3 PART II. ERASER TREATMENTS ON PAPER

3.1 EXPERIMENTAL

3.1.1 Treatment

All paper samples were �″ � 6″ strips, cut from the same roll of Whatman Chromatography paper number 1, basis weight 87 g/m2, thickness 0.16 MM., medium flow rate, supplied in �″ by 300′ rolls. Whatman Chromatography 1 is a 100% cotton paper, manufactured without fillers and with constant thickness.8 Test data for artificially aged untreated paper are given in Table III. The dry cleaning materials used for treatments were the Magic Rub, Pink Pearl, and Kneaded Rubber erasers and the Opaline Pad. For erasing, the Whatman papers were individually tacked at each end to a piece of Fome-Cor covered with a sheet of mylar which was changed between erasers. All eraser treatments were performed on the felt side of the paper.9

Table III Physical Propertiesa of Untreated, Artificially Agedb Whatman Paper

The Magic Rub and Pink Pearl erasers were used by angling the erasers so that an edge was in contact with the total width of the paper strip. These erasers were pulled at equal pressure five times along the 6″ length of the paper and five times with some overlap along the �″ width of the paper. In order to maintain as consistent an experimental procedure as possible, the Kneaded Rubber was handled first to soften, and then an edge was formed to perform approximately the action just described. It was thought that the use of the Kneaded Rubber eraser as a tamping device might disturb the paper surface less; however, McInnis used it in that way and still found it abrasive to the paper surface.4 The Opaline Pad was used by squeezing the bag to emit some of the crumbs, and by using the pad to rub five times along both the length and width of each paper.

Half of the treated samples were subjected to removal of the eraser crumbs by brushing along the length of the paper with a Japanese brush, 3�″ wide and with 1�″ bristles, until crumbs were no longer visible to the naked eye. This frequently included brushing crumbs off the untreated side of the paper. On the rest of the samples, there was no attempt to remove whatever eraser crumbs were deposited by the described treatment.

3.1.2 Aging

Paper samples were artificially aged by dry oven aging both before and after eraser treatments. Aging took place in a circulating dry air oven at 100�C. The aging categories for treated and untreated samples were as follows: unaged, aged for 7 days prior to treatment with erasers (preaged), aged for 7 days after treatment with erasers (post-aged), aged for 7 days prior to treatment and 7 days after treatment with erasers (pre- and post-aged), and aged 16 days after treatment with erasers (post-aged 16 days). All samples were equilibrated prior to testing in a constant temperature and humidity room. The range of conditions during this set of analyses was 18 � 1�C and 34 � 6% RH.

3.1.3 Photomicrography

Two sets of photomicrographs were taken to record both the eraser particulate in the paper and the surface abrasion. Photomicrographs were taken using an Olympus OM-1 camera and an Olympus 202028 stage microscope, connected by an Olympus adaptor. In the first set, magnification was 33.3, and transmitted light was from an Olympus lamp 256667 connected to a rheostat and set up for K�hler illumination.10 In the second set of photomicrographs, magnification was 13.2 and raking light was from 2 Tensor lamps.11

3.1.4 Folding Endurance12

All paper samples were tested for folding endurance on a Tinius-Olsen Model Number 2 instrument (M. I. T. Folding Tester) with a dead weight of 500 grams. The machine counts the number of double folds necessary to rupture the paper. The results, appearing in Table IV, are the means and standard deviation of six trials at the 90% level of confidence.

Table IV Folding Endurance Test Resultsa

3.1.5 Tensile Strength12

Tensile strength of all paper samples was obtained on an Instron Model TM-M 1101 Universal Tester. The data were recorded as stress versus time under a uniform increase in extension, and are reported as breaking strength in kilograms. The results appearing in Table V are the means and standard deviations of three to six trials at the 90% level of confidence.

