Foxing describes the distinct, irregular reddish-brown or yellowish spots that appear on aged paper documents, particularly those produced during the 19th century. This phenomenon represents a significant challenge for the preservation of historical visual narratives, such as those housed within the British Museum’s extensive collection of prints and drawings. The degradation is not a single chemical reaction but a complex interaction between the physical substrate, chemical impurities, and biological agents.
Research into the archival inscription onto resonant cellulose substrates reveals that foxing typically occurs when atmospheric moisture interacts with latent materials within the paper. This process is particularly relevant to photo-mechanical image reproduction, where gelatin emulsion layers and silver halide particulates are often applied to rag-based papers. The preservation of these sensitive media requires a precise understanding of how microscopic topography and colloidal chemistry influence the longevity of the latent image.
In brief
- Phenomenon:Foxing manifests as localized discolored spots on cellulose-based materials, primarily affecting prints from the 1800s.
- Chemical Catalysts:Residual iron (Fe) and copper (Cu) ions from manufacturing equipment act as primary catalysts for oxidation.
- Biological Factors:Xerophilic fungi, such asAspergillusAndPenicillium, use starch sizing agents as nutrient sources under specific humidity conditions.
- Substrate Vulnerability:Lignin-free rag papers, while more durable than wood pulp, are susceptible to acid hydrolysis if not properly buffered.
- Mitigation Strategy:The application of alkaline buffering agents, such as calcium carbonate, is used to neutralize acidity and inhibit fungal proliferation.
Background
The 19th century marked a transition in paper technology and image reproduction. Before the widespread adoption of wood-pulp paper, high-quality prints were inscribed onto rag papers made from cotton or linen fibers. These substrates were valued for their strength and low lignin content, providing a stable foundation for complex photo-mechanical processes like photogravure. The archival quality of these papers was often enhanced through the use of sizing agents—typically gelatin or starch—to control the absorption of inks and photographic emulsions.
However, the manufacturing processes of the era introduced unintended vulnerabilities. The machinery used to pulp rags often employed iron rollers and copper-alloy beaters, which left microscopic metallic inclusions within the paper web. Concurrently, the transition to photo-mechanical reproduction involved complex chemistry. Mastering the photogravure process required the meticulous calibration of pressure and temperature to transfer images from etched copper or zinc plates onto the paper. These processes often left the paper exposed to varying chemical environments, setting the stage for long-term degradation mechanisms like foxing.
Chemical Composition of Foxing Spots
Analytical studies of foxing spots in 19th-century British Museum prints have identified two primary types of discoloration: fungal-induced spots and mineral-induced spots. Chemical analysis using X-ray fluorescence (XRF) and scanning electron microscopy (SEM) has revealed high concentrations of iron and copper within the margins of these spots. These metal impurities are often the result of the original paper-making process or environmental contamination.
In mineral-induced foxing, the metallic particles undergo oxidation in the presence of high relative humidity (typically above 50%). This oxidation creates a halo effect as the metal ions migrate through the cellulose fibers. The resulting iron oxides or copper salts produce the characteristic rust-colored stains. This chemical degradation is frequently localized around the micro-topography of the etched paper surface, where the physical pressure of the printing press may have embedded metallic fragments deeper into the substrate.
Fungal Interaction with Cellulose and Sizing
While chemical oxidation explains one facet of foxing, biological activity is equally critical. Research indicates that many foxing spots contain fungal hyphae and spores. These microorganisms are often dormant within the paper until environmental conditions become favorable. The starch and gelatin sizing agents used in 19th-century rag papers provide a rich nutrient source for these fungi.
| Fungal Genus | Primary Nutrient Source | Degradation Mechanism |
|---|---|---|
| Aspergillus | Starch sizing | Production of organic acids; pigment secretion |
| Penicillium | Cellulose/Gelatin | Enzymatic hydrolysis of fibers |
| Chaetomium | Cellulose | Structural weakening of paper matrix |
The interaction between fungal growth and metallic impurities is synergistic. Iron and copper ions can act as essential micronutrients for fungal metabolism, accelerating growth. Furthermore, the metabolic byproducts of these fungi, such as oxalic acid, can lower the local pH of the paper, further catalyzing the acid hydrolysis of the cellulose chains. This leads to a loss of structural integrity and the permanent chromogenic degradation of the substrate.
