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6 Assessment of materials used for anoxic
microenvironments
Vicen Carrió and Suzie Stevenson
Abstract For various reasons, museum objects may require to be stored in environments with low oxygen levels and/
or low levels of fluctuation in relative humidity (RH). Geological specimens that are susceptible to pyrite decay
(oxidation of the mineral pyrite) or specimens that have undergone treatment for pyrite decay are in this category.
For this purpose, low-oxygen enclosures can be used. This study has assessed the effectiveness of various materials
that can be used in the manufacture of anoxic microenvironments. The results indicate that Escal barrier film is more
effective than BDF 200 barrier film, and is also a better moisture barrier. A double-skin enclosure is more effective
as an oxygen barrier. Commercially available oxygen scavengers are very effective in removing oxygen within barrier
film enclosures, while purging with oxygen-free nitrogen has no apparent benefit. Oxygen indicator eyes do not give
entirely consistent results and so should be used with care.
Keywords: storage of material containing pyrite, oxygen scavengers, oxygen indicator eyes and low-oxygen
enclosures
Introduction
When considering geological collections it is common to
assume that objects are quite robust. However, it has been
known for a considerable time that geological specimens are
subject to various types of deterioration (Howie 1979). One
such problem associated with geological material is the
breakdown of iron sulphide – a reaction commonly referred
to as pyrite decay.
Iron sulphide (FeS2), present as the minerals pyrite and/
or marcasite, is found as an accessory phase in many rock
types. It follows that many rock, mineral and fossil speci-
mens held in museums, as well as stone objects in archaeo-
logical and ethnological collections, contain significant
quantities of pyrite. Increasing and/or fluctuating relative
humidity (RH) can cause breakdown in susceptible pyrite
material (Howie 1977a,b, 1992). Pyrite breakdown involves
the release of sulphuric acid vapours. If the reaction is not
halted, the specimen may completely disintegrate. The
sulphuric acid can burn specimen labels making them brit-
tle and difficult to read; it can also burn cardboard storage
boxes or wooden cases that are in close contact with the
specimens. However, most damage to specimens results
from the volume change incurred in the hydration of the
iron(II) sulphate from the 1 hydrate to the 7 hydrate form
which could involved a volume change of over 100%
(Waller 1987, 1992). The oxidation to iron sulphate (a
white/yellow residue or powder) and the release of sulphu-
ric acid vapours is often accompanied by the loss of surface
shine on pyrite specimens. The powder produced has a
strong acidic /sulphurous smell – a common indicator of
pyrite decay. In the absence of this characteristic smell, a
simple surface pH measurement with an indicator strip
showing a pH below 5 is usually sufficient to confirm the
presence of pyrite decay (Howie 1977a).
Although pyrite can be present in many rock types, not
all specimens containing pyrite are vulnerable to decompo-
sition. Some localities are known to be the source of vulner-
able pyrite that may decompose quickly. Pyrite from other
localities may remain stable even if stored under the same
RH and temperature conditions. A list of some localities
containing vulnerable pyrite can be found in Bannister
(1937).
Fluctuating temperature and RH together encourage
pyrite to oxidise (Howie 1979) therefore controlling fluctua-
tions in temperature and humidity are important factors in
the care of fossils and minerals, as well as other stone
museum objects (Collins 1988). Generally, geological
material at risk should be stored at RH less than 40% (Howie
1992). However, the museum environment is often a com-
promise of the most suitable conditions for a range of object
types (as well as the comfort of staff!). Once pyrite decay
has started in unprotected specimens, it is irreversible, but
controlling the environment – i.e. maintaining a low RH,
providing an oxygen-free atmosphere, ensuring only small
fluctuations in temperature and using acid-free storage and
packaging material – can prevent further deterioration.
Within the National Museums of Scotland (NMS)
Geology Section, specimens considered to be at risk of
pyrite decay or specimens that have been treated for pyrite
decay are stored in laminated low-oxygen enclosures.
Oxygen levels are kept at 0.1% (maintained by the use of
PREVENTIVE CONSERVATION
33
oxygen scavengers within the enclosures). RH within the
enclosures is maintained at 30–35%, thereby significantly
decreasing the probability of pyrite decay.
