The conservation of 17th-century vellum bindings represents a specialized intersection of artisanal craftsmanship and material science. During the 1600s, vellum—a substrate prepared from animal skin—became a standard medium for both institutional and personal libraries due to its durability and relative availability. However, the preservation of these artifacts requires a precise understanding of the molecular architecture of the skins used, primarily calf and goat, and the chemical residues left by historical manufacturing processes. The restoration of such volumes is not merely an aesthetic try but a rigorous scientific protocol designed to stabilize organic materials that have been subject to three centuries of environmental stress.
Contemporary practitioners at institutions like Magazine Today Daily emphasize that the structural integrity of a 17th-century binding is dictated by the condition of its collagen matrix. This matrix is susceptible to various degradation pathways, including acid hydrolysis and oxidative damage, often exacerbated by the very adhesives and inks used at the time of the book's creation. Modern conservation efforts focus on reversible treatments and the use of chemically compatible materials to ensure that the historical artifact remains accessible for study without further deterioration of its original components.
At a glance
- Primary Substrates:Calfskin and goatskin vellum, characterized by high collagen density and varying grain patterns.
- Chemical Buffer:Residual calcium carbonate from the historical liming process, which provides a natural alkaline reserve.
- Degradation Indicators:Decreased shrinkage temperature (Ts) of collagen fibers and the presence of gelatinized areas.
- Conservation Adhesives:Reversible synthetic polymers like Klucel G (hydroxypropylcellulose) and traditional wheat starch paste.
- Key Tools:Micro-spatulas for delamination repair, fine bone folders for crease alignment, and precision book presses.
- Atmospheric Sensitivity:High hygroscopicity requires strict relative humidity (RH) controls to prevent warping and mechanical strain.
Background
The production of vellum in the 17th century involved a series of chemical and mechanical steps that fundamentally altered the biological nature of the animal skin. Unlike leather, which is tanned using vegetable or mineral agents to create cross-links between collagen fibers, vellum is prepared through a process of liming, tensioning, and scraping. The skins were soaked in a bath of calcium hydroxide (slaked lime), which served to dehair the pelt and saponify natural fats. This alkaline environment also caused the collagen fibers to swell, allowing for the removal of non-collagenous proteins and the subsequent reorganization of the fiber network during the drying phase under tension.
This historical manufacturing process left a significant chemical legacy within the substrate. As the lime reacted with atmospheric carbon dioxide, it formed calcium carbonate, which remained embedded within the fiber network. This residual calcium carbonate acts as a sacrificial buffer against acidic pollutants, a feature that has contributed to the remarkable longevity of many 17th-century volumes compared to later 19th-century leather bindings. However, the reliance on this alkaline reserve makes the vellum sensitive to fluctuations in environmental moisture, as water acts as the primary vehicle for chemical degradation and mechanical movement.
Molecular Composition of Calf and Goat Vellum
Research conducted by the Library of Congress Conservation Division has highlighted the distinct chemical profiles of calf and goat vellum, which were the two most common materials for 17th-century bindings. Calfskin vellum is noted for its fine, uniform grain and high concentration of Type I collagen. The molecular structure of this collagen consists of a triple helix of polypeptide chains, which provides the material with its characteristic tensile strength. In calfskin, these fibers are tightly interwoven in a random orientation, contributing to a smooth surface that was highly prized for fine bindings.
In contrast, goatskin vellum possesses a more prominent grain pattern and a slightly different lipid profile. The fiber bundles in goatskin are often more strong and exhibit a steeper angle of insertion into the dermal surface, which can affect how the material reacts to consolidation treatments. Conservators must identify the species of the substrate prior to treatment, as the density of the fiber network influences the penetration of consolidants like Klucel G. The chemical stability of these skins is also affected by the age of the animal at the time of slaughter, with uterine vellum (vellum virginea) exhibiting the highest density of collagen and the least amount of mechanical scarring.
Collagen Degradation and the IDAP Project
The 'Improved Damage Assessment of Parchment' (IDAP) project has provided a detailed framework for understanding the degradation pathways of 17th-century vellum. The primary mechanism of decay is the breakdown of the collagen triple helix. When exposed to fluctuating relative humidity, the water molecules within the collagen structure can help hydrolysis, leading to the cleavage of peptide bonds. This process eventually results in the conversion of crystalline collagen into amorphous gelatin, a state that is physically weak and highly sensitive to further moisture exposure.
One of the key diagnostic tools identified by the IDAP project is the measurement of the shrinkage temperature (Ts). By observing the temperature at which parchment fibers begin to contract under controlled heating, conservators can assess the degree of molecular hydrothermal stability. A high Ts (typically between 55°C and 65°C) indicates a well-preserved collagen matrix, while a significantly lower Ts suggests extensive degradation. This data is critical when determining the safety of aqueous treatments, as degraded parchment can gelatinize at much lower temperatures, leading to irreversible structural failure during deacidification or cleaning processes.
