as a chemical bridge (mordant) between cellulose and dyes; some phenol groups in tannin attach to the cellulose, and others attach to the dye.

Phenol’s chromophoric pattern of alternating single and double bonds gives it a yellow/tan colour of its own. Phenols also form strong bonds with the yellow nitrous oxides found in air pollution. Poor quality contact materials containing phenols, such as unpurified wood pulp or cheap adhesives, can literally dye cellulose yellow. Phenols in wood pulp mat boards likely contribute to the deep yellow-brown colour of mat burn.

We now have a general picture of why paper discolours. In the next issue, we will look at treatment methods that conservators use to reduce discolouration and preventive measures that can slow down the natural aging process.

Previously: Mending a Tear in an Aboriginal Drum

Next issue: Next issue: Why Paper Discolours (Part 2)


 Sat, Sep 10, 2016