The NMS currently applies a pyrite decay treatment
during which minerals and fossils are exposed to an ammo-
nium hydroxide and polyethylene glycol (PEG 400) solution
in a sealed container (Waller 1987). The length of this treat-
ment varies depending on the size of specimen. Vapour from
the solution neutralises the acidic products of pyrite decay,
precipitates iron as an iron(III) oxide and arrests further
crystal growth. Any iron oxide precipitated on the surface is
removed manually.
Once treatment has been carried out, in order to avoid
further deterioration it is necessary to store the specimen
below 40% RH or in an oxygen-free environment with RH
control. The former is both expensive and requires addi-
tional space for specimen storage since the specimens very
often cannot be stored in their original location within the
collections. Oxidation of iron sulphides in the presence of
water vapour can occur at levels as low as 30% RH.
Although the generally accepted range for material vulner-
able to pyrite decay is 30–40% RH, bone material should be
stored at about 45–50% RH (Howie 1979). Therefore,
conditions that would be desirable for pyrite decay-treated
specimens would not be suitable for bone material. (For a
list of the environmental requirements see the chapter by
Waller in Howie 1992).
The latter storage option – that of maintaining the speci-
mens in oxygen-free micro-environments – not only
discourages oxidation of the pyrite through lack of oxygen
but, with careful selection of barrier film, can also provide
RH-controlled microenvironments, making this a more
practical solution in terms of medium- to long-term storage.
In order to achieve such conditions, the specimens are
placed in low-oxygen enclosures following treatment for
pyrite decay. The specimen is placed in an envelope of oxy-
gen barrier film that is purged with nitrogen and sealed. Any
remaining oxygen within the envelope is removed using an
oxygen scavenger such as Ageless (Lambert et al. 1992;
Daniel and Lambert 1993). Oxygen levels and RH within
the envelopes are monitored using commercially available
oxygen indicator ‘eyes’ and humidity indicator strips
respectively. The specimens are returned to storage within
the collections where they are monitored. However, over the
course of three years of this systematic programme of treat-
ment and storage, it was observed that the medium-term
stability of the low-oxygen enclosures was not uniformly
reliable. Since there is a variety of materials available and
slightly differing procedures for manufacture of the enclo-
sures, an experiment was designed to test the effectiveness
of resulting enclosures from the possible combinations of
materials and processes. We have assessed various types of
oxygen barrier film and oxygen scavengers for effectiveness
over a range of RH values. The study is also investigating
the longevity of the low-oxygen enclosure in order to deter-
mine how often the envelopes require to be replaced, and the
effectiveness of these anoxic microenvironments in the
treatment of pyrite decay.
It should also be noted that low-oxygen enclosures are
also used in other areas of conservation such as pest control
(Daniel and Lambert 1993) or storage of materials such as
plastics that are susceptible to deterioration in normal atmos-
pheric conditions. Clearly, points relating to the general
effectiveness of such enclosures are relevant to all areas that
use these materials and processes.
Materials and equipment for making
low-oxygen enclosures
• Cardboard photographic film boxes 12.5 cm × 10 cm
with pH 7.
• Escal, a thick, ceramic-deposited gas and moisture
barrier polymer film. The film is supplied in rolls
allowing different sizes of enclosures to be made. Seal-
ing was done either with a heat sealer or with an Escal
clip.
• BDF 200 is a 25 µm thick, heat-sealable, transparent,
oxygen and moisture barrier film.
• Ageless products are packaged oxygen absorbers.
Ageless chemically absorbs oxygen and generates
some heat and moisture in initial reactions. Therefore
it should not be put in direct contact with the speci-
mens. The selection of the appropriate type and
number of Ageless sachets is important. A method for
calculating the oxygen volume in an enclosure and
therefore the number of oxygen absorber packets
needed is available (Mitsubishi Gas Chemical Com-
pany, leaflet 96.4). The absorbers are supplied in a
vacuum-sealed pouch. Oxygen absorption begins
immediately after the master pouch is opened so the
sachets should be used as quickly as possible.
• Ageless Z removes oxygen and one sachet will absorb
the oxygen from a litre of air. The deoxygenation time
at room temperature is one to four days depending on
the size of enclosure and the ambient temperature. The
maximum exposure time to open air is four hours.
