6th Indoor Air Quality 2004 Meeting (IAQ2004)
Padova, Italy, 10-12 November 2004
Impact of daily and seasonal Temperature and Relative Humidity
cycles on wooden artworks
Dario Camuffo, Emanuela Pagan
National Research Council (CNR)
Institute of Atmospheric Sciences and Climate
Padova, Italy
EMC in wood vs. temperature & RHEMC in wood vs. temperature & RH
Wood is strongly dependent on RH, weakly on TRH changes are dangerous, but T may change RHTwo regions are visible: 0<RH<80% & 80<RH<100%
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70 80 90 100
Relative Humidity (%)
T = 0°C
T= 40°C
T = 20°C
Eq
uil
ibri
um
Moi
stu
re C
onte
nt
(%)
Relative Humidity (%)
In conservation, the specific problems of each individual object are much more relevant than knowing average properties of materials and establishing hypothetical well-being areas.
-4
-3
-2
-1
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70 80 90 100
Relative Humidity (%)
Oak tang
Pine tang
Pine rad
Oak rad
Dim
ensi
onal
Cha
nge
(%)
Relative Humidity (%)
A relevant class of hygroscopic materials, e.g. wood, parchment, ivory, has a Moisture Content that is in equilibrium with RH. Changes in RH affect MC and generate dangerous shrinkage/swelling.
•Cycles in T and RH are responsible for dimensional changes and internal tension to the wooden coffered ceiling.
•In the long-run, the tension may have a cumulative effect, or even in some occasions it may exceed the threshold after which some structural part break.
•This is not only a consequence of the width of the cycles, but also of the synergism between cycles, if these are repeated before wood has relaxed.
•A fundamental role is played by the frequency of the largest cycles, supposing that the rare extremes which had occurred in the past have not yet concluded their action to adapt the material (i.e. to break it) to respond to their intensity or repetition.
Effect of ‘cold’ lamps and hot spotlights on the historical coffered ceiling of the Giant Hall, Padova
Radiometric measurement of
the ceilingbefore a concert,
showing the effect of cold lamps only
After the concert: the contribution of
hot spotlights is evident
Trend of Equilibrium Moisture Content (EMC) of the coffered ceiling in the cold season
Padova, Giants Hall. In the cold season, the EMC changed from 13 to 4%, with some fluctuations. The seasonal trend had superimposed some minor cycles with quasi-monthly average period, and average amplitude around 4%.
The coffered ceiling is composed of square oak panels, 2m side, thickness 1-2 cm. 9% change in EMC implies a dimensional change by 3%, i.e. 6 cm in the direction tangential to the tree rings.
Instantaneous Tangential Deformation (TD) representative of the surface layer of the coffered ceiling (pine) and 7- and 14-day
running average representative of two deeper layers Padova, Giants Hall
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
02-ott 01-nov 01-dic 31-dic 30-gen 01-mar 31-mar
Defo
rmazi
on
e T
an
g.
del le
gn
o (
%)
Deformazione Tangenziale dal valore medio
Media Mobile su 7 giorni
Media Mobile su 14 giorni
Tan
gent
ial D
efor
mat
ion
(%) dimensional change
by 3% in tangential direction
3%
Date
-1.5
-1
-0.5
0
0.5
1
1.5
2
13-ott 12-nov 12-dic 11-gen 10-feb 12-mar
Dif
fere
nza
di d
efo
rma
zio
ne
Su
pe
rf. -
Inte
rno
(%
)
deformazione tang.(ist)-media7
media7(deformazione tang.(ist)-media7)
deformazione tang.(ist)-media14
media14(deformazione tang.(ist)-media14)
Surface Stress = Surface Tangential Deformation (TD) – Inner layer (7- and 14-day running average)
Padova, Giants Hall
Sur
face
Str
ess
(%)
Scatter diagram
•Values falling outside this interval might be considered in a risky area. The practical “safety area” was based on laboratory tests on material samples, and was represented with ‘well being’ rectangles as in this example. However, this approach does not consider that wooden artworks may adapt to the ambient variability with cracks. Every new crack constitutes an adaptation to a wider environmental variability. Cumulated damage displaces thresholds for unsustainable variability.
