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CURRENT COLUMN

Why Paper Discolours (Part 1)
Why Paper Discolours (Part 1)

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

Structural
Rigid Water Gels: New Treatment Options for Paper Conservators

Structural
Structural Remedies for Canvas Paintings

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

Photos
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

Butterfly
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 Nov/Dec/Jan 2016-17

Why Paper Discolours (Part 2: Removal and Prevention)

by Rebecca Pavitt
fineartconserve.com

Kathë Kollwitz portrait, before treatment

Kathë Kollwitz portrait, before treatment

Removal of discolouration requires water-based treatments. This article outlines the general chemistry of wet cleaning, but does not detail the working specifics such as reagent concentrations and pH, limiting factors such as paper and media sensitivities or application methods

1) Humidification relaxes paper and allows even water absorption (avoiding tidelines) in the subsequent steps

2) Removal of readily soluble deterioration products using water adjusted to an appropriate range of pH and ionic concentration by adding ammonia and acetic acid.
Lower pH (for example, 5.5 – 6.0) facilitates the removal of soluble iron (Fe II) and is less of a “pH shock” to acidic papers. Higher pH (for example, 7.0 – 8.0) gently “fluffs” paper to increase permeability. Adding ions to water also reduces damage by moderating osmotic pressures between the interior of the paper fibres and the water bath

3) Removal of less soluble discolouration including metallic compounds and degraded resins (from fillers, sizes, etcetera) can, at appropriate pH levels, be dissolved and removed by chelators. Citric acid is a relatively weak chelator with less potential to damage coloured media. EDTA is a stronger chelator that solubilizes more resistant discolouration but can also affect some pigments and dyes

4) Removal of chromophores through chemical reduction. Oxidation produces chromophores, and reducing bleaches remove chromophores by adding hydrogen. This restores, to some degree, the original cellulose structure. (It cannot rejoin broken molecules.) Sodium borohydride, tert butyl aminoborane and sodium dithionite are three reducing bleaches, each targeting somewhat different chromophore structures.

Kathë Kollwitz portrait, before treatment

Kathë Kollwitz portrait, cleaned by washing in pH adjusted water, two types of reducing bleach, light bleaching and calcium based alkaline reserve

5) Removal of iron through reduction and chelation. Iron catalyzes oxidative reactions which damage and discolour cellulose; some forms can also stain paper grey and reduce overall brightness. At low pH sodium dithionite can reduce and bring into solution otherwise insoluble forms of iron (Fe III), which can then be sequestered and removed with EDTA

6 ) Removal of chromophores through oxidation. Oxidative bleaches break chromophoric double bonds by adding oxygen. This changes the chemical structure of cellulose, breaks up the sugar rings and makes the remaining molecule shorter and weaker. Used sparingly, oxidative bleaches can target discolouration remaining after previous treatments. Hydrogen peroxide is most commonly used because it is milder than chlorine bleaches. Iron (an oxidative catalyst) must first be removed through reduction and chelation and/or made insoluble by raising pH. If it is not completely removed or inactivated, bleach residues can trigger future oxidative damage. Light bleaching with UV filtered light is a slow and gentle method of oxidative bleaching.

7) Protect against future oxidation by adding a calcium-based alkaline reserve to the paper after rinsing

8) Prevention. Good quality contact materials made from alkaline cotton or chemically purified wood pulp helps to prevent future damage; additional protection is gained by using paper and matboards that contain MicroChamber xeolites. These xeolites absorb and trap volatile oxidizing agents from air pollution or from the art/artifact itself. They are also a safe way to protect artworks that cannot be water treated.

I hope this article gives the reader insight into the reasons why paper discolours and the methods conservators might use to address the problem.

Previously: Why Paper Discolours (Part 1)

Pablo Picasso portrait, before treatment

Pablo Picasso portrait, before treatment

Pablo Picasso portrait, cleaned

Pablo Picasso portrait, cleaned by washing with water adjusted to pH 7 until no further discoloration was removed, followed by light bleaching and alkaline reserve


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 Mon, Nov 7, 2016