Volume 37, Issue 2 -

Non-destructive analysis of 1,578 paper specimens made between the fourteenth and the nineteenth centuries was undertaken in an effort to better understand changes in paper composition over time and how these variations might affect paper stability during long term natural ageing. Gelatine content and colour were determined using UV/Vis/NIR spectrometry. Residual metals were measured using XRF. These components included potassium and sulfur as elements indicative of alum concentration; iron as a typical paper contaminant; and calcium, which is often associated with compounds such as calcium carbonate that can serve as alkaline reserves. The research demonstrated that papers that are lighter in colour (closer to white) are associated with higher levels of gelatine and calcium, and lower levels of iron. The survey also showed significant decreases in gelatine and calcium concentration over time with the largest differences coinciding with the rise of printing around 1500. The drop in pH over the centuries observed by other researchers is explained by the decreases in these two components rather than by a rise in alum concentration, which remained fairly stable. The craftsmanship of the specimens was evaluated using materials and workmanship ratings which showed better quality paper associated with higher gelatine and calcium concentrations and colour closer to white. Poorer quality papers were associated with higher iron levels and greater thickness.

Two aspects of consolidation of powdery paint layers are discussed in this paper: First, the colour stability of different consolidation media including cellulose and protein based materials was evaluated. Film sheet samples of the adhesives were exposed to a Xenontester for 276 hours to reach Blue Wool Standard 7. Light exposure caused loss of colour rather than yellowing in most adhesives which correlates to microfading tests. Secondly, colour changes of the pigment layer during application of consolidation media with an aerosol generator were examined. Any shift of colour results in a change of surface structure rather than a detectable shift seen in the ΔE* ab - values and the reflectance graphs. Thus, the application of aerosols is a very controllable, long-term stable method and results only in minor changes when applied in multiple layers.