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Construction of Anoxic Microenvironments –
Project Airless

Purpose

This guide shows the steps in creating an anoxic microenvironment for a specimen once any remedial treatments have been carried out. These microenvironments are being created for the re-storage of palaeontological specimens suffering from, or at risk to, pyrite oxidation, which is accelerated under high levels of relative humidity (RH). This is part of a preventative conservation project, Project Airless. These barrier enclosures can, however, be used for storage of any objects which require a reduced oxygen environment.

Why have we chosen to re-store pyritic specimens in anoxic microenvironments?

  • It is important to re-store pyritic specimens in improved conditions, as decay can continue after remedial treatment.
  • Microenvironments were decided upon to be a more sustainable, cost-effective way of conserving these specimens compared to attempting to maintain the building’s atmospheric storage conditions.
  • Re-storing these specimens under anoxic conditions, rather than eliminating RH, was deemed the best option in order to halt the decay process, as clay-based fossils may dry out causing irreversible damage through shrinkage as interstitial water is lost. In addition to this, if you were trying to just control the RH, the silica buffers would need to be conditioned to <30% RH unless all specimens are treated with ammonia vapour, which has a large cost and time implication.

Author(s)

Amy Trafford
The Conservation Centre, Core Research Laboratories
The Natural History Museum
London, UK SW7 5BD
pyrite@nhm.ac.uk

Lu Allington-Jones
The Conservation Centre, Core Research Laboratories
The Natural History Museum
London, UK SW7 5BD
l.allington-jones@nhm.ac.uk

Photo Credits: Amy Trafford and Lu Allington-Jones

Publication: 2017 

Fig. 1 - Specimens stored in a recently sealed gift-bag style anoxic microenvironment

Fig. 1 – Specimens stored in a recently sealed gift-bag style anoxic microenvironment

Description

The anoxic microenvironment shown in figs. 1-3 consists of a gas-barrier film and oxygen scavenging sachets, RPK’s. RPK oxygen scavengers are used as they maintain RH ambient at the time of sealing. Our enclosures are sealed at 40-50% RH, which is appropriate for pyritic clay fossils like the Gault. The bag is heat-sealed at the top, leaving enough additional material so that it can be cut open and re-used (Figure 1). Once a specimen has been accessed, the oxygen-scavenging sachets are replaced and the bag is resealed.

Fig. 2 - Successful anoxic microenvironment shown by the barrier film sucking inwards – oxygen has been removed from the enclosure.

Fig. 2 – Successful anoxic microenvironment shown by the barrier film sucking inwards – oxygen has been removed from the enclosure.

Fig. 3 - Specimen re-stored in a pillow-style anoxic microenvironment.

Fig. 3 – Specimen re-stored in a pillow-style anoxic microenvironment.

 

Materials, Tools & Supplies

  • Fig. 4 - Oxygen scavenging sachets (RPKs)

    Fig. 4 – Oxygen scavenging sachets (RPKs)

    ESCAL™ Neo gas-barrier film (transparent ceramic) by Mitsubishi Gas Chemicals, with inner layer of heat-sealable polyester
  • Marvelseal® gas-barrier film (foil based film)
  • Oxygen scavenger (Figure 4) – by Mitsubishi Gas Chemicals (RPK and RPA systems – RPKs were used for this project)
  • Double-sided tape – 3M #415
  • Low melt adhesive from Preservation Equipment Ltd – EVA copolymer P419-1044
  • Scalpel
  • Scissors
  • Criss Cross or Cross-Weld heat sealers
  • Perspex templates (Figure 5) or templates made from corrugated museumblueboard (Figure 6), corresponding to each storage tray size
  • Transparent polyester film – Melinex®
  • Polyester label sleeves
  • Oxygen indicator (OxyDot) (Figure 10) – optional
  • Acid-free trays (1300 micron grey/white boxboard covered with Argentia 120gsm acid-free paper, neutral pH EVA adhesive)
  • Inert foam Plastazote®
Fig. 5 - Perspex templates corresponding to various tray sizes

Fig. 5 – Perspex templates corresponding to various tray sizes

Fig. 6 - Corrugated blueboard template corresponding to tray size for barrier film

Fig. 6 – Corrugated blueboard template corresponding to tray size for barrier film

 

 

 

 

 




Construction

Re-storing in acid-free materials

Specimen is re-stored within an inert foam inlay inside a new pre-made or custom-made acid-free tray (Figure 7). The height of the edges of the tray should exceed the height of the object, to prevent contact with the barrier film.

Place any loose specimen labels in a Melinex® sleeve and attach to the outside of the storage tray using double-sided tape so that they will be visible through the bag (Figure 8). The specimen within its tray is then ready to be stored in an anoxic microenvironment.