Table V Tensile Strength Test Resultsa

3.1.6 Surface pH12

Surface pH was measured on all paper samples, using a Beckman Expandomatic SS-2 pH meter with an Ingold glass electrode. The pH meter was calibrated with buffer solutions of pH 5.0 and 7.0. Distilled water was dropped from a medicine dropper onto the paper surfaces at the reading sites. The electrode was rinsed with distilled water and wiped dry with Kimwipes between each reading. Nitrogen gas was bubbled through the distilled water to maintain the pH of the water at 8.3–8.6. Three readings were taken on each paper. Results appear in Table VI and are the means and standard deviations of 18 readings on 6 papers at the 90% level of confidence.

Table VI pH Test Results

3.1.7 Visual Examination

All paper samples were examined under illumination from a Macbeth Spectralight light box set at maximum brightness of the daylight setting. The surfaces of the papers were examined to note any alteration in texture.

A minimum of three paper samples with the same treatment and aging were viewed under ultra-violet illumination from the Macbeth Spectralight light box. Samples from sets of untreated papers, aged and unaged, were compared with the treated samples to separate changes in fluorescence due to aging from those due to treatment.

3.2 RESULTS

3.2.1 Photomicrography

Preaged and unaged papers which received the same eraser treatment could not be differentiated in the photomicrographs. The set of photomicrographs taken at a higher magnification and using transmitted light showed very clearly that attempts at complete removal of eraser crumbs were unsuccessful. All of the eraser treatments employed left eraser crumbs among the paper fibers.

The set of photomicrographs taken at a lower magnification and using raking light was moderately successful in recording observed surface abrasion caused by certain eraser treatments. The surfaces of papers treated with the Pink Pearl and Kneaded Rubber erasers are abraded; a lifting of individual fibers above the plane of the paper was clearly evident. No such disturbance was recorded on the surfaces of papers treated with either the Magic Rub eraser or the Opaline Pad. The small depth of field of optical microscopy limits it as a technique for recording surface abrasion of paper. Plans have been made to subject eraser treated samples to scanning electron microscopy in order to improve the documentation of effects of eraser treatments and of brushing on the paper surface.

3.2.2 Fold Strength

Untreated, unaged Whatman paper had a fold strength of 59 � 4 double folds to rupture which fell to 34 � 10 after 16 days of dry oven aging. Little or no reduction in fold strength is observable as a result of eraser treatments. The effect of pre-aging, post-aging, or brushing the samples was not detectable when comparison is made with untreated samples.

3.2.3 Tensile Strength

Untreated, unaged Whatman paper had a tensile strength of 4.3 � .1 kg. which was virtually unchanged after 16 days of dry oven aging. As with the folding endurance tests, no reduction in strength was observed as a result of eraser treatments.

3.2.4 Surface pH

The surface pH of untreated, unaged Whatman paper was 7.4 � .1 which was virtually unchanged after 16 days of dry oven aging. Samples treated with the Kneaded Rubber and Magic Rub erasers, whether or not brushed to remove surface crumbs, retained that pH within .2 pH unit upon aging. Papers treated with the Opaline Pad and Pink Pearl eraser increased in pH by about 1.0 � .2 pH units. No effect of aging or brushing was detectable. A spot check on the unerased sides of papers treated with these two erasers showed pH values lower than the erased sides but slightly higher than untreated papers.

Since both aged and unaged Art Gum erasers, chemically identical to Opaline crumbs, had an acid pH, the magnesium silicate in the Opaline Pad (see Table I) was indicated as the alkaline agent. Magnesium silicate deposited on an unaged Whatman paper sample increased the paper's pH from 7.2 to 8.2, an effect consistent with that observed on Opaline treated paper sample, suggesting that the magnesium silicate is the alkaline agent.

3.2.5 Wetting

The procedure for measuring the surface pH involved dropping distilled water on to the surface of the paper. It was noted that certain treated papers exhibited a resistance to wetting. The papers exhibiting the most resistance to wetting were those treated with the Pink Pearl eraser. Papers erased with the Kneaded Rubber eraser were resistant to wetting after post-aging and pre- and post-aging. The Opaline Pad caused a slightly less dramatic effect, though upon wetting, papers both pre- and post-aged with no attempt to remove crumbs had a spotty appearance. Only the Magic Rub eraser caused no incidence of altered wettability. After 16 days of aging, only the papers treated with the Kneaded Rubber eraser continued to resist wetting.

