Volume 2, Number 6
Dec 1989

The Use of the Archivist's Pen and Universal pH Solution for Estimating the Surface pH of Paper

*Conservation Department, The Museum of Modem Art, 11 West 53 St., New York, NY 10019. [In 1989, Antoinette King is still at this address.]
**Bell Telephone Laboratories, Murray Hill, NJ (Consultant to the Museum of Modem Art)

Reprinted with permission from Studies in Conservation volume 15 (1970).

Rapid and facile methods of estimating the acidity or alkalinity of the surface of paper are of considerable interest to paper conservators and others with allied interests. Among the possible choices of a method to fulfil this need is the Archivist's Pen (1), a pH-sensitive marker which may simply be applied to the test surface. Another potentially useful procedure is to apply a drop of universal pH solution to the paper (2). Although this dye mixture is designed to be used in solution, it may prove useful for measurements on paper. To evaluate the rage of applicability of these two methods, they were compared with each other and with a recently developed physical technique for measuring surface pH using a modified Beckman 39142 combination glass electrode (3).

The Archivist's Pen employs bromcresol green indicator solution as a sensor. For pH >5.2 the blue color of the dye is retained. For more acidic surfaces, the color changes through green (pH about 4.5) to yellow, when pH is 3.5 or below. Observations using the Archivist's Pen were made not less than one nor more than five minutes after marking the paper. With some papers, the color of the test spot changed during several hours or days after making the mark, possibly because of the influence of the acidity in the body of the paper an the dye, rather than of the surface pH. No systematic study of these long-term color changes has been made.

The universal pH indicator solution used for this study was "Harleco Wide Neutral Range Indicator Solution" (4), although the findings should be equally applicable to other commercially available products. The indicator solution was calibrated with buffer solutions of known pH. The calibrations indicated that the color scale suggested by the supplier gave pH values which were too high by about 0. 5 pH units. All surface measurements made using the Harleco solution were adjusted accordingly. Again, observations were made one to five minutes after wetting the surface, and any long-term color changes were neglected. Comparative data using the Archivist's Pen, the indicator solution, and the surface electrode are collected in Table I for a variety of papers. The samples were selected to encompass widely differing physical characteristics. All tabulated points are averages of at least three concurring observations on different portions of the test sample. Because of the relative insensitivity of the color changes observed using the Archivist's Pen and the Harleco solution, the precision of these observations is m better than ± 0.5 pH units, depending somewhat on the nature of the specimen under investigation. The colors of the solution are somewhat obscured if the paper is colored. The precision of the electrode measurement is better, ± 0.3 pH units, but requires equipment which is more elaborate and may not be readily available. Although a precision of ± 0. 5 pH units is not high, it is nevertheless sufficient for most applications encountered in paper conservation.

The quantitative significance of the absolute pH value assigned to the paper surface is difficult to assess, since the concept of pH is defined in term of concentration of acid or alkaline centers. However, the qualitative significance is quite apparent. The further the pH reading is from 7, the further the surface is from neutrality. Although it is not meaningful to discuss the absolute accuracy of pH measurements of paper surfaces, the relative ordering of a series of papers would be a valuable aid in their evaluation for various functions. The agreement . the three independent measurement techniques is within their expected precision. Thus the reliability of any one of these methods for measuring surface pH is quite high. However, it does not follow that the measured surface pH is representative of the pH of the bulk sample. Comparisons of surface measurements with hot and cold aqueous extractions have shown significant differences (5) which may be due to the effects of the surface coating of the paper or the use of paper fillers and binders.

TABLE I

This study has shown that either the Archivist's Pen or a calibrated universal pH solution can provide reasonable estimates of the pH of paper surfaces. One noteworthy disadvantage of both methods is that permanent stains are left an the paper following a measurement. Nevertheless, these may be useful techniques for assessing the pH of matboards, lining and hinging papers and any other materials used in conservation framing and storage of works of art for which expendable samples are available.

Notes and References

1. Available from Process Materials Corporation, 329 Veterans Boulevard, Carlstadt, NJ 07072. Cost approximately $3.50 per pen. [Note: In 1989, address has changed and the company no longer carries the pen.] The manufacturer provides an instruction leaflet with the pen describing its use. No other literature on the pen has been published to date.

2. "An Investigation into the use of pH Indicators an Paper" evaluates four different indicator solutions. ICA [Intermuseum Conservation Association] Newsletter, vol. 7, no. 1. Oberlin, Ohio June 1969.

3. Available from Beckman Instruments, Inc., U.S. Highway 22 at Summit Road, Mountainside, New Jersey 07091.

4. Available from Process Materials Corporation. Cost approximately $1.95 per 4 oz. bottle. There is no literature or data sheet provided with the Harleco solution. A brief description of its use, including a color scale, is attached to the bottle.

5. Falconer, W.E., King, A., and Pelikan, A., unpublished results.