Figure 1 -- Construction dlagram showing cross section of a painting on paperboard.
Varnish Paint Paper
Canvas Paperboard
TYPICAL DEFECTS
Rigid supports other than wood panels (discussed under the topic, Paintings on Wood, Ivory, or Bone) have unique problems.
Usually they are eonstructed of low-grade, acidic material and become dry and brittle. Their layered construction makes them
prone to delamination, especially along the edges. Because of their sensitivity to moisture, they may warp and distort. Their
paint and ground layers are prone to the usual problems of paint- ings on canvas, see Selection Criteria for Paintings on Canvas or Paper Mounted on Canvas.
BACKING BOARD
Like paintings on canvas, paintings on rigid supports should have a rigid backing board attached to the reverse of the frame.
Although the rigid support of the painting helps to guard against impact, as it becomes embrittled and dries out, the added protec-
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tion of a backing board can offer inexpensive protection. The rigid backing board prevents dirt accumulation and moderates changes of relative humidity. Thin, pressed wood, foam core pa- per sheets, or heavy laminated paperboard are used for backing boards. Hardware should be corrosion resistant materials such
as brass or chromed steel. Do not use nails or staples as they are likely to loosen and fall out.
Ascertain that there is no history of an unstable support or paint layer, such as:
¯ Detachment of paint layer (tenting paint, flaking, cup- ping, or losses of the paint layer)
Recent treatment for cleavage or flaking paint RIGID SUPPORT
There is a variety of rigid supports used for paintings. They may be reinforced fiber glass used for contemporary paintings, academy boards constructed of a layer of paper or canvas glued to an acidic wood pulp board, inexpensive plywood with water soluble glue between its laminates, pressed woodboard, or com- mon cardboard. Each of these has individual problems although many of their faults are common.
The rigid support should not be delaminating and sepa- rating between layers. With all of these materials, the edges are most vulnerable to damage from impact, abra- sion, and aging. Layered structures such as these are sen- sitive to relative humidity changes that can cause the layers to move.
There should be no unrepaired checks or splits in the support. Plywood panels will check with age showing
small cracks with raised edges which may be over the en- tire surface. Water soluble glues in the plywood may dete- riorate with age.
There must be no insecure repairs of the support. Often broken corners may be poorly reglued without reinforce- ment and are easily rebroken.
Canvas or paper attached to a rigid support assumes the character of the rigid support and may become damaged as the rigid support deteriorates. Paper glued to plywood or a wooden panel will often tear as the wood below moves in response to relative humidity changes. However, an alu-
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minum honeycomb cored support is not susceptible to di- mensional changes from temperature or relative humidity.
The support should be of sound construction without signs of insects (such as silverfish, wood worms, ter- mites), fungus activity, or dry rot.
There should be no signs of internal stress within the support such as pronounced warpage, severe checks, or splits. Care should be taken when cross-grain additions are found since they may be incompatible with the origi- nal support. Pressed wood panels are prone to warping unless both sides of the support are coated with the ground layer.
PAINT/GROUND
The painting should not have loose or tented
paint/ground or be delaminating within the paint/ground layers. Often the detached paint is hidden as a "blister"
(blind cleavage) and is not readily apparent. If delamina- tion within the paint/ground structure is suspected, have the painting examined by a conservator.
The painting should not have a history of chronic insta- bility in the support or paint layer such as recurring
paint flaking. Such chronic problems indicate an unstable structure which is more responsive to environmental
changes making the painting more vulnerable and raising loan risks.
VARNISH
Although the varnish may be discolored or flaking from the paint layer, rarely does this cause losses in the paint. Although this condition may not put the paint layer at risk, the painting should not be lent if it has flaking varnish.
SENSITIVITIES
¯ Sensitivity to Relative Humidity--Moderate range of tolerance, usually 50% or 55% +5%
Rigid supports are sensitive to relative humidity extremes which cause dimensional movement of the materials leading to warping and delamination.
¯ Sensitivity to Temperature--Low response to tempera-
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ture changes except for extremes
Paint films become more brittle at low temperatures. The re- lationship of temperature and its effect on relative humidity must be considered when selecting the packing materials. Gener-
ally, if the moisture content in the air is held constant, lowering the temperature raises the relative humidity and raising the tem- perature reduces the relative humidity.
¯ Sensitivity to Vibration--Very low response to usual transit vibrations
Recent research has shown that the vibrations normally en- countered in various transit modes does not reach the same natural harmonic as paintings on rigid supports and does not pose a threat to the paintings. Note: fragile paintings with loose paint are extremely vulnerable to any vibrations.
¯ Sensitivity to Shock--High sensitivity to shock and impact
All paintings are extremely vulnerable to the shock gener- ated by high impacts such as dropping a painting or case on its corner, especially if the temperature and humidity are low. As the condition of the painting becomes more fragile, its vulnerabil- ity becomes greater. Cracked paintings are at greater risk than uncracked paint layers and paintings with separating support layers are especially vulnerable to high shock loads.
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