The Structural Conservation of Panel Paintings

supply and the bag containing the object. This allows the “dry” nitrogen

from the cylinder to be humidified to the object’s optimal RH before the

gas flows into the bag. The humidification system functions by dividing

the gas flow from the nitrogen cylinder into two valve-controlled lines.

One stream of nitrogen is bubbled through water contained in a stout

polypropylene bottle. The second stream flows directly to a second (dry)

bottle, which is also connected to the water-filled bottle. The mixing of

the dry and humidified gases is controlled by valves, which regulate the

flow rate into each bottle. To monitor the RH of the resulting combined

gas stream, a third bottle is used that contains an RH sensor and that also

acts as a final mixing chamber before the humidified gas passes into the

plastic bag containing the object (Fig. 7).

An important aspect ofthe design ofthe nitrogen-supply-and-

humidification system is the use of leakprooffittings that minimize the

influx ofoxygen into the system. All fittings from the nitrogen cylinder to

the entrance ofthe bag use^1 / 4 -in. (approx. 6 mm) brass O-ring-sealed

Swagelok fittings. These fittings connect the polypropylene tubing, which

is used to pipe the nitrogen gas from the gas cylinder, through the humidi-

fication system, and into the bag. Swagelok O-ring-sealed fittings are

inserted into holes that are precisely drilled in the lids of the humidifica-

74 Hanlon and Daniel

Aclar bag

Humidification system

Nitrogen

supply

Figure 6
Schematic of the dynamic system for creating
a low-oxygen environment.