Escal combined with Ageless Z will retain the mois-
ture given off during the active period of the Ageless
Z (Conservation by Design, February 1999).
• Ageless RPA-5 removes oxygen, moisture and corro-
sive gases. Escal combined with Ageless RPA-5 will
maintain low RH. It can therefore be used where high
humidity adversely affects the specimens and where a
low RH is required. It is also used where oxygen and
moisture are primary sources of deterioration.
• The Ageless Eye is an in-package colour change moni-
tor for oxygen levels. A pink colour indicates 0.1% or
less oxygen inside the enclosure. A bluish colour indi-
cates the presence of 0.5% or more oxygen in the
enclosure. The lower the temperature, the more slowly
the colour of the eye will change. The ‘eyes’ are stored
under refrigeration in oxygen-free vacuum packs prior
to use (Mitsubishi Gas Chemical Company 1994).
• Audion Sealmaster 620 electric heat sealer.
• Escal clip: a clip designed to seal an open bag effec-
tively.
• Humidity indicator cards provide a simple indication
of RH within 10% bands.
CONSERVATION SCIENCE 2002
34
Method for making low-oxygen enclosures
• Select a container (cardboard box with or without a
mineral/fossil specimen) and estimate its dimensions
to calculate the volume of oxygen.
• Select the oxygen barrier film, cut and make a bag to
enclose the container.
• Heat-seal three sides of the bag.
• Place the container in the bag.
• Choose the type of oxygen scavenger required.
• Calculate the number of oxygen scavengers required.
Place them inside the bag.
• Put the oxygen indicator eye(s) and the humidity indi-
cator card inside the bag.
• Heat-seal the bag, leaving a small gap for possible
flushing with oxygen-free nitrogen.
• Purge oxygen from the bag using oxygen-free nitrogen
for around 30 seconds.
• Seal the bag using heat or clip as required.
The following points were considered:
• Estimation of the colour shown by humidity indicators
can vary from one observer to another, hence a single
observer was chosen for all colour demarcation estima-
tions.
• The colour of the Ageless Eye was similarly assessed.
• Exposure of the oxygen scavenger Ageless to oxygen
prior to use could make the scavengers ineffective.
• Inappropriate quantities of the oxygen scavengers
could have been used.
• Escal and BDF 200 need to be completely flat for per-
fect sealing.
• Enclosures may not be well sealed. Creases or foreign
substances on the seal area could render the seal faulty.
• Barrier film used could be faulty or damaged.
• Oxygen indicator eyes could be faulty or inaccurate.
• Oxygen scavenger sachets could be faulty.
• Boxes could have higher or lower initial humidity
levels depending on where they were stored prior to the
experiment (boxes were taken from different stores).
• UV could affect the barrier film since sealed contain-
ers were stored in an area open to diffused natural light.
• Escal and BDF 200 film could deteriorate after a period
of time.
• Ambient humidity and temperature varied throughout
the experiment: 30–40% RH and 15–30 °C.
The experiment involved preparing and monitoring 13
different combinations of these variable factors (see Table
6.1). In order to test variability due only to materials and
procedures, no specimens were included in the enclosures.
Experimental results
The different combinations and results for the whole experi-
ment are set out in Table 6.2. They can be subdivided into
three groups:
• G.1 Samples 5, 8, 9, 10 and 13
These enclosures showed little fluctuation in RH over
the course of the experiment (Table 6.3). Enclosures 8,
10 and 13 indicated low to no oxygen presence. Enclo-
sures 5 and 9 indicated low initial oxygen levels but a
sudden increase in oxygen between readings suggest-
ing a possible breach in the seals/barrier films during
the course of the experiment.
• G.2 Similar patterns of fluctuation in RH levels were
exhibited by enclosures 1, 2, 3, 4 and 12 with RH
showing an initial decrease of 10% to 20% within a
few weeks and then increasing to an acceptable 30–
40% (Table 6.4). The oxygen levels in each of these
bags however showed variable patterns.
• G.3 Enclosures 6, 7 and 11 showed RH decreasing to
zero over the course of the experiment. The oxygen
levels are generally low (Table 6.5).
Table 6.1 All combinations of materials and procedures used.
