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Active or inactive?

Mould exists in one of three states – it is actively growing, inactive, or non-viable (dead). While it is understood that active mould is hazardous to both the collection and those accessing it, it is also important to be aware that dormant and non-viable mould also present a risk to health (Fig. 1). The effects of exposure to mould are cumulative and around 2% of the population may develop an adverse reaction to it.

A diagram showing the three states of mould: active, dormant, and non-viable. Arrows between them show how active mould can become dormant mould, which in turn can become non-viable, but also how only dormant mould can return to being active. Above this is a double-headed arrow which spans the width of the diagram - it is labelled 'risk to collection', and ranges from very dark red to yellow as it follows the three stages from active (highest risk) to non-viable (least risk). There is another double-headed arrow beneath the three stages, which relates to health and safety risk. This arrow is completely orange, denoting that the risk is the same no matter the state of the mould.

Figure 1, Mould lifecycle stages and risk (produced by The National Archives)

When growing and reproducing, mould requires an organic food source, like paper, parchment, adhesive and other materials found in archival collections. It also needs moisture and thrives when the surrounding environment has a sustained relative humidity of +65%. When such conditions occur, germination and growth begin. Microscopic spores – or conidia – are released into the air and as they land on a surface that can support growth, they form chains called hyphae. These thread-like chains release digestive enzymes which degrade materials and subsequently absorb nutrients from it. These chains form the visible body of the fungus, known as a mycelium.

A diagram showing a cyclical spore process. Four stages are each connected by clockwise arrows - at the top is 'hyphal growth', then 'spore production', then 'spores', followed by 'spore germination', which returns to 'hyphal growth'.

If disturbed by air movement or by physical contact, spores are released into the surrounding area and, given the right environmental conditions, will go on to form further mould colonies.

When active mould growth is present, you may notice:

  • A perceptible musty odour, both within a storage space and on a document itself
  • A damp feel to the document surface
  • Noticeable changes to the appearance and condition of the document
  • Fluffy or slimy growths, which may smear when touched

Identifying and responding to an active mould infestation is essential. When left untreated, active mould can spread quickly, posing a risk to the ongoing condition of your wider collection, as well as to those who access it.

As humidity levels decrease, active mould becomes dormant and it:

  • Desiccates, becoming dry, powdery, or crusty and – like active mould – can be disturbed by movement
  • Can remain inactive for many years but may reactivate once humidity levels create a suitable environment
  • Can appear as mottling or staining on the document surface, often with dry, powdery deposits in the box – though appearance can vary widely

Because you cannot visually assess the difference between dormant and dead mould, (Personal Protective Equipment (PPE) should always be worn while handling affected collection material.

The science of mould has additional information on how mould grows.

What does mould look like?

Due to age, storage and usage, archive and library collections may be soiled and discoloured, making it difficult to tell the difference between mould growth and historic dirt deposits. As such, identification of mould is not always straightforward. However, there are several key characteristics to look out for which may indicate its presence within your collections.

Common visual indicators can include:

  • Staining, mottling and/or discolouration of the affected surface, frequently in a circular pattern and in a variety of colours (Fig.2-4)
  • Water damage and/or tide marks – where contact with moisture has occurred, mould growth is common (Fig. 5-6)
  • Surface deposits – mould that is actively growing, or powdery accretions of dormant mould (Fig. 7-8)
A bound document, showing the front cover and spine of the closed book. The covering is black, and has been scratched and damaged. Across the surface are also clear small white blooms of mould.

Figure 2 – catalogue reference FO 277/215

A selection of original paper records in a pile. Some have been lifted up to reveal the extent of the dark mould staining present - the stain patterns are the same on the underside of the document being lifted, and the topside of the document directly beneath it.

Figure 3 – catalogue reference WO 78/4369

An aged manuscript book, opened to reveal severe discolouration and mould staining.

Figure 4 – catalogue reference E 190, piece number unknown

Two severely damaged bound documents, shown side by side. Clear tide marks can be seen on both.

Figures 5 and 6 – IR 58/21229 and ADM 38/3722

Close-up of an original paper document, displaying significant blooms of black mould.

Figure 7 – catalogue reference AO 3/877

Close-up of original planning papers, with a hand at the top of the image lifting part of the pile to reveal significant black mould staining.

Figure 8 – catalogue reference WO 78/4369

The science of mould has additional information on how mould grows.

What does mould damage look like?

During mould growth, digestive enzymes cause weakening of the surface. This can impact significantly on the condition of a document and lead to irreversible damage, including:

  • Adhesion and/or blocking – through the addition of moisture, pages are stuck together and cannot be turned or separated easily (fig. 9-10), leading to loss of information
  • Breakdown in structural strength of surface – a document becomes difficult to handle without causing further damage (fig. 11-12)
  • Losses – where significant portions of a document have been detached or completely dissociated (fig. 13-14), leading to loss of information
An original paper manuscript, with a page lifted up to show the damage between the pages. They are greyed and deteriorating severely at the edges. Severe discolouration can also be seen.

Figure 9 – catalogue reference E 190/5/4

A close-up of the original manuscript that appears in Figure 9. The greyed and deteriorating top edge of the document can be seen, as a gloves hand uses a thin tool to prise two pages apart.

Figure 10 – catalogue reference E 190/5/4

A corner of an original manuscript, showing holes in the paper caused by mould damage.

Figure 11 – catalogue reference C 101/1246

A corner of what appears to be a land survey, showing discolouration and deterioration of the edges.

Figure 12 – catalogue reference IR 58/21229

A yellowed paper document, with large holes on the top left part of it. The holes extend through several leaves of paper.

Figure 13 – catalogue reference C101-1246

An original handwritten and tabulated record. Holes in the document and deterioration of the edges is very apparent.

Figure 14 – E 190/585/21

The science of mould has additional information on how mould grows.