3.2.6 Color

No color change was detected between unaged and aged, untreated samples. Color changes did occur in treated samples, and these generally increased with post-treatment aging time. The color changes, which were too subtle to be documented by Munsell color chips, may be attributable to: 1) a loss of brightness resulting from the disturbance of the erased surface, and 2) deposition of eraser material on the surface and within the fibers of the paper. The erasers are discussed in the order of decreasing color change on the treated papers. The papers erased with the Pink Pearl eraser lost their brightness immediately. With increased aging, papers with no brushing appeared warmer and brushed papers appeared grayer and dirtier. To a lesser extent than Pink Pearl, the Kneaded Rubber eraser resulted in a dirty appearance on all papers tested, which increased with longer aging. All papers treated with the Opaline Pad became increasingly warmer in appearance with aging. Papers treated with the Magic Rub eraser were unchanged except for pre- and post-aged and 16 days aged papers with no brushing for crumb removal. The pre- and post-aged papers were slightly warmer and grayer than unaged, untreated papers. The 16 days aged papers were even grayer.

3.2.7 Fluorescence

It was expected that there would be a correspondence between the fluorescence of an eraser and of the papers treated with that eraser. However, no significant differences in fluorescence as a result of eraser treatments were found. The intensity of fluorescence depended upon the aging time of the paper, with all unaged papers fluorescing the least. Previous experimenters found that paper decreases in fluorescence after being heated at 105�C for one hour.13

3.2.8 Surface Abrasion

One week of pre-aging had no influence on the susceptibility of papers to abrasion by erasers. Papers erased with the Pink Pearl eraser showed the most dramatic abrasion in that the surface fibers of the Whatman paper were lifted up from the plane of the paper. The Pink Pearl crumbs adhered most tenaciously and required more brush strokes for removal from the paper than the other erasers.

The Kneaded Rubber eraser also caused abrasion in the treatment. It seemed to pull on the fibers as it was pulled across the paper surface. Very few crumbs were visible from the Kneaded Rubber eraser, and the least number of brush strokes was required to remove visible crumbs.

Papers erased with the Magic Rub eraser exhibited a small amount of abrasion, particularly along the edges of the papers. Fewer brush strokes than were used with the Pink Pearl were necessary to remove visible Magic Rub crumbs.

The Opaline Pad was the least abrasive cleaner, such that no change in the texture of the paper was visible to the naked eye. The greatest amount of brushing was necessary to remove visible crumbs.

3.3 DISCUSSION

There are some interesting parallels between the results of McInnis' study and the current study. McInnis used many materials in common with this experiment: Whatman Chromatography #1 paper, Kneaded Rubber and Art Gum erasers, a dry cleaning pad identical to Opaline, and a white Mars Plastic synthetic eraser, which, like Magic Rub, is a vinyl. Preaging conditions were identical; eraser treatments were gentler: four strokes compared with ten performed in this work. The crumbs from the dry cleaning pad were manipulated with a brush and not with the pad itself as in this study. In McInnis's report, examination of treated papers was exclusively optical, but a scanning electron microscope was employed. The parallels in the findings are as follows: 1) preaged and unaged papers respond identically to eraser abrasion; 2) the Kneaded Rubber eraser was particularly damaging to paper fibers; 3) the dry cleaning pad was unabrasive; 4) the Mars Plastic eraser could be used without abrasion; 5) all of the erasers leave particulate matter among the paper fibers.

The earlier McCrone findings are more well known, since they are abstracted in Carolyn Horton's Cleaning and Preserving Bindings and Related Materials, and the text of this book recommends materials based on McCrone's work.14 The Opaline Pad, Pink Pearl, and Magic Rub erasers were included in this report, and treatments were applied to both rag and wood pulp paper samples. Aging was 100 � 1�C for 27 days in a circulating air oven. Tests performed were microscopic examination for fiber damage, cold extraction pH, and fold and tensile strength.