Starting Presence (1) Presence (1) O2O2Presence (1) Presence (1) Heat Ageless RH (%)
date and or absence or absence scavenger scavenger or absence or absence sealer Eye: at
sample (0) of Escal (0) of Bdf Ageless Ageless (0) of N2(0) of setting colour start
numbers: ceramic 200 barrier Z: Number RPA-5: Number Escal clip when
28 June 2000 barrier film film of sachets used of sachets used started
1 0 1 2 1 0 0 5 Pink 30–40%
2 0 1 2 1 1 1 5 Pink 30–40%
(eye broken)
3 0 1 2 1 1 0 5 Pink 30–40%
4 0 1 0 1 0 0 5 Pink 30–40%
5 0 1 2 0 1 1 5 Pink 30–40%
6 1 0 0 1 1 1 5 Pink 30–40%
7 1 0 0 1 1 1 5 Pink 30–40%
8 1 1 2 1 1 1 5 Pink 30–40%
(eye broken)
9 1 0 2 0 1 1 5 Pink 30–40%
10 0 1 4 0 0 0 5 Pink 30–40%
11 1 0 0 2 0 0 5 Pink 30–40%
12 0 1 0 2 0 0 5 Pink 30–40%
13 1 0 4 0 0 0 5 Pink 30–40%
PREVENTIVE CONSERVATION
35
Table 6.2 All results.
Starting Presence (1) Presence (1) O2O2Presence (1) Presence (1) Heat Ageless Final RH (%) and Ageless Eye Colour
date and or absence or absence scavenger scavenger or absence or absence sealer Eye:
sample (0) of Escal (0) of Bdf Ageless Ageless (0) of N2(0) of setting colour
numbers: ceramic 200 barrier Z: Number RPA-5: Number Escal clip when
barrier film film of sachets used of sachets used started
28 Jun 3 Jul 10 Jul 17 Jul 27 Jul 23 Aug 19 Sep 2 Mar 28 Jun 18 Mar
2000 2000 2000 2000 2000 2000 2000 2001 2001 2002
1 0 1 2 1 0 0 5 Pink 30 P/B 20 B 10 B 10 B 10 B 20 B 20 B 30 B 30 B
2 0 1 2 1 1 1 5 Pink 30 P 20 P 20 P 20 B/P 20 B 20 B 20 B 30 B 30 B
(eye broken)
3 0 1 2 1 1 0 5 Pink 30 P 10 P 10 P 10 P 10 P 30 P+ 20 P+ 30 P+ 30 P
4 0 1 0 1 0 0 5 Pink 20 B 10 B 10 B 10 B 10 B 20 B 20 B 30 B 30 B
5 0 1 2 0 1 1 5 Pink 40 P 40 P/B 40 B 30 B 20 B 30 B 30 B 30 B 30 B
6 1 0 0 1 1 1 5 Pink 20 P 10 P 10 P 10 P 10 P 10 P+ 0 P/B 0 P/B 0 P/
B
7 1 0 0 1 1 1 5 Pink 20 P 10 P 10 P 10 P 10 P 10 P+ 0 P/B 0 P/B 0 B
8 1 1 2 1 1 1 5 Pink 30 P 30 P 30 P 30 P 30 P+ 30 P+ 30 P+ 30 P+ 30 P+
(eye broken)
9 1 0 2 0 1 1 5 Pink 50 P 50 P 50 P+ 50 P+ 50 P 50 P+ 50 B 50 B 50 B
10 0 1 4 0 0 0 5 Pink 50 P+ 50 P+ 50 P+ 50 P 40 P+ 40 P+ 30 P+ 40 P+ 40 P+
11 1 0 0 2 0 0 5 Pink 10 P/B 10 P/B 10 P/B 10 P 10 P/B 10 P+ 0 P+ 0 P+ 0 P
12 0 1 0 2 0 0 5 Pink 10 P/B 10 B/P 10 B/P 10 B/P 10 P/B 10 P+ 10 P+ 40 P+ 40 P+
13 1 0 4 0 0 0 5 Pink 50 P+ 50 P+ 50 P+ 50 P+ 50 P+ 50 P+ 50 P+ 50 P+ 50 P+
P+ = 1, completely oxygen-free; P = 0, oxygen-free; P/B = –1, slight oxygen presence; B/P = –2, intermediate oxygen presence; B = –3, full oxygen presence
CONSERVATION SCIENCE 2002
36
Table 6.3 Group 1 results for samples 5, 8, 9, 10 and 13. Sample 9 chosen as representative result.
