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Pastels, charcoal- and chalk drawings: a porous research project

Did you know that a chalk drawing looks like velvet when seen under a microscope? Read here about a research project which will help safeguard some of the Museum’s works of Unique National Importance.

Georges Michel, Landskab med træer, før 1843

Georges Michel, Landscape with trees, before 1843

Works in charcoal, chalk and pastel
The Royal Collection of Prints and Drawings possesses over 245.000 works representing a wide array of techniques and materials – right from 15th century drawings on hand-made paper to the digital art of our time. There are different problems concerning conservation attached to each type of material.

Water-colours, for example, have a tendency to fade, and ink, in the worst case, can eat away the paper. One particularly vulnerable group of works is pastel-, chalk and charcoal drawings, due to their loose and porous layer of colour. They are called dry colours and the illustration below shows microscope pictures of chalk, pastel and charcoal respectively.


Charcoal is burnt wood cut to shape, while chalk and pastel are mainly produced by mixing a water-soluble binding material with ground pigment, forming a paste which is rolled or pressed and finally dried, giving the typical chalks we know.

Loose colours
Works of art with a porous layer of colour can be very delicate, as the colour layer does not bind with the underlay as in watercolours or oils, for example, but only in respect to the strength with which it has been applied. The colour pigments are therefore often very loose on the surface of the paper and have a tendency to powder when handled. At the same time, the conservation of porous layers of colour is similarly extremely delicate and difficult.

Colour loss and rubbing out
To the naked eye, the colour layer can appear stable and resilient, but on a closer study the porous surface looks like a velvet landscape that can alter and change its character at the slightest touch.

Typical damage to works with a porous colour layer is thus loss of colour and rubbing out as a result of unsuitable handling and storage. The picture on the left below illustrates the loss of colour in the chalk drawing, and the picture on the right shows rubbing out in the charcoal drawing.

The picture lower left shows colour loss in the chalk and the picture on the right shows rubbing out in the charcoal drawing.

Colour loss in the chalk

Rubbing out in the charcoal drawing.

How can we hold the colours?
Artists are and have always been aware of the problems associated with porous layers of colour. Some artists have attempted to fix the colour with a diffusible consolidation medium like size, for example. Unfortunately the fixing effect is often minimal and sometimes it contributes to an alteration of the colour tones, discolouration and accelerated decomposition of the work.

New research project
There is great focus on finding out how porous colour layers on paper are best safeguarded. The Department of Conservation at Statens Museum for Kunst has in 2008 initiated a research project to investigate the problem. The research takes its starting point in the collections of the Museum and involves a thorough survey of the extent of the problem.

We expect to be able to find suitable methods of securing colour layers and of developing techniques for handling works of art when they are studied, exhibited and transported. In this way, Statens Museum for Kunst can ensure that works of Unique National Importance executed in pastels, chalk or charcoal can also be preserved for the future.

Written by
Karen Esser
B. Sc. in conservation
Conservation Department

Karen Esser by the microscope.

Updated: 15.oct.2014
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