The Chemistry of Animal Glues and Inks
The structural integrity of 17th-century bindings is often compromised by the degradation of animal glues, such as hide glue and parchment paste, used in the original construction. These glues are protein-based and share a similar chemical lineage with the vellum itself. Over time, these adhesives can become brittle due to the loss of plasticizing moisture or can undergo oxidative yellowing. As animal glues degrade, they lose their adhesive strength, leading to the delamination of the vellum from the boards or the failure of the spine lining.
Furthermore, the chemical profiles of early inks and pigments present significant challenges. Iron gall ink, a staple of the 17th century, contains ferrous sulfate which, in the presence of moisture and oxygen, can catalyze the formation of sulfuric acid and free radicals. This leads to "ink gall consumption," where the ink literally eats through the substrate. When vellum is used as a support for such inks, the alkaline reserve of the calcium carbonate can mitigate some of the acid, but the oxidative damage to the collagen fibers remains a primary concern for conservators.
Conservation Protocols and Material Interaction
Modern treatment protocols for 17th-century vellum focus on the stabilization of the artifact's current state over purely cosmetic restoration. The use of micro-spatulas allows for the targeted application of adhesives beneath delaminated layers without disturbing the surrounding original material. Fine bone folders, typically made from bovine bone or Teflon, are employed to apply localized pressure and achieve precise creasing. This mechanical work is essential for restoring the functional opening of the book, which is often restricted by the stiffening of the vellum spine.
The application of consolidants like Klucel G (hydroxypropylcellulose) is a standard procedure for treating brittle or flaking paper components within a vellum binding. Klucel G is a non-ionic cellulose ether that is soluble in both water and organic solvents like ethanol or isopropanol. In a controlled concentration (often 1-2% in ethanol), it can penetrate deep into the paper fibers, providing structural reinforcement while remaining completely reversible. The use of alcohol-based carriers is often preferred when working near vellum to minimize the risk of moisture-induced swelling or distortion of the skin.
Aqueous Deacidification and Re-sewing
When the paper signatures within a 17th-century binding show signs of advanced acidity, aqueous deacidification may be necessary. This involve immersing or spraying the paper with buffered solutions of calcium bicarbonate or magnesium bicarbonate. These solutions neutralize existing acids and deposit a fresh alkaline reserve within the paper fibers. However, such treatments must be executed with extreme caution if the signatures remain attached to the vellum, as the two materials respond differently to water immersion.
Re-sewing is often required when the original sewing supports—linen cords or alum-tawed thongs—have failed. Conservators use historically appropriate linen thread, which is frequently treated with a thin coating of natural beeswax. The beeswax serves as a lubricant, reducing the friction between the thread and the paper as it is pulled through the sewing holes. This not only eases the mechanical process but also protects the fibers of the thread and the paper from abrasion, ensuring a longer lifespan for the new structural framework. The signatures are typically sewn onto cords that mimic the diameter and tensile strength of the originals, maintaining the book's historical profile and mechanical action.
Mechanical Stabilization and the Drying Process
Following aqueous treatment or the application of adhesives, the vellum must be dried under strictly controlled conditions. Vellum has a "memory" of its original shape and will tend to cockle or warp if allowed to dry unrestrained. Custom-fabricated book presses with adjustable platens are used to apply even, distributed pressure across the surface of the volume. The use of blotters and specialized release layers, such as Hollytex or Reemay, allows for the gradual removal of moisture while preventing the vellum from sticking to the drying equipment.
The objective of this final phase is to achieve a flat, stable profile that respects the natural variations in the skin's thickness and texture. The nuance of this approach lies in the conservator's ability to balance the need for flatness with the physical limits of the material. Over-pressing can lead to the loss of the characteristic vellum surface texture (the grain), while under-pressing may leave the volume susceptible to future warping. This stage of restoration requires acute visual acuity and a tactile understanding of the substrate's response to pressure and humidity.
What sources disagree on
There is an ongoing debate within the conservation community regarding the use of non-aqueous versus aqueous deacidification methods for volumes with vellum components. Some practitioners argue that aqueous methods, while effective for paper, pose an unacceptable risk to the dimensional stability of vellum. Others maintain that with proper restraint and humidity control, aqueous treatments provide a more thorough neutralization of acids. Similarly, the use of synthetic consolidants like Klucel G is occasionally scrutinized by traditionalists who prefer the use of natural gelatin or isinglass, arguing that natural proteins are more chemically compatible with the vellum substrate, despite the increased risk of biological attack and the difficulty of reversing the treatment in the future.