Scatter diagrams represent the history of the past T and RH cycles that have interacted with the artwork. The interval of safe variability in T and RH lies between 0 and a critical threshold in T and RH.
safety area
T(°C/day)
R
H(%
/day
)
Yearly daily cycles Padova, Giants Hall, 1 m
Most of the points lie outside the safety area and this explains why so many cracks were found in the coffered ceiling.
The interval 6% for RH and 1.5°C for T determined from laboratory tests can doubled after field survey.
0
5
10
15
20
25
30
35
40
45
50
0 1 2 3 4 5 6 7 8
T(°C)
RH
(%)
In fact, these values are close to the modes of the observed data and have been experienced many times. New cracks have damaged the ceiling, widening the area of environmental variability
T(°C/day)
R
H(%
/day
)
The histograms show the frequency of distribution of the daily variability either in T, or in RH, grouped by classes of intervals.
The mode M represents the most frequent cycle to which artefacts have adapted (with cracks).
The upper tails UT (i.e. right side) in plots are constituted by rare and risky departures from the typical values. These have not yet concluded their potential impact, and should be carefully avoided.
Yearly frequency of T and RH daily cycles Padova, Giants Hall
T
RH
M
M1
UT
UT
M2
Uffizi Gallery,Florence
30
35
40
45
50
55
60
65
70
5-1-980:00
5-1-9812:00
6-1-980:00
6-1-9812:00
7-1-980:00
7-1-9812:00
8-1-980:00
8-1-9812:00
9-1-980:00
14
16
18
20
22
24
26
28
RH(1m)
RH(3m)
T(1m)
T(3m)
10
20
30
40
50
60
70
80
90
28-1-980:00
28-1-9812:00
29-1-980:00
29-1-9812:00
30-1-980:00
30-1-9812:00
31-1-980:00
31-1-9812:00
1-2-980:00
12
14
16
18
20
22
24
26
28RH(1m) RH(3m)
T(1m) T(3m)
Heating with exceeding moisture compensation
The best situation is found during the closure days when the HVAC is off
Heating without moisture compensation
Yearly frequency of T and RH daily cycles (1m height) Uffizi Gallery
0
2
4
6
8
10
12
14
16
18
2.1 2.7 3.3 3.9 4.5 5.1 5.7 6.3 6.9 7.5 8.1 8.7 9.3 9.9 10.5
Temperature (°C)
Fre
qu
ency
dai
ly c
ycle
s
0
5
10
15
20
25
30
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Relative Humidity (%)
Fre
qu
ency
Dai
ly C
ycle
s
T
RHM
UT
M
UT
0
5
10
15
20
25
30
35
40
45
50
0 1 2 3 4 5 6 7 8 9 10 11
T(°C)
RH
(%)
T(°C/day)
R
H(%
/day
)
0
5
10
15
20
25
2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9
Temperature (°C)
Fre
qu
ency
dai
ly c
ycle
s
Yearly frequency of T and RH daily cycles at 4.5m height near the altar
The histograms show the frequency of distribution of the daily variability either in T, or in RH, grouped by classes of intervals.
0
5
10
15
20
25
30
35
40
45
50
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Relative humidity (%)
Fre
qu
ency
Dai
ly C
ycle
s
Daily cycles at the mode M are typical and the material has adapted, possibly with cracks. Cycles from M to 2M may be still sustainable, but they fall in the attention area because they are not so frequent. They might still deepen existing cracks not yet concluded and fatigue may be accumulated for new ones.
T
RHM1
M1
M2
M2
In the case that past cycles were not sustainable, they caused some cracks in the critical constraints to adapt the artwork to the environmental T and RH cycles.
Cracks created new degrees of freedom to respond to the environmental variability. The sustainable T and RH span is widened at the expenses of a worsen and worsen damage to artworks.
Conclusion 1
Daily cycles at the mode M are typical and the material has adapted, possibly with cracks. Cycles from M to 2M may be still sustainable, but they fall in the attention area because they are not so frequent. They might still deepen existing cracks not yet concluded and fatigue may be accumulated for new ones.
Conclusion 2
Daily cycles greater than two times the mode are quite exceptional and may be responsible for the largest cracks that are visible, or may provoke new ones.
M
2M