G. Cadelano, P. Bison, A. Bortolin, G. Ferrarini, M.Girotto, F. Peron, M. Volinia

Conservation of historical frescoes by timed infrared imaging analysis

Istituto Tecnologie della CostruzioneConsiglio Nazionale delle Ricerche

Università IUAVdi Venezia

AITA 2013

12th International Workshop on Advanced Infrared Technology

and Applications

Politecnicodi Torino

Introduction | Case study | Experimental procedure| Data collection | Conclusions

Water presence is considered the primary threat to the cultural heritage conservation.

The salt efflorescence/subflorescence is the effect of the crystallization of solvated salt occourring after the evaporation of the solvent (water).

This phenomenon is associated with the presence of recurrent cycles of wet and dry inside the walls.(=solvatation and crystallization)

water damage to mural paintings

Introduction | Case study | Experimental procedure| Data collection | Conclusions

The conservation of cultural heritage requires the monitoring of the phenomena involved in the process of decay of artworks.

In most circumstances quantitative local methods are not appropriate, and often are prohibited, to be applied to cultural heritages due to their invasive or destructive characteristics.

Literature shows that quantitative thermography is an outstanding instrument for accurate thermal analyses of buildings, also in historical architectures, in order to monitor indoor and outdoor surfaces and to evaluate the distribution of moisture.

This work represents an attempt to combine some data acquisition and signal processing methods typical of the NDE applications with an advanced infrared acquisition system.

IR thermography and wall moisture

Introduction | Case study | Experimental procedure| Data collection | Conclusions

St. Eldrad’s chapel is an isolated structure of the Abbey of Novalesa complex, located about 60 Km west of Turin, Italy.The building measures 8 x 5.8 meters inside and dates back to the 11th century and is preceded by an atrium built in the second half of the 17th century.

St. Eldrad’s chapel

XI secXVII sec

The internal walls and ceiling are completely decorated with fresco paintings that date back to the second half of the 11th century, depict the iconographic cycles of St. Nicholas and St. Eldrad.

Introduction | Case study | Experimental procedure| Data collection | Conclusions

No conservation problems for many centuries, until recent times.

Serious damages started to be evident soon after restoration works, which included:

• the removal of the outer plaster layer;

• the closure of a window in the South side;

• a trench was escavated along the northern external wall.

degraded wall paintings

NORTHSOUTH

Introduction | Case study | Experimental procedure| Data collection | Conclusions

The stimulus in this active thermography application is the solar radiation that can be considered as a periodic stimulus with period equal to one day. The corresponding physical phenomenon is a thermal wave that propagates from the outer surface of the building toward the inside of the chapel.

Observing the temperature of the wall from inside, assuming evenly space distributed solar radiation, and wall thickness and stratigraphy as homogeneous, the water content of the wall is expected to have a main role in eventual thermal anomalies.

active thermography technique

day 11

day 10

day 9

day 8day 7

day 6

day 5

day 4

day 3day 2

day 1

North

South

East

West

T [°C]

t

Indoor air temperature

Introduction | Case study | Experimental procedure| Data collection | Conclusions

The inner surface temperature oscillates with the same period of the heat flux. The amplitude of the oscillation depends on the thermophysical properties of the wall; its phase is delayed by wall thickness and characteristics.

T [°C]

tday 10 day 11

detrend(Tblue,Tgreen)

active thermography technique

0 24 48

Introduction | Case study | Experimental procedure| Data collection | Conclusions

dry vs wet

material λ [W m-1 K-1] ρ [Kg m-3] Cp [J Kg-1 K-1]

Clay bricks 0.5 1800 840

Marble 2.8 2600 800

Pietra Serena 2.45 2540 811

Concrete 1.0 2100 880

Water 0.6 1000 4182

wet

wetwet C)(

total

void

V

Vf

0 24 48

0 24 48

24 hrs period

t

t

t

Wet wall

Heat flux

IR camera

)cos( t

OHdrywet f2

OHdrywet CfCC2

)()()(

drydrydry C,,Dry wall

T

T

Thermal diffusivity

Penetration depth

Phase shift

OUTSIDE INSIDE

Introduction | Case study | Experimental procedure| Data collection | Conclusions

aIRview is a robotic monitoring system that consists of optical sensors for the detection of heat flows and special sights designed to be used in the infrared range.

aIRview can be used to automatically acquire and process thermal images of wide areas in short time, with 1 pixel=1 cm resolution.

Geometrically correct reconstruction of surface temperatures is obtained through the correction of optical distortions and automatic mosaiking process.

robotized thermographic system

Introduction | Case study | Experimental procedure| Data collection | Conclusions

aIRview can be remotely programmed and controlled.

The processed data are automatically uploaded in the internet and are downloadable by the users as ready to use results in real time through the aIRview website.

aIRview has been used to monitor three inside walls of St. Eldrad’s chapel (South, West and North), every 15 minutes for 11 days.

remote monitoring

signal processingTEM

PER

ATU

RE [

°C]

NORTHSOUTH WEST

Introduction | Case study | Experimental procedure| Data collection | Conclusions

The algorithm applied to the sequences of thermal images has been a Fast Fourier Transform. Analyzing the function from the time domain, shown in red, to the frequency domain, in blue,the component frequencies, spread across the frequency spectrum, are represented as peaks in the frequency domain.

Introduction | Case study | Experimental procedure| Data collection | Conclusions

image processing resultsNORTHWESTSOUTH

PH

ASE

AM

PLI

TU

DE

VIS

IBLE

Introduction | Case study | Experimental procedure| Data collection | Conclusions

South wall results

VISIBLEPHASE

Introduction | Case study | Experimental procedure| Data collection | Conclusions

The chosen case study has been particularly challenging due to its peculiar context that required special hardware.

Analyzing the damaged South wall with FFT amplitude and phase, has been possible to attempt to explain the decay mechanism related to moisture presence, as effect of wet/dry cycles that cause alternate salt solubility and crystallization.

The particular shape of the boundaries of the deteriorated areas and the strong presence of salt efflorescence is conducible to wall moisture phenomena, such as the capillary action from the ground and the rain infiltrations.

goals

Introduction | Case study | Experimental procedure| Data collection | Conclusions

Seasonal measurements (changes in the pattern of moist areas, test repeatability)

Comparison with software modeling (input data)

Microclimate analysis (moisture contribution from tourists and air humidity variations, air flows)

extras

Thank you for your attention

contact: gianluca.cadelano@itc.cnr.it

Novalesa

Abbey