Fig. 7 - Specimens re-stored in acid-free materials

Fig. 7 – Specimens re-stored in acid-free materials

Fig. 8 - Loose labels inserted into label sleeve and attached to outside of tray

Fig. 8 – Loose labels inserted into label sleeve and attached to outside of tray

Construction of barrier film enclosure

Decide on a style most suitable for your specimen. We have found that a gift-style bag such as shown in fig.1 is used for smaller specimens while a pillow-style bag (Figure 3) is more suited to larger specimens. The gift-style bag is also the best one for keeping expansion space to a minimum, though it does use more barrier film. For trays containing larger and heavier specimens, a foil-based film, Marvelseal®, is used for the base of the bag as this material has more resistance to tearing.

Gift-style bag 

  1. Cut a piece of barrier film large enough to fold up over the tray, plus enough to re-seal a few times.
  2. Choose a card or Perspex template appropriate for the tray size (figures 5 and 6) (use internal guide, not outer edge). Or just mark out the shape below, 1.5cm wider than your tray.
  3. Draw around the template on barrier film using a soft pen (felt-tip), or cut around a solid template directly with a scalpel. If drawn, cut along the inside of the pen lines using a metal ruler. You will now have the foundation of your bag (Figure 9).
    Fig. 9 - cut out barrier film

    Fig. 9 – cut out barrier film

  4. Oxygen indicator (optional) – Adhere OxyDot to barrier film by applying silicone to the glass side, not the rough side (Figure 10). Position in bag where it will be readable once sealed. Be careful not to touch the surface of the OxyDot with bare hands.
    Fig. 10 - OxyDot adhered to inside of barrier film bag

    Fig. 10 – OxyDot adhered to inside of barrier film bag

  5. Fold in each corner and use a single seal with a broad heat-sealer or double-seal with a narrow heat-sealer (Figure 11). If the barrier film is from a roll, it will curl inwards – the inside of this curl will be the inside of the bag. This is the heat-sealable layer of the barrier film. Be careful to seal near the edges or the bag will be too small. Check your seals, if there are any leaks then re-seal at a slightly higher setting.
  6. Check any loose specimen labels are visible without having to open the bag.
    Fig. 11 - Heat seal edges

    Fig. 11 – Heat seal edges

Pillow-style bag

  1. Cut barrier film to a suitable size to accommodate both the specimen within its tray and oxygen scavenging sachets. Fold film in half.
  2. Heat-seal two of the open sides.
  3. Slide in the specimen tray and oxygen scavengers (see the ‘Comments’ section for the calculation of number required).
  4. Heat-seal the final side, leaving an excess of barrier film on one side to allow bag to be opened and re-sealed for future access (Figure 3).

Sealing anoxic microenvironment

  1. Place your specimen and sufficient oxygen scavenger(s) in the bag. Fold in the barrier film along the shorter sides of the tray (Figure 12).
  2. Squeeze out as much air as possible and heat seal the top of the bag. Turn the bag around and seal from the other side. Check there are no leaks in your seal.
    Fig. 12 - Expel as much excess air as possible before sealing

    Fig. 12 – Expel as much excess air as possible before sealing

Comments

Calculation of oxygen scavenger needed from the volume of the bag:

  • Measure the width and length of the tray and estimated height of the finished bag in centimetres (not the height of the squashed flap, but the height of the void).
  • Multiply the three figures and this will give the volume in millilitres.
    For example:  tray width: 10cm  tray length 20cm  estimated finished bag height 7cm  10cm x 20cm x 7cm = 1400ml  Oxygen scavenging sachets come in different sizes. RP3K is for 300ml of air, RP5K is for 500ml of air and RP20K is for 2000ml (or 2 litres) of air.  So for 1400ml bag use 3 RP5K sachets.

A few hours after the bags are sealed the scavengers will cause the sides to suck inwards (Figure 2). If this does not occur then the seals or RPKs have failed.

As the barrier film enclosures and oxygen scavengers cause the object’s footprint to expand, we have kept expansion to a minimum with larger specimens that consist of multiple disassociated parts by creating tiered storage boxes (Figure 13).

Fig. 13 - Specimen stored in a tiered storage tray

Fig. 13 – Specimen stored in a tiered storage tray

For heavy specimens, use Marvelseal® for the bottom of the bag to prevent the corners of the storage box causing tears in the thinner ESCAL™ when lifting. These two different barrier films can be heat-sealed together.

 

Adapted From

The design has been developed and tested by various staff at The Natural History Museum, London.

 

Keywords

anoxic microenvironment, oxygen scavenger, barrier film, preventative conservation

Special Purposes

Microenvironment

Collection Type

Natural Science

 

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