Plastics 207
to create millions of microscopic pores between the carbon atoms, giving a
huge surface area density of 300–2000 m^2 per gram. Activated charcoal can
be obtained in the form of woven textile, impregnated card or paper, pellets
of various size and powder. Although cheaper, pellets and powder are likely
to adhere to tacky objects. Its high adsorption capacity for a wide variety of
atmospheric pollutants, organic acids, odours and water vapour has been applied
to both industrial and domestic situations, including absorbent mats for cooker
hoods. Activated charcoal cloth or charcoal-impregnated paper used as pack-
ing materials for cellulose nitrate objects will readily adsorb nitrogen oxide
degradation products, rendering them unable to contribute to further degrad-
ation of the plastic. When all the pores of the active charcoal are filled, no fur-
ther adsorption is possible and the adsorbent should be replaced. At the time
of writing, there has been no research into the volume of gases which can be
adsorbed by activated charcoal before exhaustion, and possible desorption
that takes place in a museum environment. As a precaution, active charcoal
packing materials should be renewed every three years.
The ability of cellulose nitrate to produce corrosive gases that subse-
quently affect metals in the surrounding area suggests that it should be stored
away from sensitive objects in museum collections. Sulfonepthalein indicators,
commonly known as Cresol red and purple, detect the presence of acidic degra-
dation products and can be used to reveal the presence of actively-deteriorating
cellulose nitrate in a mixed collection. Sulfonepthalein changes colour when
in contact with small quantities of nitrogen and sulfur oxides thus allowing
early detection of degrading cellulose nitrate. It can be used to draw an
outline on a sheet of paper around a degrading object to highlight areas
producing acidic emissions. Subsequent prompt removal of the offending
artefact helps to preserve the quality of storage for the surrounding collec-
tions. The advantage of using sulfonepthalein indicators is their high
sensitivity.
5.2 Conservation of Cellulose Acetate
Zeolites, also known as molecular sieves, are a range of hydrated silicates of cal-
cium and aluminium, and have proved effective for inhibiting the rate of deteri-
oration of cellulose acetate. The removal of water from the lattices under
intense heating produces pores of particular sizes, enabling the resulting struc-
ture to accommodate small gaseous or liquid molecules. Molecular sieve
Type 4A seen in Figure 4, has been used to retard the ‘vinegar syndrome’,
which destroys movie film and cellulose acetate objects stored in sealed
metal cans or containers, by taking up the acetic acid produced during degrad-
ation. Zeolite pellets can be enclosed in semi-permeable polyethylene sachets
before placing in the metal cans or containers. With acetic acid effectively