Aqueous deacidification represents a cornerstone of modern paper conservation, particularly when addressing the structural and chemical vulnerabilities of 17th-century volumes. These artifacts, often featuring vellum bindings and handmade rag paper text blocks, suffer from a progressive accumulation of acidic compounds derived from environmental pollutants, iron gall ink degradation, and internal chemical reactions. The restoration process for such items requires a calculated intervention that neutralizes existing acidity and introduces an alkaline reserve to safeguard the substrate against future deterioration.
Conservation protocols use various buffered solutions, most notably calcium bicarbonate and magnesium bicarbonate, to address these issues. Each chemical pathway offers specific advantages regarding solubility, penetration depth, and long-term stability of the cellulose fibers. The choice of agent is dictated by the specific material profile of the artifact, including the presence of fugitive pigments, the degree of paper embrittlement, and the intended mechanical tension of the rebound volume within its restored vellum casing.
What changed
The methodologies governing paper stabilization have undergone a significant transition from the early 20th-century techniques pioneered by William Barrow. Barrow's original two-stage deacidification process involved sequential immersion in calcium hydroxide and calcium bicarbonate. While effective at neutralizing acids, this method was often criticized for its potential to leave excessive deposits on the paper surface and its labor-intensive nature. Modern conservation has moved toward more detailed, single-stage treatments using refined buffered solutions that provide a more controlled deposition of the alkaline reserve.
- Shift in Buffering Agents:Increased adoption of magnesium bicarbonate for its higher solubility in water compared to calcium bicarbonate, allowing for a more substantial alkaline reserve in fewer applications.
- PH Regulation Standards:Transition from simple neutralization (pH 7.0) to targeting a residual pH between 7.5 and 8.5, as recommended by the Library of Congress, to counteract ongoing atmospheric acidification.
- Tooling and Precision:The integration of micro-spatulas and custom-fabricated book presses with adjustable platens has allowed conservators to manage the physical stresses placed on paper during the wetting and drying cycles of deacidification.
- Adhesive Compatibility:A movement toward reversible, synthetic consolidants like KLUCEL G (hydroxypropylcellulose) to stabilize brittle fibers before and after aqueous treatment, replacing less stable traditional starches where appropriate.
Background
The 17th century was a period of transition in papermaking and bookbinding. Books from this era typically feature paper made from linen and cotton rags, which are inherently more durable than later wood-pulp papers. However, the use of iron gall ink—a mixture of iron salts and tannic acids—introduced a localized source of acidity that often causes the ink to "eat" through the paper over centuries. Furthermore, the vellum used for bindings is a highly hygroscopic material, meaning it reacts sensitively to changes in humidity, often leading to mechanical stresses that can crack the paper signatures it protects.
Restoration requires addressing the degradation pathways of traditional animal glues. Hide glue and parchment paste used in original 17th-century bindings tend to become brittle and acidic over time, losing their adhesive properties and staining the paper. Consequently, the conservation of these volumes involves the delicate removal of old adhesives using controlled moisture and micro-spatulas, followed by the re-sewing of signatures. This mechanical restoration is only sustainable if the paper fibers themselves are chemically stabilized through deacidification.
Chemical Reaction Pathways in Deacidification
The primary objective of using bicarbonate ions is the neutralization of sulfuric acid residues and other acidic byproducts found in 17th-century paper pulp. Sulfuric acid is formed when environmental sulfur dioxide reacts with moisture and metallic catalysts within the paper. The chemical reaction between these acids and the bicarbonate solutions results in the formation of neutral salts, water, and carbon dioxide.
Calcium Bicarbonate Treatment
Calcium bicarbonate [Ca(HCO3)2] is often preferred for its historical compatibility with the natural minerals found in many 17th-century water sources used during the original papermaking process. When calcium bicarbonate reacts with sulfuric acid [H2SO4], it produces calcium sulfate [CaSO4], water [H2O], and carbon dioxide [CO2]. The calcium sulfate remains within the paper matrix as a stable, neutral salt. A secondary benefit is the deposition of calcium carbonate [CaCO3] as the solution dries and reacts with atmospheric carbon dioxide, providing a lasting alkaline reserve.
Magnesium Bicarbonate Treatment
Magnesium bicarbonate [Mg(HCO3)2] offers a higher degree of solubility, which allows conservators to achieve a higher concentration of bicarbonate ions in the treatment bath. This is particularly useful for heavily degraded papers that require a strong alkaline reserve. Magnesium ions also have a smaller atomic radius than calcium ions, which some research suggests leads to deeper penetration into the primary cell wall of the cellulose fibers. However, magnesium can occasionally cause a slight shift in the color of certain 17th-century pigments, requiring pre-treatment testing for colorfastness.
PH Stabilization and Library of Congress Guidelines
The Library of Congress preservation guidelines provide the industry benchmark for deacidification efficacy. For 17th-century artifacts, the goal is to reach a stabilized pH level that prevents the acid-catalyzed hydrolysis of the glycosidic bonds in cellulose. Hydrolysis is the primary mechanism by which paper becomes brittle and loses its folding endurance.
| Parameter | Target Benchmark | Significance |
|---|---|---|
| Initial pH | 4.5 – 5.5 | Typical state of untreated 17th-century paper. |
| Post-Treatment pH | 7.5 – 8.5 | Optimal range for long-term fiber stability. |
| Alkaline Reserve | 1.5% – 2.0% | Percentage of CaCO3 or MgCO3 by weight. |
| Cold Extraction pH | Measurement Standard | Standardized testing method for verification. |
Integration with Vellum Binding Restoration
The stabilization of the text block through aqueous deacidification is only one aspect of 17th-century restoration. Once the paper is treated and dried under controlled pressure in custom presses, the signatures must be reassembled. This process involves sewing the signatures onto cords using linen thread. To minimize friction and prevent the thread from cutting through the deacidified paper, the thread is typically treated with natural beeswax.
The vellum binding itself requires a different set of conservation skills. Because vellum is processed skin, it does not respond well to the same aqueous treatments as paper. Conservators must ensure that the moisture content of the deacidified text block is fully stabilized before it is reattached to the vellum cover. Any residual moisture can cause the vellum to warp or develop "cockling." The use of bone folders allows for the precise creasing of paper and vellum without abrading the surface, ensuring that the mechanical action of the book remains fluid after restoration.
Synthetic Consolidants and Reversibility
In cases where the 17th-century paper is too fragile for immediate immersion in a bicarbonate bath, conservators employ targeted consolidation. KLUCEL G (hydroxypropylcellulose) is a widely used synthetic adhesive for this purpose. It is soluble in both water and organic solvents like ethanol, allowing for great flexibility in application. By applying KLUCEL G in low concentrations (typically 0.5% to 2%), the conservator can strengthen brittle edges and delaminated layers, providing the physical integrity necessary for the paper to survive the deacidification process. The principle of reversibility is critical; any consolidant used must be removable in the future without damaging the original artifact.
Visual Acuity and Deterioration Detection
A critical component of the conservation process is the acute visual assessment of the artifact. Deterioration is often subtle in 17th-century materials. Conservators look for "tide lines" which indicate past water damage and localized acid migration, as well as the characteristic "halo" effect around iron gall ink, which signals active acid corrosion. The tactile feedback provided by fine bone folders and the precise control of micro-spatulas allow the conservator to feel the resistance of the fibers, guiding the intensity of the chemical and mechanical treatments. This detailed approach ensures that the structural integrity of the book is restored while its aesthetic and historical authenticity remains uncompromised.
