The Structural Conservation of Panel Paintings

tions in this example, the gesso layer will yield with a drop in RH from

70% to 64%, and the paint will yield when the RH drops to 60%. Crushing

or cleavage of the design layer could occur at about 35% RH if the gesso

ground is not sound. A panel equilibrated to a high level of RH will suffer

some permanent deformation if subjected to the well-controlled environ-

ments found in many institutions. In addition, a smaller increase in RH,

(6–8%), is needed to cause tensile yielding when compared to a panel

equilibrated to 50% RH.

How realistic is the example above? At such a high RH level, there

is a strong potential for biological attack that should be observed and

noted. For a panel’s RH to equilibrate to a high annual mean, RH levels

during the more humid periods of the year must also be high. Evidence of

536 Richard, Mecklenburg, and Tumosa

0 20 40 60 8 0 100

RH (%)

St

rai

ni

ng

es

so

an

do

ilp

ain

tla

ye

rs

Longi tudinal

direction

Equil ib ri um

environ ment

Tangent ia l directio n

1 0.00 4

2 0. 00 4

2 0. 00 25

1 0. 00 25

Cleavageof

designla ye r

Cra cking of

design lay er

0 .01 6

0 .01 4

0.01 2

0.01 0

0 .00 8

0.00 6

0.00 4

0.00 2

0 .0 00

2 0.00 2

2 0.00 4

2 0.00 6

2 0.00 8

2 0.01 0

2 0.01 2

2 0.01 4

2 0.01 6

0 20 40 60 80 100

RH(%)

St

rai

ni

ng

es

so

an

do

ilp

ain

tla

ye

rs

Longitu din al

dir ecti on

Equil ibriu m

envi ro nment

Tangentia l direc tion

1 0. 004

2 0.004

2 0.002 5

1 0.002 5

Cl eavage of

design laye r

Cr acking of

design la ye r

0.016

0 .014

0.012

0.010

0.008

0 .006

0.004

0.002

0 .000

2 0.002

2 0.004

2 0.006

2 0.008

2 0.010

2 0.012

2 0.014

2 0.016

Figure 10
Calculated strains in gesso and flake white oil
paint applied to an unrestrained, tangentially
cut white oak panel versus RH. The panel
painting is assumed to be equilibrated to 64%
RH. The paint still has a fairly large allowable
RH fluctuation, even on the tangentially cut
wood, but the gesso is now confined to a
more restricted RH range.

0 20 40 60 80 100

RH (%)

Str

ain

in

ge

ss

oa

nd

oil

pa

int

lay

ers

Lon git ud inal

direct ion

Equ ilib ri um

envir on men t

Ta ngen tia l dire cti on

1 0.0 04

2 0. 00 4

2 0. 00 25

1 0.0 025

Cle ava ge of

desig n laye r

Cra ckingof

desig n layer

0.0 16

0.0 14

0.0 12

0.0 10

0.0 08

0.0 06

0.0 04

0.0 02

0.0 00

2 0 .0 02

2 0 .0 04

2 0 .0 06

2 0 .0 08

2 0 .0 10

2 0 .0 12

2 0 .0 14

2 0 .0 16

Figure 11
Calculated strains in gesso and flake white oil
paint applied to an unrestrained, tangentially
cut white oak panel versus RH. The panel
painting is assumed to be equilibrated to
70%RH. Both the gesso and paint are now
confined to a very restricted RH range in the
tangential direction. This painting would be at
serious risk if subjected to low RH levels.