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Off-gassing Woodblock Prints-Storage Impact Considerations and Mitigation Strategies -

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT The storage environment of a Japanese woodblock print collection was assessed for organic acids. The active air sampling method was used to collect organic acid emissions in the low microgram range from areas of a selected woodblock print with different pigments, following which an off-gassing mitigation strategy based on the fan filter unit(FFU) system was investigated. Research findings revealed that the off-gassing behavior of woodblock prints is significantly impacted by storage practices and to a lesser degree by the pigments. The FFU system can be used as a mitigation strategy, but the permanence of the results depends on the storage conditions.


INTRODUCTION
Organic acids-particularly acetic acid and formic acidare known to be released as degradation products from paper and wood materials, and they are often found in comparatively higher quantities inside libraries (Gibson et al., 2012;Mašková et al., 2017). When no mitigation strategy is employed, the released gases may accumulate, especially in enclosed spaces, and eventually attack collection materials (Tétreault et al., 1998;Tétreault et al., 2003;Brokerhof and Van Bommel, 1996;Gibson and Watt, 2010;Oikawa et al., 2006;Malagodi et al., 2017). To minimize potential damage, many museum institutions have issued guidelines to monitor organic acids and other volatile organic compound (VOC) emissions. The Tokyo National Museum(TNM), for instance, recommends the maintenance of acetic acid and formic acid concentrations below 123 µg/m³ and 19 µg/m³ indoors, respectively.
Being common gases released from paper materials, organic acids tend to accumulate inside paper-made storage boxes, especially if the boxes are used to store paper-based collections. As such, woodblock prints, which are typically stored in wooden or cardboard boxes and sandwiched in Japanese traditional paper folders, are likely to be exposed to organic acid-concentrated microclimates. It is therefore important to evaluate the storage conditions in terms of
In this paper, we present the results of an investigation on the storage conditions of a Meiji period  woodblock print collection in terms of the concentration of gaseous organic acids. We also assess the print's off-gassing behavior in different colored areas, which were characterized by an X-ray fluorescence(XRF) analyzer. In addition, we describe an organic acid rinse experiment performed on a selected woodblock print using the fan filter unit(FFU) system. The FFU system had been used in a previous experiment to successfully reduce organic acid off-gassing in Japanese folding screens by 85% (Matsui et al., 2018). In this experiment, the results obtained from gas rinsing were evaluated after a 20-25 day follow-up period.

MATERIALS: WOODBLOCK PRINTS
The woodblock prints studied in this research belong to a three-print set titled "Yōjo Reishiki Kyōiku-no-zu", which was attributed to Toyohara Chikanobu   and red), were selected for the off-gassing measurements.
The location of the sampling points is shown in Figure 1.
The color materials of the selected sampling points were characterized using a Brucker Tracer III-SD handheld X-ray fluorescence(XRF)(Tracer III-SD, Brucker, USA) analyzer equipped with a rhodium target X-ray tube set at 40 kV and 11 µA. Each sample was scanned for 60 seconds.

Measuring organic acids
Measurement of organic acid content(acetic acid and formic acid) was performed via active air sampling using Kitagawa™ precision gas no. 910 detector tubes. These detector tubes exhibit a detection range of 5 µg/m³ and 400 µg/m³ for acetic acid, and 5 µg/m³ and 800 µg/m³ for formic acid, and are equipped with a measuring scale printed alongside the tubes that is calibrated to acetic acid concentrations at 20°C. Measurements recorded under different ambient air temperature conditions required the application of temperature corrections to the obtained readings, based on the temperature reference chart provided by the manufacturer. To perform the measurements, the detector tubes were mounted on an ASP-1200 air sampling pump. As the tubes were designed to collect air at a flow rate of 200 ml/min for 60 min, the pump settings were adjusted to match the tubes' specifications, and a total amount of 12 L of air was sampled for each measurement.
Off-gassing measurement in the prints was conducted at the sampling points(M-01 to M-06) as previously described.
To sample the print, an apparatus composed of two acrylic sheets(3 mm thickness), with 3.5 mm diameter holes for sampling and a metal rack support, was constructed as illustrated in Figure 2. This allowed for a measuring surface area of 7 mm 2 . The print was adjusted between the acrylic sheets so that the area to be sampled interfaced with the sampling hole, where the tip of the detector tube was inserted.
The measurement was performed through the rear side of the print.
In addition to measuring off-gassing from the prints, the wooden box(in closed state) and the hōsho paper folder were Figure 2. Design and outlook of apparatus constructed for the measurement.
Off-gassing Woodblock Prints / Ana Teresa Guimaraes Romero, Toshiya Matsui, Eriko Nagahama | 31 sampled for volatile organic acid content, along with the air in the storage room( Figure 3). Measurements in the wooden box and paper folder were not conducted in situ; instead, the materials were taken to another room with a low organic acid concentration for sample collection(blank = 20±3 µg/m 3 ).

