9 SUCTION TABLE

THE SUCTION TABLE or suction disk consists of a flat porous surface, sealed on top of a plenum chamber that is connected to a vacuum pump. The reduced pressure below the surface allows liquids to be drawn through an artwork placed on top. Soluble stains and adhesives can be removed in the process.

The major advantage of removing pressure-sensitive tape by suction is that suction limits solvent spread. If the tape lies over solvent-soluble design, the conservator can work around, and so not bleed, the fugitive color. Many papers, especially those of the 19th and 20th centuries, contain solvent-soluble dyes and sizing agents. Control of solvent spread helps to prevent disfiguring rings and stains caused by movement of those components in the paper.

Difficulty arises when the solvents that remove the tape also move color in the paper—creating tide lines of liberated dyes and sizing. One solution is to employ two solvents sequentially: a strong one to dissolve the adhesive, and a weaker, misable solvent to flush the stronger solvent and solubilized adhesive from the paper. Employing the restraining solvent can reduce the amount of the strong solvent needed and help localize its effects. The weaker solvent could be any solvent with no or little adverse effect on components of the paper or media. In practice, the strong solvent is applied to the adhesive, the area ringed with the weaker solvent, and then the center flushed with the weak solvent to carry dissolved material into the underlying blotter.

Another advantage of the use of suction is that much less mechanical action is involved, than occurs with other local removal techniques, such as rolling with cotton swabs. Thus, abrading solvent-sensitized inks and crushing the paper surface texture are avoided.

A typical situation in which suction is useful is characterized by the following example. A cartoon drawing executed in graphite and red pencils had small pieces of cellophane tape lying on the front over both types of pencil. The carrier was still present and adherred to the paper; the adhesive had partially sunk into the paper, causing translucent stains on the reverse, but had reached the hard, gold, crusty stage in only a few places. Tests with solvents that might be expected to remove the adhesive residues caused no movement of sizes, fillers, or dyes in the paper. The graphite was extremely friable, and even the mechanical action of a dry swab was enough to lift it. The red pencil was very soluble in toluene. Given this sensitivity of the image, no removal technique except suction could be used on this drawing. In actual treatment, the carrier was removed locally by painting a mild solvent on the verso and lifting the cellophane with tweezers. Sticky surface adhesive was removed from non-design areas with a crepe square. Then the solubility of the adhesive residues was tested. In this case, toluene dissolved the oily residue within the paper and acetone dissolved the crusty residue.

9.1 Equipment

The suction table used for many years at the Library of Congress was built in the library and consists of a small fritted plastic top supported by a channeled aluminum base, and connected to a wet/dry shop vacuum. Other designs for homemade suction tables and disks (small, fritted-glass surfaced funnels set flush in a countertop) have been developed. Suction tables are also available commercially.5 Regardless of what table is used, all exhaust air and solvent fumes should be vented into a fume hood.

9.2 Procedure

Cover the suction table with polyethylene except for the small area to be used. Keep the area as small as possible for best suction. Lay thin blotter over the whole area exposed to suction. This method makes it easy to move the blotter as it becomes dirty without rearranging all the plastic mask. (Note: Never allow the object to be subject to suction while “bent” over the edge of a thick blotter. When the blotter covers the entire suction area, this can't happen.) A thin blotter is preferable because a thick blotter reduces the air flow needlessly.

Lay the art object on the table. Where possible, place the side to which the tape was applied against the blotter. Any surface adhesive will then reach the blotter directly. When working near soluble media as in the case of the cartoon drawing, it may be necessary to keep the image face up so solvent can be applied precisely. In some cases soluble media may be placed face down if a polyester film (Mylar) mask is traced from the drawing, cut out, and laid over the verso to coincide with the design. Mask any exposed blotter with more polyethylene film.

Where it was not possible to crepe away surface adhesive because of underlying friable media or soft-surfaced paper, dot on mild solvent and blot from the top while under suction. The mechanical action of blotting helps to remove sticky adhesive.

Next, with either a brush, eye dropper, or air brush, apply solvent to remove adhesive that has penetrated the paper fibers. The size of the drop or the amount of solvent applied at once is important. The drop must be big enough to penetrate the paper and carry adhesive into the blotter below or the technique will not be effective. On the other hand, big drops spread unnecessarily and will form adhesive or paper-dye-laden tide marks far away from the original tape stain.

For fine work, touch the paper surface lightly with the dropper while squeezing the bulb very gently. Release solvent only as it is absorbed by the paper. Move the dropper continuously to dry areas. Work several pieces of tape, or a sizable area, at once, because solvent must be added to dry paper. If the paper or blotter below is soaked with solvent, then additional applications will spread laterally in the object. Replace the underlying blotter as it becomes soiled. Be PATIENT; old adhesives that have increased in molecular weight take a while to soften and go into solution. This is normal behavior, so don't give up too soon!

When all the adhesive seems to have been removed from a given area, test by brushing solvent evenly over the dry paper. Watch the spot as the solvent evaporates. Any remaining resin will trap the solvent and so dry more slowly than clean paper. This is a good way to check for adhesive tide lines that may not be visible in dry paper. If resin persists in spots, try more of the same solvent, or try both stronger or weaker solvents.

Crusty patches can be rolled with tiny cotton swabs on the suction table. The slight mechanical agitation hastens break-up of the mass. Remove adhesive tide lines as they form. Brush or dot solvent directly on the line, then even the area by brushing outward from the center in a star pattern. Lightening the pressure on the brush as you move away from the adhesive puts an ever diminishing amount of solvent in the paper and ensures that no new tide lines will form. An air brush adjusted to deliver a very small, controllable, cone-shaped mist of solvent is ideal for this operation.

When the adhesive has been removed from an area, a gray line may remain at the perimeter. This is surface dirt trapped by the sticky adhesive at the edges of the carrier. Dry cleaning with a vinyl eraser, off the suction table, may remove it. A golden stain may also remain after all adhesive has been removed if the tape carrier or adhesive has attacked the paper chemically. Such discoloration may respond to washing with water or water raised to pH 9 with ammonium hydroxide, and thus be removed as a final operation locally on the suction table.

When the tape crosses areas or lines of soluble media, remove the resin first in the largest open spaces. Gradually bring the solvent up to the edge of the soluble design. Adhesive cannot be removed from behind these areas and will remain in the art work.