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    DOCUMENT CONSERVATION :: leaf casting  
  Leaf-casting is an effective means of repairing a paper object with numerous holes and tears. Patching of losses can be done by hand very successfully, but when confronted with many book pages, a semi-automated process is more efficient. The item must first be washed to reduce soluble discoloration, acidity, and other degradation products. Once that occurs, it is ready for the leaf-casting process.
Preparing Pulp

Pulp is prepared to replace areas of loss. The conservator has many options available, in terms of colors and fiber type, in order to get desired results. On occasion one may try to match the original as closely as possible, other times a clearly different, yet sympathetic color is preferred to highlight areas of original paper versus replacement.

Paper pulp is placed in an industrial blender to break apart and separate fibers in order to suspend them in water. Measurements are taken to determine the density of this solution. This number will be of use later when one needs to know how much pulp to add to the casting machine.


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Photographing the original

A digital photograph is taken of the item on a black background. This image is then examined by computer software to determine the ratio of loss (in this case, by counting black pixels) to existing paper. That figure, when combined with a measure of the paper thickness, gives a determination of how much pulp is needed for replacement.

Although this system counts printed text as loss, in most cases its contribution is negligible.
Casting 

The leaf-casting basin is first pumped full of water from a storage tank below until it reaches a level of the casting surface. It is then placed on the casting surface.


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The object is misted with water to wet it out and placed face down on a non-woven polyester support. All wrinkles and tears are flattened and aligned. A plastic grid placed on top stabilizes the object, then water is turned on to fill the basin.

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A pre-measured amount of pulp slurry is added and mixed to distribute.

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The pump is turned on and water is drained from the basin causing the paper fibers to collect around the areas of loss; where the downward flow is greatest.

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Click image to watch video 		Sizing

Once all water is drained away, another sheet of non-woven polyester is placed on top and the sandwich is shifted to the suction table. Although the new pulp fibers will adhere to the original paper through hydrogen bonding, a sizing agent is brushed through the polyester support on both sides to ensure a strong bond and durable finished product. This sizing can be either dilute methyl cellulose or purified gelatin. The suction table aids in sizing application, and keeps the fragile pulp repairs in place while being brushed.
Drying

The leaf-cast object is now placed between blotters and pressed to dry.


Click image to watch video 		Trimming

Once fully dry, the object is trimmed of excess pulp around the edges. At this point any areas where the filled bond is weak can be locally repaired with wheat starch paste.
   
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