LEAF CASTING ON THE SUCTION TABLE
1 LEAF CASTING
IN THE LATE 1950s, Esther Alkalay and Ulia Petrovna Nyuksha began experimenting with the concept of filling missing areas in paper with fibers suspended in a liquid medium. By 1961, equipment for this purpose had been developed and was in use in laboratories located in Eastern Europe and the U.S.S.R. The process has come to be known as “leaf casting,” and is best described in the preprints of the 1980 Cambridge Conference (see bibliography). Today the Library of Congress and the Northeast Document Conservation Center both use leaf casting machines based on Alkalay's design. Other models, varying in cost and sophistication, have been developed in recent years and are in use in laboratories throughout the world.
1.1 Steps of traditional leaf casting
- The damaged paper should be thoroughly wet. (Extending the soak time and increasing the wetness of the sheet aids in later bonding.)
- The paper is placed on a permeable surface at the bottom of the leaf casting tub. A hold-down mechanism is usually employed to restrict movement of the damaged sheet when water is added to the area above the paper.
- A precise amount of pulp of appropriate fiber type and color is stirred into the water bath. It is important that measurement of the missing area and paper thickness be accurate and the pulp addition be correct so that the cast fills will equal the thickness of the original paper. Simple graph paper or a planimeter can facilitate this calculation.
- Suction, whether from an electric pump or gravity, is applied to the space below the damaged work. Water in the top portion drains through the voided spaces only, depositing an even layer of pulp across the loss.
1.2 Characteristics and Qualities of Leaf Casting
Leaf casting has distinct advantages over other methods of filling losses. If one's work is organized and equipment is properly adjusted, damage to paper can be repaired very quickly. It is possible to create a smooth, flexible and unobtrusive transition from the fill to the original paper. And if fibers of similar dimensional characteristics are selected and prepared, distortion can be minimized or avoided. However, there are areas of concern and potential problems in leaf casting, and they should be understood in order to take full advantage of this technique.
The bonding of the fill material to the old paper can be problematic. The quality of connection will depend on a) the state of deterioration of the original sheet; b) the bonding characteristic of the pulp prepared for casting; c) the edge of the loss and the degree of fiber extension; and d) the speed that water drains through the losses.
In the past, various kinds of adhesives have been added to the casting liquid to improve bonding. Unfortunately, adhesives affect the entire object during the period of immersion and drainage, not just the fill. This changes the nature of the paper—at the very least—and if the adhesive material proves to have poor aging characteristics, the entire artifact will suffer. Therefore, the addition of adhesives, though helpful in bonding, should be carefully considered and avoided when possible.
Conventional leaf casting requires the complete immersion of the paper in water. Clearly, many items cannot undergo this action without risking loss or alteration of the media. As a remedy, application of fixing agents has been suggested. However, it seems imprudent to impose such an altering measure just for the purpose of repair when other satisfactory methods are available. Hand pulp technique and localized leaf casting methods do not require total water immersion.
During leaf casting, there is some pulp deposition onto the surface of the original paper. As the fluid level in the tank gets low during the final stage of drainage, some of the fibers in the solution will become affixed to the surface of the original. This may be minimized if the pulp-in-fluid concentration is kept low, though in certain cases some cleanup may be necessary. (If the work is one-sided and the casting takes place with the work face down, then slight deposition on the verso may not be an issue.)
In an early publication too obscure to remember, reference is made to the employment of chronic gum chewers for major restoration ventures. Here, beating and fibrilation of fibers with cuspid and molar action is gentle but thorough, with chemical processing provided by enzymatic mechanisms (spit). Unfortunately, with leaf casting activity on the rise, it has become more and more difficult for conservators to locate eager chompers to fill increasing demand.
Many conservators have turned to the use of household blenders for conversion of old paper to pulp. Recently, blender preparation has been criticized in the literature in favor of beating devices especially designed for the purpose. Certainly, beating machines do provide precise control over fiber length and hydration and they will accommodate the use of raw fibers. Petherbridge points out that bonding and dimensional stability is very dependent on pulp quality and preparation. However, the use of expensive equipment is not always practical or necessary. If care is shown in the selection of paper for repulping and the fiber-to-water ratio is properly controlled during beating, the quality of blender-beaten fills may be acceptable for many purposes.