Microclimate monitoring of Ariadne's house (Pompeii, Italy) for preventive conservation of fresco paintings

Abstract

Background: Ariadne's house, located at the city center of ancient Pompeii, is of great archaeological value due to the fresco paintings decorating several rooms. In order to assess the risks for long-term conservation affecting the valuable mural paintings, 26 temperature data-loggers and 26 relative humidity data-loggers were located in four rooms of the house for the monitoring of ambient conditions.

Results: Data recorded during 372 days were analyzed by means of graphical descriptive methods and analysis of variance (ANOVA). Results revealed an effect of the roof type and number of walls of the room. Excessive temperatures were observed during the summer in rooms covered with transparent roofs, and corrective actions were taken. Moreover, higher humidity values were recorded by sensors on the floor level.

Conclusions: The present work provides guidelines about the type, number, calibration and position of thermohygrometric sensors recommended for the microclimate monitoring of mural paintings in outdoor or semi-confined environments.

Figure 1

Plan of Ariadne’s house. Lodgings marked as 1 to 4 are the only roofed ones (parallel tilted lines delimit the covered area). Data-logger #1 was located on the top of an outside wall next to room 3 at 3 m from the ground level and it was covered with a ceramic tile.

Figure 2

Calibration of RH and temperature sensors. Difference between RH and temperature recorded by each probe with respect to the average from all probes in two different calibration periods: first, from 18^th to 21^st July 2008 (a: RH; b: temperature) or second, from July 31^st to August 28^th 2009 (c: RH, d: temperature). Trajectories were smoothed using a moving average with a window size of one day.

Figure 3

Smoothed time series of RH and temperature. Average RH (a) and temperature (b) recorded by all probes (except #1) during one year (day 0 corresponds to July 23^rd 2008). The time series was smoothed using a moving average with a window size of one day. Horizontal lines in red correspond to the ranges recommended by the standard DM 10/2001 for the conservation of mural paintings.

Figure 4

Trajectories of RH (a) and temperature (b) recorded by all probes during three summer days (23^rdto 25^thJuly 2008). Vertical lines correspond to midnight (0:00 hr).

Figure 5

Temperatures in room 4. Temperatures recorded by data-loggers in room 4 (#2, #3 and #4) during four summer days (4^th to 8^th August 2008). Values of #1 are also shown for comparison.

Figure 6

Location of probes #2, #3 and #4 in room 4. The picture was taken at 1:05 PM. The brighter zone at the lower right corner corresponds to sunlight incident on the frescoes that affects #3 from 2:00 PM to 8:00 PM.

Figure 7

Maximum daily differences of temperature between data-loggers #3 and #4. Calculated for the 372 days of the period under study (day 0 corresponds to July 23^rd 2008).

Figure 8

Abnormal peaks of temperature recorded in room 4 and 3. Temperatures recorded by probes in room 4 (a) and five probes in room 3 (b) during four winter days (19^th to 23^rd February 2009). Values from #1 are also included for comparison.

Figure 9

Mean daily trajectories of temperature and RH in summer. a) Temperature at rooms 1 and 3; b) temperature at rooms 2 and 4; c) RH at rooms 1 and 3; d) RH at rooms 2 and 4. Summer period: 7/23/2008 to 9/20/2008. Color codes indicate the location of probes: room 1 (green), room 2 (blue), room 3 (violet), room 4 (red). The thicker black line corresponds to the average time series of all probes.

Figure 10

ANOVA results (room 2). ANOVA results showing the differences among probes in room 2 on the daily mean RH recorded in summer 2008. For each data-logger, the average and 95% LSD interval is depicted. Codes indicate the wall orientation where each sensor is facing (N: north; S: south, E: east, W: west) and the height: f (floor level), u (upper) or i (intermediate).

Figure 11

ANOVA results (rooms 1 and 3). ANOVA results (average and 95% LSD intervals) showing the differences among probes in room 1 (a) and 3 (b) on the daily minimum RH recorded in summer 2008. The codes indicate the wall orientation (N: north, S: south, E: east, W: west, and M: mosaic) and the height: f (floor level), i (intermediate) or u (upper).

Figure 12

Mural frescoes in Ariadne’s house (western wall of room 1). Three probes can be observed: #22, #25 and #26, which remain at 240, 54 and 15 cm from the floor, respectively.

Figure 13

Evolution along the year of daily mean parameters: temperature, AH and RH. Data recorded by all probes during the daytime (8:00 AM to 8:00 PM) and night-time (8:00 PM to 8:00 AM). Day 0 corresponds to 24^th July 2008 (days <60: summer; 60–151: autumn; 152–239: winter; 240–331: spring). Trajectories were smoothed using a moving average with a weekly window size.

Figure 14

Bivariate plot of daily average RH vs. temperature. a) Bivariate plot of daily average RH from each probe with respect to daily average temperature, corresponding to the year 2008–2009 (372 days). b) Theoretical function relating RH and temperature (Equation (2)), which is also depicted inside the bivariate plot.

Figure 15

Bivariate plot of daily minimum RH vs. temperature. Bivariate plot of daily minimum RH from each probe vs. daily minimum temperature, averaged for all the 372 days. Horizontal lines in red (45–60%) correspond to the range of RH recommended by the standard DM 10/2001 for the conservation of mural paintings.

Figure 16

Bivariate plots (summer vs. winter). a) Bivariate plot of daily maximum temperature (T_max) averaged for the summer period (7/24/2008 – 9/22/2008) vs. T_max averaged for the winter period (12/21/2008 – 3/31/2009). b) Bivariate plot of daily minimum RH (RH_min) in summer vs. RH_min in winter. Axes were inverted in the right plot to ease the comparison. Lines in red (45–60%) correspond to the range of RH recommended by the standard DM 10/2001.

Figure 17

Position of thermohygrometric probes. Room code as in Figure 1. Probe #5 was installed at the center of room 3 under a glass box on the ground protecting a mosaic. Probes #6, #8 and #11 were located on the floor level under a ceramic tile that was placed to protect them from accidental stepping.

Figure 18

Daily time series of probe #22. Temperature and RH values registered by one specific probe (#22) during a day randomly chosen (February 16^th 2009) as well as average values collected by the 26 probes. One measurement was recorded every 30 min. According to the manufacturer, the accuracy is ±0.5°C and ±5% for temperature and RH data-loggers, respectively.