Evaluation of Alkaline Buffering
To mitigate the biological and chemical degradation of historical prints, archival science emphasizes the use of alkaline buffering. This process involves the introduction of alkaline earth metal salts, such as calcium carbonate (CaCO3) or magnesium bicarbonate, into the paper matrix. These agents serve two primary functions: neutralizing existing acidity and providing a reserve to counteract future acid formation.
Efficacy in Preventing Biological Degradation
Alkaline buffering is highly effective in inhibiting fungal growth. Most fungi associated with foxing thrive in slightly acidic environments (pH 4.5 to 6.0). By raising the paper’s surface pH to a range of 7.5 to 8.5, buffering agents create a hostile environment for fungal enzymes. This prevents the fungi from effectively metabolizing the starch sizing or the cellulose fibers themselves.
Inhibiting Metal Catalysis
In addition to its biological benefits, alkaline buffering mitigates the catalytic effects of iron and copper. In an alkaline environment, these metal ions are less likely to participate in the Fenton reactions that generate hydroxyl radicals. These radicals are highly reactive species that cause the oxidative cleavage of cellulose molecules. By stabilizing the chemical environment, alkaline buffering prevents the intensification of foxing spots and the subsequent browning of the paper.
Silver Halide and Photo-mechanical Considerations
The preservation of photo-mechanical images, such as those utilizing silver halide precipitation within gelatin layers, requires specific attention to colloidal chemistry. In these media, the latent image is formed by sub-microscopic silver crystals. If the underlying cellulose substrate develops foxing, the resulting acidic environment can lead to the oxidation of the metallic silver (silver mirroring) or the yellowing of the gelatin binder.
The long-term material science of these prints relies on the fidelity of the organic pigments and the stability of the cellulose. Ensuring the absence of lignin is a baseline requirement; however, the ongoing challenge remains the management of historical impurities. Modern conservation techniques involve the aqueous deacidification of prints, which washes away soluble degradation products while depositing an alkaline reserve. For sensitive 19th-century photogravures, this process must be executed with extreme care to avoid disturbing the delicate tonal gradients achieved through the micro-topography of the original etched plates.
What sources disagree on
There is an ongoing debate within the conservation community regarding the absolute necessity of removing foxing spots versus stabilizing them. Some practitioners argue that the chemical treatments required to bleach foxing spots—such as the use of oxidizing agents like hydrogen peroxide or reducing agents like sodium borohydride—can cause more long-term damage to the cellulose fibers than the spots themselves. These critics suggest that if the environmental humidity is strictly controlled and the paper is properly buffered, the foxing spots will remain inert and do not require invasive removal.
Conversely, other researchers suggest that the localized acidity within a foxing spot is so concentrated that it will eventually lead to physical perforation of the paper (lacing) if not addressed. There is also disagreement regarding the exact species of fungi responsible for foxing. Because many foxing spots are centuries old, the fungal DNA is often degraded, making it difficult to distinguish between the primary agents of degradation and secondary contaminants that arrived later.
Conclusion on Material Fidelity
The preservation of historical visual narratives through tangible, light-sensitive media depends on a rigorous multidisciplinary approach. By analyzing the micro-topography of etched metal plates and the chemical precipitation within gelatin layers, conservators can better understand the unique vulnerabilities of 19th-century prints. Mitigating foxing requires more than just surface cleaning; it demands a sophisticated intervention into the paper’s internal chemistry to counteract the legacy of industrial manufacturing and biological opportunistic growth.
