If you are interested in the conservation of paper artifacts, you may have encountered gelatin-sized rag paper, which is a type of paper commonly used for fine art prints and drawings. Gelatin sizing is a process of applying a thin layer of animal glue to the surface of paper fibers, which improves the paper's strength, durability, and resistance to abrasion. But how does gelatin sizing affect the paper's interaction with water, and what are the implications for the preservation of paper artifacts? In this blog post, I will discuss the effect of water on gelatin-sized rag paper, and some of the challenges and opportunities that conservators face when working with this material.
Gelatin sizing is a natural substance made from animal bone that is applied to paper to act as a protective filler or glaze. It has been used as an external sizing for centuries in European and early American papers. One of the benefits of gelatin sizing is that it reduces the paper’s absorbency and prevents water-soluble components from leaching out of the paper. This can help preserve the original appearance and color of the paper and the media applied on it. For example, gelatin sizing can prevent watercolors from bleeding or fading over time. Gelatin sizing can also protect the paper from acidic deterioration caused by atmospheric pollutants or acidic inks.
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However, gelatin sizing also makes the paper more sensitive to changes in relative humidity and temperature, which can cause dimensional changes and mechanical stresses in the paper. Gelatin-sized paper tends to expand and contract more than unsized paper when exposed to moisture or dryness, which can result in curling, cockling, cracking, or tearing of the paper. Gelatin sizing can also make the paper more susceptible to mold growth and insect damage, especially in humid environments.
Another challenge of gelatin sizing is that it can interfere with some conservation treatments that involve water, such as washing, deacidification, or humidification. Gelatin sizing can reduce the effectiveness of these treatments by limiting the penetration of water into the paper fibers, or by dissolving and redepositing on the paper surface. Gelatin sizing can also react with some chemicals used in conservation treatments, such as alkaline solutions or enzymes, and cause damage to the paper.
Therefore, conservators need to be aware of the presence and condition of gelatin sizing on rag paper artifacts and choose appropriate methods and materials for their examination and treatment. Some of the techniques that can be used to identify gelatin sizing include visual inspection, microscopy, staining tests, infrared spectroscopy, or mass spectrometry. Some of the factors that can be considered when selecting conservation treatments include the type and extent of damage, the nature and value of the artifact, the availability and cost of resources, and the expected outcome and risks of the treatment.
For more information about gelatin sizing and its effect on rag paper artifacts, I recommend reading Paper and Water: A Guide for Conservators by Gerhard Banik and Irene Bruckle, which is a comprehensive and practical resource for conservation professionals working with paper materials.
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