The Structural Conservation of Panel Paintings

a permeability, or transmission rate, of 50 cm^3 m^22 per day per atmos-

phere. Aclar is a plastic laminate sandwiched between layers of Mylar and

polyethylene. Other plastic composite films are available with a lower oxy-

gen permeability (such as Marvelseal), but these are either very expensive,

unavailable in suitable sizes, or coated with an aluminized layer that pre-

vents visual inspection of the object inside the bag.

Bags are fabricated by heat-sealing sheets of the Aclar plastic film

(which has a heat-sealable inner coating ofpolyethylene) to create a bag or

pouch that conforms to the shape of the object (Fig. 1). The seals can be

made with a heated, handheld spatula or a clamping heat sealer. When

the painting is placed into the bag, it is recommended that some form of

spacer be used so that the bag does not rest on the painting’s surface.

As a panel painting is essentially a two-dimensional object, even if

the panel is enclosed in an integral frame, it is easy enough to construct a

simple bag or pouch that closely conforms to the shape of the painting

and is of a volume comparable to the panel painting. This results in an

efficient enclosure for the subsequent reduction of the oxygen contained

within the bag, whereas bags constructed for three-dimensional objects,

such as furniture, are often much larger than an object’s total volume.

Creating a low-oxygen environment

After the object has been encapsulated in an Aclar bag, the oxygen concen-

tration in the bag must be reduced to less than 0.1%. As air is composed of

approximately 20.9% oxygen, with the bulk of the remaining gases being

nitrogen, the amount of oxygen to be removed or replaced is approxi-

mately 20% of the total volume of the bag. To produce and maintain the

low-oxygen atmosphere, the bag is continuously purged with an inert gas

such as nitrogen or with an oxygen scavenger, such as Ageless. Based on

the studies by Rust and coworkers (1996), the authors recommend a treat-

ment time of fourteen days at an oxygen concentration of 0.1%. This pro-

vides a safety margin, as the study found seven days to be the maximum

time required to kill the most resistant species.

M A T  I I 71

Figure 1
Heat-sealing of the Aclar plastic film to
createa bag to encapsulate a panel painting
for treatment.