Anoxic treatment

Anoxic treatment kills insects by removing the oxygen they need rather than by chilling them. It takes longer than freezing and needs more equipment, but it puts no thermal or mechanical stress on the object, which makes it the method of choice for material that cannot safely be frozen.

How it works

The object is sealed inside an enclosure made from a barrier film with very low gas permeability, and the oxygen inside is brought down to a fraction of a percent and held there for several weeks. Every life stage suffocates, including eggs and pupae, which are the stages that survive inadequate treatments.

Two routes reach the low oxygen level.

Nitrogen flushing displaces the air with nitrogen from a cylinder or a generator. It reaches the target quickly and suits large volumes, but it needs gas supply, regulators and, for anything sizeable, a controlled flow over time to compensate for permeation through the film.

Oxygen scavengers are sachets of a reactive material, usually iron-based, that consume the residual oxygen chemically inside a sealed package. They are far simpler, need no gas supply, and suit small to medium packages. They have a finite capacity, so the quantity has to match the package volume with a margin.

In practice many treatments combine the two: flush to remove the bulk of the oxygen, then include a scavenger to absorb what permeates in during the hold.

Getting the humidity right

This is the step that catches people out. Cylinder nitrogen is essentially dry, and flushing an enclosure with it can take the interior humidity down to a level that stresses hygroscopic material more than the insects ever did.

Two remedies, usually combined: humidify the incoming gas, and include conditioned silica gel in the package set to the humidity the object is used to. A logger inside the package records whether it actually held.

Verifying it

An anoxic treatment that was not verified is an assumption. Three things are worth measuring.

Oxygen concentration, with an oxygen indicator or a meter. Indicator tablets that change colour at a threshold are the cheap option and give a yes-or-no; a meter gives a number and lets you watch for a slow leak.

Humidity and temperature, with a logger inside the package.

Seal integrity over time. A slow leak shows up as oxygen creeping back up, and the whole treatment fails silently if nobody is watching. This is the most common reason an anoxic treatment does not work.

What it suits

Anoxia is the alternative when freezing is contraindicated: painted, gilded, varnished or lacquered surfaces; composite objects with very different materials; panel paintings; brittle or previously damaged material; some plastics; and objects with loosely attached applied decoration.

It also suits material too large for available freezer capacity, since a barrier film enclosure can be built to almost any size, and objects on open display that can be tented in place.

What to weigh against freezing

	Freezing	Anoxia
Duration	Days	Weeks
Equipment	Freezer, bags, logger	Barrier film, gas or scavengers, oxygen indicator
Mechanical stress	Some, on composites	Essentially none
Humidity risk	Condensation on rewarming	Excessive drying during flushing
Scale	Limited by freezer size	Limited only by film and sealing

Where both are viable, freezing is usually faster and cheaper. Where the object is delicate, composite or painted, the extra weeks are the price of not introducing a second problem.

As with any treatment, killing what is present is only half the job. The source has to be found, or the material will be reinfested in the same store, and trap monitoring is what tells you whether that happened.