Starting Presence (1) Presence (1) O2O2Presence (1) Heat Presence (1)
date and or absence or absence scavenger scavenger or absence sealer or absence
sample (0) of Escal (0) of Bdf Ageless Ageless (0) of N2setting (0) of
numbers: ceramic 200 barrier Z: Number RPA-5: Number Escal clip
28 June 2000 barrier film film of sachets used of sachets used
501 2 0151
811 2 1151
910 2 0151
10 0 1 4 0 050
13 1 0 4 0 050
50
40
30
20
10
0
RH (%)
RH
oxygen indicator
colour
28 Jun 00
03 Jul 00
10 Jul 00
17 Jul 00
27 Jul 00
23 Aug 00
19 Sep 00
2 Mar 01
28 Jun 01
18 Mar 02
1 Pink
0 Pink
–1 Pink/Blue
–2 Blue/Pink
–3 Blue
Date of reading
Sample 9
Oxygen indicator colour
Table 6.4 Group 2 results for samples 1, 2, 3, 4 and 12. Sample 3 chosen as representative result.
Starting Presence (1) Presence (1) O2O2Presence (1) Heat Presence (1)
date and or absence or absence scavenger scavenger or absence sealer or absence
sample (0) of Escal (0) of Bdf Ageless Ageless (0) of N2setting (0) of
numbers: ceramic 200 barrier Z: Number RPA-5: Number Escal clip
28 June 2000 barrier film film of sachets used of sachets used
101 2 1050
201 2 1151
301 2 1150
401 0 1050
12 0 1 0 2 0 5 0
50
40
30
20
10
0
RH (%)
RH
oxygen indicator
colour
28 Jun 00
03 Jul 00
10 Jul 00
17 Jul 00
27 Jul 00
23 Aug 00
19 Sep 00
2 Mar 01
28 Jun 01
18 Mar 02
1 Pink
0 Pink
–1 Pink/Blue
–2 Blue/Pink
–3 Blue
Date of reading
Sample 3
Oxygen indicator colour
PREVENTIVE CONSERVATION
37
Discussion
From the work carried out here, it is clear that low-oxygen
enclosures can be an effective way to maintain low oxygen
levels and relatively stable RH conditions.
Using BDF 200 as an internal film barrier and Escal as
the external film produced the best results. However, where
specimens are enclosed in anoxic bags there may be a risk
of boxes and sharp specimens breaking the thin Bdf 200 film
thereby reducing the effectiveness of the process.
Sample 8 shows the results nearest to the combination
that was deemed optimal for pyrite decay-treated specimens.
The experiment suggests that it can be difficult to achieve
consistent results with respect to low-oxygen conditions.
This would appear to be more associated with human error
in the sealing of the bags and/or possible breach of the
barrier films, as noted by previous studies (Burke 1992,
1996). The risk of this can be partially offset by double-
bagging the enclosures. It is therefore very important to
monitor the conditions within the enclosures on a regular
basis. The reliability of oxygen indicator eyes is perhaps not
quite as high as the manufacturer’s literature would suggest,
and it is important to include more than one ‘eye’ in any
enclosure.
The experiments carried out in this study used empty
enclosures. Obviously the nature of the objects within enclo-
sures in collections will have a significant effect on the
speed and effectiveness of oxygen removal.
With respect to RH, the experiment suggests that the low-
oxygen enclosures are reasonable in terms of maintaining
stable RH conditions. For all objects placed in low-oxygen
enclosures, it is important that they are maintained at the
appropriate RH for that object prior to being placed in the
enclosures. This will ensure that the ‘target’ RH is not
compromised. Alternatively, conditioned silica gel could be
used within an enclosure for objects requiring very specific
RH conditions (King 1982, 1983). The humidity indicator
cards are a simple, relatively inexpensive method of moni-
toring RH variation, though they do not give absolute RH
values. Again, it is necessary to monitor the enclosures on
a regular basis for deterioration in the materials and/or
breach of the enclosure.
