2 ERASER COMPOSITION AND LITERATURE REVIEW

THE SEVEN ERASERS—Absorene, Art Gum, Tapeten Reiniger, Pink Pearl, Mars Plastic, Magic Rub, and Kneaded Rubber—can be divided into three categories: starch, rubber or oil, and vinyl-based (see table 2).

Table 2 Evaluation of Erasers in Paper Conservation

Absorene is a pink, malleable material made primarily for cleaning wallpaper. Its manufacturer, Absorene Company in St. Louis, reports its ingredients to be flour, salt, water, and mineral spirits.2 A study done at the Canadian Conservation Institute (CCI) reported the composition as starch, salt (NaCI) and trace quantities of aluminum.3 Both Horton and Banks recommended Absorene for the removal of dust from books and paper objects.4 The only reservations stated were that the crumbs, when dried out, adhered to surfaces and were somewhat difficult to remove. The CCI report found that Absorene left many sticky pink particles on paper which became hard and brittle on drying.5

Art Gum was studied by McInnis, who found that it abraded paper fibers extensively, and by Pearlstein.6 Pearlstein identified it as factice, which is vegetable oil vulcanized or cross-linked with sulphur bonds. The principal method of manufacturing factice is by heating a drying oil (usually rape seed oil, although linseed, whale, and soya bean oil are also used) with 10%–30% sulphur for several hours at 150�C–170�C.7 The purpose of this process is to form cross-links between the long polymer chains of oil, and the result is a dark, elastic product. A lighter product can be made using 15% sulphur monochloride at room temperature. Other materials are frequently added to enhance its properties or speed its manufacture. Antioxidants retard degradation. Reinforcing agents (such as carbon black and, less frequently, colloidal silica) improve the tensile strength and durability. Some forms of vulcanized products are susceptible to attack by mold.8

Tapeten Reiniger appears to be a vulcanized rubber and/or vegetable oil product9 and may contain calcium carbonate, which is frequently included in rubbers as a filler and extender. (Positive microchemical results were obtained for CaCO3.) In C.V. Horie's study and literature review of conservation materials, vulcanized rubber is discussed generally: “Being tacky, the very small particles are held in fibrous material. Over time and exposure to light, the powder adheres to itself, forming a film, yellowing slightly at the same time. The rubber is almost totally insoluble in solvents.”10

Pink Pearl 101 is a pink rectangular eraser composed of factice. Pearlstein reported its components as vulcanized vegetable oil, rubber, antioxidants, softeners, pumice and coloring agents, while CCI reported its inorganic components as: silicate, magnesium, iron, aluminum, potassium, chloride, sulphur, and possibly titanium.11 The inclusion of pumice, approximately 65%–75% silicon oxide and 10%–20% aluminum oxide, is presumably to act as a filler, absorbent, and adherent for rubber products, in addition to being an abrasive. Pink Pearl was recommended by both Horton and Clapp,12 although Horton did note its abrasive quality. Pearlstein's study found that it readily abraded the surface, altered the color and texture of the test paper, and decreased its wettability. She further noted that its crumbs were the most difficult to remove and deemed it the worst eraser of those she tested.13

Mars Plastic is a white vinyl eraser, composed of poly (vinyl chloride)(PVC), a dialkyl phthalate, calcium carbonate, chloride, and possibly titanium.14 Poly (vinyl chloride) is dimensionally stable and resistant to moisture, most acids, fats, petroleum hydrocarbons, and fungus. The presence of chlorine in the organic molecule increases its density, viscosity, and chemical reactivity, but decreases its specific heat, solubility in water, and flammability. It is readily compounded into flexible or rigid forms by the addition of plasticizer, stabilizers, or fillers. The material decomposes at 148� C, and toxic hydrogen chloride fumes are evolved.15

Concern about PVC-based erasers has revolved primarily around the inclusion of plasticizers and their possible effect on the painted or printed portion of a work of art, since they are known to interact with materials in contact with them. In addition, PVC liberates hydrochloric acid as it degrades.16 CCI felt that both Mars Plastic and Magic Rub, the two PVC-based erasers examined here, contained enough calcium carbonate to neutralize at least some of the acid.17

Other concerns relate to the specific characteristics of the eraser, such as its particular abrasive material and the percentage and particle size of that material. The McInnis study found that Mars Plastic broke unsized paper fibers but did not affect sized paper. CCI felt that quantification of the damage for the purposes of comparison was not possible, but noted that low to moderate amounts of residue remained on the surface.18

Magic Rub (or Peel-off Magic Rub) is a white vinyl-based eraser. Its composition as reported by the manufacturer is 30% (weight) poly (vinyl chloride), 35% calcium carbonate filler, and 35% dioctyl phthalate as a plasticizer.19 It was recommended by Horton, Banks, Clapp, and Pearlstein. Horton noted that it did not leave harmful residue, but that it did contain abrasives and should not be used vigorously.20 Banks described it as “extremely gentle and [having] the quality of tending to pick up [its] own crumbs.”21 Pearlstein found that Magic Rub altered the surface characteristics but not the mechanical properties of paper, nor did it affect the paper's pH or wettability. It did leave some residue on the surface, however, it altered the paper the least of all the erasers she tested.22

Kneaded Rubber is made up of natural and synthetic rubbers, vulcanized vegetable oil, antioxidants, mineral oil, pumice, calcium carbonate, titanium dioxide, and carbon black.23 CCI reports its composition to be poly (isobutene), calcium carbonate, titanium, chloride, sulphur and trace amounts of magnesium, aluminum, silica, and potassium. Banks described it as being among the gentlest erasers; however, further investigation has shown it to be quite abrasive. Pearlstein noted that the use of kneaded erasers increases the wettability of the paper, and that the residual crumbs are particularly difficult to remove and are not visible to the eye.24