Transcript Impact of daily and seasonal Temperature and Relative Humidity

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
Equilibrium Moisture Content (%)
EMC in wood vs. temperature & RH
40
Wood is strongly dependent on RH, weakly on T
RH changes are dangerous, but T may change RH
Two regions are visible:
0<RH<80% & 80<RH<100%
35
30
25
T = 20°C
20
T = 0°C
15
10
T= 40°C
5
0
0
10
20
30
40
50
60
70
80
90
Relative
Relative Humidity
Humidity (%)(%)
100
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.
7
Oak tang
Dimensional Change (%)
6
5
Pine tang
4
Oak rad
3
2
Pine rad
1
0
-1
-2
-3
-4
0
10
20
30
40
50
60
70
80
RelativeHumidity
Humidity(%)(%)
Relative
90
100
In conservation, the
specific problems
of each individual
object are much
more relevant than
knowing average
properties of
materials and
establishing
hypothetical wellbeing areas.
•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 ceiling
before 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
(%)
del legno(%)
Deformazione
Deformation
TangentialTang.
4
3.5
3
dimensional change by
3% in tangential direction
3%
2.5
2
1.5
1
0.5
Deformazione Tangenziale dal valore medio
0
Media Mobile su 7 giorni
-0.5
Media Mobile su 14 giorni
-1
02-ott
01-nov
01-dic
31-dic
Date
30-gen
01-mar
31-mar
Surface Stress = Surface Tangential Deformation (TD) – Inner
layer (7- and 14-day running average)
Padova, Giants Hall
Differenza diSurface
deformazione
Superf.
(%)- Interno (%)
Stress
2
deformazione tang.(ist)-media7
media7(deformazione tang.(ist)-media7)
1.5
deformazione tang.(ist)-media14
media14(deformazione tang.(ist)-media14)
1
0.5
0
-0.5
-1
-1.5
13-ott
12-nov
12-dic
11-gen
10-feb
12-mar
Scatter diagram
safety area
RH(%/day)
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.
T(°C/day)
•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.
Yearly daily cycles
Padova, Giants Hall, 1 m
RH(%/day)
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.
T(°C/day)
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
Yearly frequency of T and RH daily cycles
Padova, Giants Hall
M2
M1
T
UT
M
RH
UT
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.
Uffizi Gallery,Florence
Heating without moisture
compensation
The best situation is
found during the
closure days when the
HVAC is off
Heating with exceeding
moisture compensation
Yearly frequency of T and RH daily cycles
18
M
Frequency daily cycles
16
(1m height) Uffizi Gallery
T
14
12
10
8
6
UT
4
2
0
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
Frequency Daily Cycles
RH(%/day)
M
30
Temperature (°C)
RH
25
20
15
10
UT
5
T(°C/day)
0
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 (%)
M1
Frequency daily cycles
25
20
15
10
5
0
2.1
2.3
50
M1
45
Frequency Daily Cycles
2.5
40
35
30
25
20
15
10
5
0
0
2
4
6
Yearly frequency of T and RH daily cycles
at 4.5m height near the altar
T
The histograms show the
frequency of distribution of the
M2
daily variability either in T, or in
RH, grouped by classes of
intervals.
Daily cycles at the mode M
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)
are typical and the material
has adapted, possibly with
RH
cracks. Cycles from M to
2M may be still sustainable,
but they fall in the attention
area because they are not
M2
so frequent. They might
still deepen existing cracks
not yet concluded and
may be
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46fatigue
48 50
Relative humidity (%)
accumulated for new ones.
Conclusion 1
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 2
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.
M
2M
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.