Historical papers vary widely in their apparent stability. Knowledge of elemental concentrations can sometimes explain why some papers are found in browned and brittle condition and others are not. High concentrations of K, Al and S in such papers, for instance, are likely associated with alum; an acidic compound. Papers still durable and light in color may contain high levels of Ca; likely in association with calcium carbonate or other compounds known to act as an alkaline reserve in paper, helping them maintain an alkaline pH over time. Determination of these elemental concentrations in artifacts on paper can help the preservation specialist make more informed decisions about the efficacy of various aqueous intervention treatments or storage protocol options.
XRF is an especially effective non-destructive technique for elemental analysis where the density of elements of interest is high, as in the analysis of modern metals. XRF analysis of historical paper, however, is made difficult by its minimal thickness, lack of density, and usually low concentrations of key elements of interest; Al, K, S, Mg, Fe, Cu and Ca. Quantitative analysis is made more difficult by the wide variation in thickness and density of papers made in Europe between 1300 and 1800. A technique was developed to correct for the variation in paper thickness and density by placing a thin film impregnated with Cr and Br behind the paper specimen. Variations in Cr and Br signal strength and backscatter in a selected region of the spectrum allowed us to generate a ‘thickness/density’ value that became part of the final calibration. A special acrylic accessory was constructed and mounted to a Bruker TRACeR III-V handheld instrument to position the Cr/Br thin film in the same location behind paper specimens during calibration and analysis of unknowns.
40 historical paper specimens were used in the calibration set; 36 to establish the calibration, and 4 to validate the accuracy of the final calibration. Samples from 10 spots were taken from each specimen and combined into one sample for destructive inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis.. The ICP-OES data for each specimen therefore represent an average result for the 10 spots. XRF analyses near each of the 10 spots used for the ICP-OES sampling were collected and then summed before being used in the calibration. We assume this summed scan corresponds to the average result obtained through ICP-OES.
Using this technique, our calibration allows us to estimate ppm concentrations of K, S, Ca, and Fe with a precision of +/-20% at a 90% confidence level when concentrations are close to 1000ppm. Precision improves at higher concentrations, and is poorer at lower concentrations. Compositional variation must also be considered. For instance, precision is generally better with Ca, due to its high concentration. Fe was present in the calibration specimens in a narrower range making the accuracy outside of this range poorer. The precision for S is not as good as for K due to decreased instrument sensitivity for lighter elements. However, we believe this level of precision is adequate for documenting trends in the concentrations of alum (potassium aluminum sulfate or aluminum sulfate), Ca compounds, and Fe in hundreds of specimens over the centuries, or for documenting the average concentrations in large collections or collection subsets.
Our parameters for precision result from a review of repeated measurements made on 4 samples held back from the calibration. The precision reported with the results is the propagated variance and agrees with the direct measurements. The accuracy of the results on the 4 samples not used in the calibration, for which we had ICP-OES results, showed variations distributed fairly evenly above and below the ICP-OES values indicating that the accuracy was much better than the precision.
We are currently evaluating results from a new experiment designed to determine if this technique can be used to monitor changes in single artifacts during aqueous treatment. The precision may improve over what we obtained when analyzing widely differing unknowns because single artifacts provide the same or a very similar paper thickness/density from analysis to analysis. In the meantime, we believe the method is useful at its present level of development for collections surveys. Again, such data can provide valuable information to preservation specialists who make collections care and treatment decisions.