Rinsing organic acids: experimental design
The following experiment plan was designed to evaluate the influence of storage environment on organic acid off-gassing and assess the short-time impact of using the FFU system as a mitigation strategy. Data collection was conducted in three steps: during preliminary measurement (before rinse experiment), after Phase 1 of the experiment, and after Phase 2 of the experiment. For this experiment, the right-side woodblock print was used, and its four sampling points(M-01, M-02, M-05, M-06) were measured for changes in organic acid content before the experiment, after Phase 1, and after Phase 2.
The rinse experiment involved two Phases. In Phase 1, the organic acid content was rinsed off of the woodblock print, following which the print was returned to its original storage place, where it remained for 20 d, after which organic acid re-emission was measured. In Phase 2, executed after Phase 1 was concluded, the woodblock print was rinsed again; this time, it was subsequently stored in a different storage place, i.e., inside a closed polypropylene container with an Art-Sorb® silica cassette for humidity control, in a room with low organic acid content in air(<22 µg/m³) for 25 d. Figure   4 shows the procedure followed in the experiment.
The organic acid rinse procedure was conducted via an

Pigment identification
The elements detected by XRF scanning and subsequent identification of pigments are listed in Table 1 The black pigment is most likely made of organic material.

Preliminary measurement of organic acids
Organic acids measured in the storage room were found to be below the TPM's recommended limit, which is   Despite the optimal values in the storage room, we found a significantly higher concentration of organic acids inside the wooden box: approximately 3.8 times higher than the concentration in the storage room. The concentration inside the folded hōsho(measured at the fold) was less than half of the concentration inside the box(refer to Table 2 for results regarding storage materials). The listed values were all corrected for temperature to facilitate data assessment.
Temperature and relative humidity conditions during sampling are also listed for reference.

Measurement of organic acids in Phase 1 and
Phase 2 Organic acid re-emission was observed after the follow-up periods in both phases of the experiment, as shown in Figure   7. The lowest off-gassing values corresponded to Phase 2 of the experiment as expected, although Phase 1 results also showed that mitigated emissions were maintained in the woodblock print. Nevertheless, compared to storage in a clean environment, the original wooden box storage condition seems to increase off-gassing from the woodblock print by 32±19%.

Storage environment
A somewhat organic acid-concentrated microclimate was found inside the wooden box that the ventilation system of the University of Tsukuba Library did not reach. It is known that wooden boxes historically used in document storage tend to emit moderate to high levels of organic acids, particularly acetic acid (Gibson and Watt, 2010), although the wood species more commonly used in Japanese storage materials (paulownia and Japanese cedar) have been noted to emit comparatively lower rates of acetic acid (Oikawa et al., 2005). Comparing the preliminary measurement values with the Phase 1 and Phase 2 results, it seems clear that the main source of organic acid off-gassing in the right-side woodblock print is not endogenous but exogenous, i.e., the storage environment provided by the wooden box. The contrastingly low off-gassing levels of the middle print though suggest that the paper folder and the prints sandwiching it provide some level or protection by serving as obstacles to the migration of VOCs emitted from the box.
It has been suggested in published research that acidic volatile compounds can migrate between stacked sheets of paper but tend to accumulate and become trapped in the sheets that are further away from "escape areas"-for instance, released organic acids from cellulose degradation would accumulate at the bottom and middle sheets of a stack, the upper part of which is open to the atmosphere (Carter et al., 2000). In this case, we suspect that the organic acids released from the wood box accumulate on the paper folder and the first print, and the remaining organic acids that managed to migrate through these obstacles may eventually become trapped in the paper folders that are stacked in the middle.

Influence of the color materials and degradation in off-gassing behavior
The VOC migration theory mentioned earlier might also apply to the relative location of the sampling point measured in the print, as the off-gassing values obtained from the right-side print were not uniform. In particular, sampling  which is believed to contain the same pigments. It is possible that there is an additional endogenous source of off-gassing in the area surrounding sampling point M-06, such as an ongoing chemical reaction that is releasing organic acids as a by-product.

FFU system as a mitigation strategy
The FFU system can be used as an innocuous method to reduce VOC emissions from paper and wood materials. of molecular sieves to organic acid adsorption, but Grøntoft et al.(2015) found that activated carbon cloth could be used to significantly reduce organic acid concentrations inside showcases. The caveat here is the potential corrosive properties of the chloride present in activated carbon cloth; therefore, the application of these materials in cultural heritage pieces of art should be carefully considered.

CONCLUSION
We were able to ascertain organic acid emission values from different areas of a woodblock print and observe that the storage environment and storage practices(particularly the stacking order) seemed to significantly influence on the off-gassing behavior of these artifacts. The slight variation in acidic gases emitted from different areas of the print can be explained by differing VOC adsorption and retention properties of the color materials and possibly ongoing degradation processes as well. The FFU system was found to be an effective mitigation strategy for quick and temporary solutions; however, for long-term preservation, other methods of combating re-emission and storage microclimates should be considered. where the FFU is equipped with filters only targeting acidic gases, which could allow for non-acidic VOCs to be retained in the rinsed object as an alkali reserve.