8 POULTICE

THE USE OF A POULTICE is quite simple. It is a relatively old-fashioned technique and one which we could not do without. A poultice consists of a solvent to dissolve the adhesive and an absorbent. The absorbent can be selected from a wide variety of materials, such as clays (Fuller's earth), siliceous materials (diatomaceous earth or fumed siliceous materials (diatomaceous earth or fumed silica), or cellulosic materials (cellulose powder, filter paper, or torn-up blotters). These two components work like a mud pack. The absorbent prevents the solvent from evaporating too fast while keeping it close to the surface of the adhesive. This allows the adhesive to take on solvent and soften or go into solution. During drying, the dissolved adhesive is pulled into the poultice away from the object's surface, and can be discarded easily.

8.1 Procedure

Dry clean the artifact in the traditional fashion by using vinyl or powdered drafting erasers. Next, mechanically or with a solvent, remove the tape carrier and as much adhesive as possible. Test the adhesive mass remaining on the object to identify the solvents that will remove it. Then test the paper and all nearby inks, dyes, and pigments for their solubility in these solvents. Finally, select the most effective absorbent by experimenting on a test strip of adhesive. Fuller's earth is most frequently selected for objects treated in the Conservation Office.

To prepare for poultice applications place the object in a fume hood on top of a sheet of five mil polyester film. Not only does the underlying film focus evaporation of the solvent upward, it also aids in after-treatment clean up by providing a surface onto which the used poultices can be swept. Build a one-half inch powdery dam of Fuller's earth around the outside perimeter of the adhesive. Place additional Fuller's earth in a beaker and mix it with the solvent of choice. The proportion of solid to liquid that seems to work best is slightly more than three parts Fuller's earth to two parts solvent. The resulting poultice should be relatively dry. A dry poultice works better and results in less lateral movement of the solvent than does a more liquid poultice. A poultice that is too dry will not adhere to the paper well enough to draw the adhesive out. Spread the solvent-rich poultice on the adhesive area and allow it to dry thoroughly.

To insure that the adhesive is fully extracted from the paper, make sure all solvent has evaporated away from the interface of Fuller's earth and paper before the poultice is removed. When the poultice is dry brush it away. To determine how adhesive removal is progressing, brush a quickly-evaporating solvent such as methylene chloride over the entire area of adhesive residue. The solvent will evaporate from the paper more slowly where adhesive remains. Repeat the entire poultice procedure several times if necessary.

8.2 Advantages and Disadvantages

Localization of solvent application, the swelling time needed for solvation, and the capillary action that extracts the dissolved adhesive from the paper make the poultice a very effective means of removing pressure-sensitive tape adhesives from paper. Another advantage of the poultice technique is that the rate of solvent evaporation can be altered by changing the environment around the poultice. For example, the rate of evaporation can be slowed by building a polyester tent over the poultice, by putting weights on top of the poultice, or by mounding up additional poultice material.

The poultice does have disadvantages. One is that poultice debris can be caught in crevices of porous paper. This debris is especially apparent against dark paper or pigments. One way to remove trapped debris is to rub the area with a soft vinyl eraser or a kneadable eraser. Another disadvantage is that lateral movement of solvent and solvent-carried adhesive can occur, creating tidelines in the paper. The formation of tidelines can be avoided if the amount of liquid in the poultice is reduced. If a drier poultice is used and tidelines still persist, gently swab them with a solvent. The vacuum suction table can be helpful in this process; and, in fact, a combination of treatments is often necessary and more beneficial than a single method.