1 INTRODUCTION

Fungi are known to damage artworks on paper in two principal ways. First they utilize the paper cellulose as a carbon source, weakening and eventually destroying the paper fibers. Fungi also need the trace metals found in the paper or often in the inks of the artwork. The most obvious manifestations of damage are the multihued, fungal-induced stains that mar many works. These stains may arise from colored fungal bodies or, more commonly, from their metabolic waste products that generate and bind tightly to the paper fibers. In other cases, such as foxing, fungal metabolism changes the oxidation state of trace metals, such as iron, which are already present in the paper, to produce colored stains from colorless forms.

While the fungal stains can sometimes be extracted with appropriate solvents, there are few effective solvents that do not dissolve the ink or damage the paper fibers, and many stains resist solvent extraction entirely. Developing new solvent systems is time consuming and requires a great deal of trial and error, since the chemical structure of the pigment stains is not generally known (Szczepanowska and Lovett 1992). Mechanical stain removal is also problematic in that it is not selective between ink and stain; often produces abrasion of the paper fibers, markedly deteriorating the art work; and is extraordinarily tedious.

For these reasons, we decided to examine an alternative physical technique that might be more specific and flexible. Lasers, unlike conventional light sources, produce a monochromatic pencil-like beam of intense light that is capable of vaporizing colored materials. If the stain or pigment absorbs the laser light more effectively than the substrate, it should be possible to remove the pigment while leaving the substrate undamaged. Research scientists have used this technique to remove adsorbed substances from surfaces, and doctors have successfully removed birthmarks and tattoos nonsurgically with lasers (Lawrence 1986; Muncheryan 1979). To our knowledge, no one has used lasers successfully to remove stains from paper.Barger (1991) experimented with lasers to clean old daguerreotypes about 10 years ago, but the work was not recorded in any repeatable fashion and was not published.Asmus (1978; Asmus et al. 1973) used a laser in cleaning surfaces of marble statuary and has mentioned that an attempt to clean paper with a laser was unsuccessful (Asmus 1986). Preliminary trials of cleaning with lasers have also been carried out (Vitkus and Asmus 1976). We decided to test the capability of a high-energy laser to remove stains caused by several common, cellulolytic fungi from paper and from an actual stained etching. Our findings suggest that the technique of laser stain removal (LSR) represents a significant addition to the techniques available to art conservators.