An environment without oxygen is more important than
lower levels of humidity. Fluctuation in humidity will be
potentially more harmful than a high or low humidity. Of
course, specific conditions will apply depending on the
nature of the specimens/objects to be enclosed in the
microenvironments. For pyrite decay treatments, storage
will be more effective in an environment with no oxygen but
levels of humidity at 30–40%.
Conclusions
• Both Escal barrier film and BDF 200 barrier film per-
form similarly.
• Escal appears to be better suited to maintain the RH
levels suitable for pyrite decay-treated specimens.
• BDF 200 does not appear to have the ability to keep
humidity stable for long periods and in fact, humidity
increases over time. After periods of one to three
Table 6.5 Group 3 results for samples 6, 7 and 11. Sample 6 chosen as representative result.
Starting Presence (1) Presence (1) O2O2Presence (1) Heat Presence (1)
date and or absence or absence scavenger scavenger or absence sealer or absence
sample (0) of Escal (0) of Bdf Ageless Ageless (0) of N2setting (0) of
numbers: ceramic 200 barrier Z: Number RPA-5: Number Escal clip
28 June 2000 barrier film film of sachets used of sachets used
610 0 1151
710 0 1151
11 1 0 0 2 0 5 0
50
40
30
20
10
0
RH (%)
RH
oxygen indicator
colour
28 Jun 00
03 Jul 00
10 Jul 00
17 Jul 00
27 Jul 00
23 Aug 00
19 Sep 00
2 Mar 01
28 Jun 01
18 Mar 02
1 Pink
0 Pink
–1 Pink/Blue
–2 Blue/Pink
–3 Blue
Date of reading
Sample 6
Oxygen indicator colour
CONSERVATION SCIENCE 2002
38
months, the moisture barrier becomes less effective
and the environment reverts to atmospheric humidity.
• Using Bdf 200 as an internal film barrier and Escal as
the external film produced the best results. The use of
a double barrier film gives a more robust anoxic bag.
• Escal clips are efficient in sealing anoxic bags.
• The experiment has been carried out without speci-
mens in the enclosures. Clearly, specimens in the
microenvironments can vary the results in terms of
length of time taken to stabilise the anoxic environ-
ment and the danger of abrasion to the bags them-
selves.
• Available manufacturer’s information about Ageless
Eye suggests that the colour first turns blue when in
contact with oxygen, and then returns to pink after the
container becomes oxygen-free. The oxygen indicator
eyes in the majority of our experiments did not
conform to this. From the 13 samples, only two of the
indicator eyes (samples 11 and 12) turned blue before
returning to pink. During the experiment we worked
quickly transferring the indicator eye from its oxygen-
free package to the enclosure. The enclosure was
immediately sealed. Perhaps these procedures affected
the behaviour of the indicator eye colour.
• Oxygen indicator eyes that were broken but remained
sealed within their packet gave results that were
consistent with the unbroken ‘eyes’.
• Oxygen scavenger sachets worked well. The addition
of oxygen scavenger sachets above the recommended
numbers produced faster results.
• Surprisingly, purging with oxygen-free nitrogen
appeared to have no significant benefit to either the
speed or efficiency of oxygen absorption.
• The use of nitrogen in anoxic bags may decrease the
risk of abrasion by ballooning the bags away from
internal sharp edges.
Acknowledgements
We sincerely thank D. Suzanne Miller and Brian Jackson for help-
ful comments and discussions.
Suppliers
Ageless products, Escal, Escal clip, Bdf 200 barrier film and heat
sealer: Conservation by Design Ltd, Timecare Works, 60 Park
Road West, Bedford MK41 7SL, UK
Mitsubishi Gas Chemical Company, Inc, New Business Planning
and Development Division RP Team, 5-2, Marunouchi 2-
chome, Chiyoda-ku, Tokyo 100, Japan
Nitrogen gas: BOC, PO Box 12, 12 Priestley Road, Worsley,
Manchester M28 2UT, UK
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Authors’ addresses
*Vicen Carrió, Department of Geology and Zoology, National
Museums of Scotland, Chambers Street, Edinburgh, EH1
1JF,UK (v.carrio@nms.ac.uk)
Suzie Stevenson, Department of Geology and Zoology, National
Museums of Scotland, Chambers Street, Edinburgh, EH1
1JF,UK
(*address for correspondence)
