Evaluation of Various Test Strips for the Identification of Copper Ions in Paper-Based Objects

Abstract Copper ions accelerate the degradation of organic materials; therefore, their identification in historic paper objects is essential for assessing damage risks and support conservation decision-making. In this study, three different test strips for copper ion detection were evaluated: the commercially available Cuprotesmo test, a laboratory-prepared test following Hulthe’s method (Hulthe, Peter. 1970. “A Bathocuproine reagent-paper for the Rapid Semi-quantitative Determination of Copper in the 1 to 70 P.P.M. Range.” Analyst 95 (1129): 351–5), and a test strip prepared by Han Neevel (Neevel, H. 2006. “The Development of in-situ Methods for Identification of Iron Gall Inks.” In Iron Gall Inks : On Manufacture Characterisation, Degradation and Stabilisation, edited by J. Kolar, and M. Strlič, 147–69. Ljubljana: National and University Library). The copper tests were evaluated on model papers, historical papers, and solutions containing various metal salts. With the highest limit of detection of 0.22 ± 0.05 % copper, Cuprotesmo is less suitable for determinations in historical papers, as this is comparable to typical copper levels in iron gall inks. Additionally, the test causes pronounced staining on both model and historical materials. Neevel’s tests showed lower limit of detection (0.06 ± 0.04 %) when freshly prepared but produced false-positive responses in the presence of other metal ions, limiting their suitability for routine use. Tests prepared according to Hulthe’s method demonstrated comparable sensitivity to Neevel’s test without ion interference and caused minimal discolouration. Despite a subtle colour change and the need for fresh preparation, Hulthe’s method proved the most suitable under the testing conditions for application to cultural heritage paper objects. Careful assessment of ink or pigment migration remains essential to avoid misinterpretation of the results.