CHEMICAL WATERMARKING OF PAPER
8 SUMMARY OF EXPERIMENTAL RESULTS
CHEMICAL WATERMARKS are complex structures that defy simple interpretation. Their behavior is further complicated by the paper on which they are placed. However, some preliminary conclusions can be drawn.
Erasers do not appear to damage the watermarks.
Organic solvents may be dissolving various components of the chemical watermark, altering the refractive index between the paper and the watermarked area. The solvent-treated watermarks appear unaffected when viewed under ultraviolet illumination, suggesting that the particular components responsible for the ultraviolet absorption of the watermarks are not being moved. Paper additives such as dyes, intensifiers, brighteners, sizing agents, and clays, minerals, and other fillers may also dissolve in various organic solvents, especially when present in combination with the chemical watermarks.
It appears that the areas of chemical watermarks should not be treated with organic solvents. Even the laid and chain lines of a paper with a chemical watermark should be carefully examined and tested before any treatment, as it is possible such lines may also be chemically produced (Carlson 1990).5
Neutral aqueous, alkaline, and bleaching solutions dramatically change the visual and fluorescent properties of the watermarks. The chemical watermarks diffused into the papers when placed in aqueous and alkaline solutions and air dried, much like the effect seen with paper chromatography. Under ultraviolet illumination the watermarks on the side of impregnation gain an opaque white fluorescence, compared with the dark absorption seen prior to immersion. After immersion the chemical watermarks on the side opposite impregnation appeared blurred, dull, and dark under ultraviolet sources.
Bleaches and antichlors seem to alter the paper additives (such as fluorescent agents, sizes, and dyes) and the chemical watermarks. The chemical watermarks and some papers seemed to lose intensity and appeared significantly changed when viewed under short-wave ultraviolet illumination. Many marks exhibited a white opaque fluorescence on the side of impregnation, and a dull absorption on the side opposite impregnation as seen in the water-immersed samples.
These findings conflict with statements from Browning (1977) and Yankoski (n.d.). Browning claimed chemical watermarks to be “a polymerizable organic material which is not easily removed by solvents.” Yankoski cited a chemical watermark as a “translucent mark that is indelible, tamper proof and permanent.” However, Carlson (1988) stated that a chemical watermark is most definitely not indelible or tamperproof and that organic solvents would probably disrupt the marks.
Artificially aging the samples furthered understanding of the material. Humid oven aging turned the watermarks an opaque brown that was darker than the surrounding paper and diminished the translucency of the mark. The watermarks aged under dry-heat conditions diffused into the paper, yet they remained translucent. The paper also darkened significantly. The artificially aged marks appeared sharper under ultraviolet illumination than under normal illumination.
In the humid oven-aged samples, moisture or moisture in combination with heat may have cured or cross-linked a component of the watermark mixture, thus changing the color and refractive index properties of the mark. A possible explanation for the loss of clarity in the dry oven-aged watermark sample is that the lower-weight molecular components were being volatilized or melted into the surrounding paper structure.