Anoxia

Tamara Jaeger <tamarajaeger [at] hotmail__com> writes

>We are considering purchasing a large enclosure similar to the
>Rentokil "Bubble" for anoxic pest eradication treatments of objects
>and archival materials that cannot be frozen. I am interested in
>hearing what systems other institutions are using as well as their
>associated benefits and drawbacks.

About 12 years ago, staff at Hampshire County Council Museums
Service put together an anoxic chamber, on a shoestring budget and
it's still functioning today!

A resin and fibreglass chamber was purchased and mounted on
furniture casters.  The size of the chamber: 110 (across) 80 (front
to back) and 94 (high); all measurements are in centimetres.

A 1.5 cm thick sheet of Perspex, bedded onto a neoprene gasket set
into the upper surface of the tank, formed a sealable lid which
could be clamped down using fixed but adjustable tension clamps.

Holes were drilled in the tank sides to admit nitrogen gas at the
bottom and to let out the nitrogen/air mixture at the top of the
tank.  The latter was connected to a percentage oxygen meter to
monitor the gradual reduction in oxygen levels.

Three interconnected plastic containers, the left half-filled with
water, were placed in line with two valves  to adjust gas moisture
content levels. The gas and moisture mix in the middle pot and the
right-hand one is connected to a hygrometer if required.  These were
added so that the nitrogen gas would pass through the water to bring
its low RH (5%) up to a level that could be tolerated by the
material being treated.

For this to work effectively and economically, the chamber had to be
filled to capacity with treatment material so as not to use up too
much gas.  Since this meant that smaller items had to wait until
they could be slotted into the programme, a smaller tank (63 x 48 x
43 cms) was constructed from 1.0 cm thick Perspex.

Nitrogen gas was then passed into the bottom of the container, via a
bicycle valve, for one week.  The displaced gas was passed out via
the oxygen meter and then into a bubble tray, so that an audible
signal was always present to show that the in-running gas had not
run out.

The gas was then turned off after this time and when the oxygen
level was below 0.03%, the material was left in the tank for a
further week as the oxygen level slowly crept up to about 0.1% by
the end of the week.

The process was found to kill Anthrenus larvae to a depth of at
least 20 cm, some in dense plumage.

No material treated has shown any signs of deterioration or
molecular change.

RH levels were monitored by placing a digital 'hygrotemp' meter in
the chamber.

For a fast and more effective kill, the humidity can be reduced if
the treated material can sustain lower RH levels.

N.B.  The combination of low oxygen and RH levels is what kills the
infesting insects: the low oxygen levels cause their spiracles to
open wider than usual and this causes them to lose vital body
moisture so that they die more rapidly from dehydration/desiccation
rather than asphyxiation.  So the lower the chamber RH, the more
effective and rapid the insect kill.

Since nitrogen makes up most of the air we breathe, no-one has yet
complained of ill-effects from either the bubbling nitrogen or from
the chamber when the lid is removed.

The equipment described has been in use for about 10 years.
Maintenance has been low, the work level achieved has been high and
the insect kill has been 100% as far as can be monitored.

The cost of gas used per average load in the main chamber is about
UKP25.00.

Simon Moore, MIScT, FLS, ACR
Senior Conservator of Natural Sciences
Hampshire County Council
Recreation and Heritage Department
Museums and Archives Service
Chilcomb House, Chilcomb Lane
Winchester SO23 8RD, UK
+44 1962 826737


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                  Conservation DistList Instance 22:21
                 Distributed: Sunday, October 12, 2008
                       Message Id: cdl-22-21-004
                                  ***