No change in fold or tensile strengths or pH's was found; and there was no fiber damage observed though there is no description of how the erasers were used. The report states that Pink Pearl and Magic Rub erasers, which contain CaCO3, are “ideal abrasives for book materials, especially if they remain in the book material” due to their alkalinity.1 The report says that the effect of CaCO3 on the cold extraction pH performed was invisible because CaCO3 is not water soluble.1 However, in discussing dry cleaners used on vellum in another section of the same report, the Pink Pearl and Art Gum erasers are recommended over the Magic Rub eraser because the pH of the vellum treated with Magic Rub was .1 of a pH unit lower than vellum treated with the other two erasers.1

The difference between McCrone's uniform cold extraction pH findings for all of his aged, treated papers and the variation in pH obtained in the current study may be due to the great dilution of eraser residue in McCrone's cold extraction solution. How much eraser material was deposited on McCrone's paper samples is not stated. Cold extraction and surface pH measurements often produce different values.15

Among the questions generated by the current study is whether a more effective means for eraser crumb removal can be developed. A consequence of eraser particulate in paper may be that solvents used for tape and mount removal may swell eraser particulate while it is enmeshed in the paper fibers. Art Gum, Opaline, and Pink Pearl erasers, soluble in warmed acetone-hexane mixtures, may swell in either solvent used alone. Brushing is clearly not effective for removal of the particulate material, and it may even enmesh the particulate deeper into the paper fibers.

A potential major drawback of the Magic Rub eraser which requires further investigation is the thermal degradation of its constituent plasticized poly(vinyl chloride). The eraser immediately became soft at 100�C, and Paul Banks noted the dissolution of the yellow paint on pencils stored in a drawer in direct contact with a vinyl eraser.3 Dioctyl phthalate is a paint solvent. At 370�C heating for 30 minutes, plasticized poly(vinyl chloride) may be more hazardous than the scope of our tests has shown.

It should be noted here that erasers have been used in the conservation of materials other than paper, such as in the cleaning of stone statuary or of metallic threads, and in the use of absorbent crumbs to remove surface dirt from a variety of media. The results of the study conducted here cannot be generalized to evaluate erasers for these other uses, but these results demonstrate the necessity for evaluating these products in all of their conservation applications.

3.4 CONCLUSIONS

1. All products tested using several dry cleaning procedures altered surface characteristics but not mechanical properties of Whatman Chromatography paper.
2. All the erasers tested showed marked alterations upon dry oven aging. Dimension change was greatest for the Pink Pearl eraser, and least for the Magic Rub eraser. Weight change was greatest for the Art Gum eraser, and least for the Magic Rub eraser. The surface pH of the aged erasers changed by about one unit except for the Art Gum eraser, which exhibited negligible surface pH change upon aging. All erasers exhibited color change, and became more odorous.
3. The eraser treatments regardless of aging or brushing produced no measurable alterations on the folding endurance or tensile strength of the paper. The Opaline Pad and Pink Pearl eraser treatments caused increases in the surface pH of the paper; treatment with the Magic Rub and Kneaded Rubber erasers left the surface pH unaffected. Subtle color changes were detectable as a result of treatment with each of the erasers. The wettability of the paper was decreased by all eraser treatments except those using the Magic Rub eraser.
4. Treatments using all eraser samples left detectable amounts of eraser material in the paper. Photomicrographs showed that attempts at complete removal of eraser crumbs by brushing were unsuccessful.
5. The Magic Rub eraser was the least altering to the paper, and the eraser itself was the most stable to aging. Papers treated with the Magic Rub eraser suffered negligible abrasion and color change, and had no change in either surface pH or wetting ability.
6. The Opaline Pad was even less abrasive to treated paper samples than the Magic Rub eraser. Slight warming of the color of the paper surface after treatment and aging was noted and is probably due to the color change in the eraser crumbs. The surface pH of the paper increased after treatment.
7. The Kneaded Rubber eraser was judged abrasive as used. It adversely decreased the wettability of the paper after treatment. The eraser changed dimension upon aging. The crumbs were particularly difficult to remove, as they were small, not visible to the naked eye, and clung to the paper fibers.
8. The Pink Pearl eraser was judged the worst eraser because it readily abraded the paper surface and required working over the paper excessively to remove visible crumbs. The abrasion and pink particulate matter altered surface color and texture of the paper. Paper wettability decreased after treatment.