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Mould in Archives and Heritage Collections: When Cleaning Alone Is Not Enough

By Anca Haralambie, Founder, Aridom Sanex. Last reviewed 6 October 2026.

Most archive mould problems begin with a simple observation: a bloom on a binding, a smell that comes back on Monday morning, a box that does not look right. The natural question is how to deal with it without harming the material. There is another question that should come first. Why is it here, and is what you can see the whole of it?

Mould on a collection is often connected to something beyond the collection. Moisture from an old leak, condensation on a cold wall, a bay of shelving with almost no air movement, a void nobody looks into. Clean the visible growth without understanding that and you may deal with today's problem while leaving its explanation untouched.

At Aridom Sanex we look at three things together: the collection, the environment around it and the root cause. A sensible approach to archive mould remediation only comes after that. If you are dealing with mould in an archive now, talk to us before deciding how the collection or the room should be treated.

What to do first when you find mould in an archive

Avoid unnecessary handling and limit access to affected material while the situation is assessed. Record where the mould is and how far it seems to extend, with photographs taken without disturbing it. Tell whoever is responsible for collection care, and your estates or facilities team. If water is still getting in, deal with that promptly. Do not start brushing, vacuuming, spraying or fogging collection material without a plan, suitable controls and conservation advice.

Why the visible mould may not be the whole problem

The first instinct is to clean what you can see. With historic or irreplaceable material that deserves a pause. An affected volume may be one point in a wider pattern, and handling it more than necessary can disturb mould and spread it. Treating individual items without looking at their surroundings can also leave an environmental or building problem exactly where it was.

Some questions are worth answering before anyone decides on treatment. What kind of material is affected? Is it a few items or several places? Has mould appeared here before? Is there a history of leaks, flooding or condensation? Are some shelves or areas worse than others? Does a musty smell return after the room has been shut? Is there damp elsewhere in the room or next door? Is a conservator already involved?

None of these gives a diagnosis. They define what needs investigating.

Is it active mould, inactive mould or something else?

Old collections accumulate marks. Foxing, dust, tide lines from past water damage and other deposits can all resemble mould, and genuine mould can sit among them. Conservation guidance distinguishes active, inactive (dormant) and non-viable growth, but appearance alone does not always settle which you have, particularly on material that is already stained or deteriorated.

That uncertainty matters. Inactive mould is not harmless: it can keep its allergenic properties and may reactivate if conditions change. Active growth changes what can safely be done to a collection. Both are reasons to assess before treating, and to be wary of any decision made on appearance alone when the material is valuable.

Collection, environment and root cause

It helps to separate the problem into three connected parts.

The collection

The books, registers, documents, photographs and bindings that are affected or exposed. Age, construction, condition and significance all count, and decisions about what can be done to the material itself belong with conservation expertise.

The environment

Shelving, surfaces, the air, the room and, where relevant, neighbouring spaces, ducts, voids and other parts of the building that may form part of the contamination picture.

The root cause

Whatever allowed mould to develop: moisture ingress, a previous leak, condensation, poor ventilation, restricted airflow, building defects. Often more than one.

Work on the collection without the environment and part of the problem may stay behind. Work on the room without the collection and the affected material is ignored. Neither corrects a building fault. Those faults remain the responsibility of the building owner or the appropriate contractor or engineer, but they need to be identified and recorded, because recurrence usually depends on them.

Could this be happening in your collection?

Two stores can show similar-looking mould and need very different responses. What matters is where the mould is, how it is distributed and how that relates to the building. These are the kinds of observation that make us ask more questions:

  • heavier growth on lower shelves than upper ones
  • mould returning after previous cleaning
  • one part of a store much worse than another
  • a musty smell that returns after the room has been closed
  • staining, bubbling plaster or damp at wall bases
  • a past leak or flood
  • compact shelving with little air movement
  • growth near ducts, hatches or shafts, or related problems in adjoining rooms
  • collections and estates teams disagreeing about where it is coming from
  • uncertainty about whether the records can be moved, or concern about exposing fragile material to chemicals

No single sign identifies a cause. Together they can show whether the problem looks confined to the collection or whether the surrounding environment or building deserves investigation. That judgement is made on site, not from a list.

Recognised one or more of these signs? Photograph what you have found and talk to Aridom Sanex before deciding how the affected collection or environment should be treated.

When should you ask for specialist assessment?

The size of an outbreak matters, but it is not the only measure. Guidance from the Canadian Conservation Institute groups outbreaks by visible area and suggests professional remediation may be needed once they reach roughly 4 square metres. Those bands are a rough guide to containment and protective equipment. They were not derived from health effects, and they say nothing about the value of what is affected.

A handful of mould-affected registers can need more judgement than a much larger area of replaceable material. Assessment is worth considering when mould keeps returning, the source of moisture is unclear, several areas seem affected, growth may extend into hidden areas, the collection cannot easily be moved, earlier treatment has not worked, there is doubt about the condition of the material, or different teams disagree about the source.

If mould has returned after previous cleaning, we can assess whether the collection is the only issue or whether moisture, airflow or the surrounding building is contributing.

Why archive mould remediation needs a site-specific plan

Established conservation guidance provides the framework, but the response has to be adapted to the collection, the contamination, the building and how it is used. No single protocol fits every archive. The right response depends on the material and its condition, whether growth appears active, inactive or uncertain, how far it extends, the moisture history, how air moves through the space, how the building is constructed, whether the collection can be moved, what the conservator advises and what the building has to keep doing while work happens. Change one of those and the right answer can change.

So we do not arrive with a fixed treatment and fit the site to it. We assess first, with you and, where appropriate, your conservator, and then develop a protocol for that collection, that environment and that building. Environmental remediation and collection conservation are related disciplines. They are not the same one, and where conservation judgement is needed we work within it.

Where HOCl and automated dry fogging fit

Aridom Sanex works with hypochlorous acid (HOCl) and automated dry fogging as part of its environmental decontamination capability. Where an assessment identifies a role for them, they can form one part of a site-specific protocol alongside other measures. They are tools within a strategy, not a substitute for investigation, and many archives will not need them.

They do not stop a leak, correct a ventilation fault, replace the physical management of contaminated material or take the place of conservation expertise.

Two points of care. First, performance and compatibility are separate questions. Evidence that a system performs under defined test conditions, such as the airborne room disinfection standard BS EN 17272, which is tested on non-porous surfaces, says nothing about how paper, leather, parchment, inks, bindings or photographs will respond. Second, conservation guidance is cautious about chemical treatment of heritage material itself, and notes that killing mould does not remove its allergenic or toxic properties. Any proposed chemical treatment has to consider whether collection materials could be exposed, including when the surrounding room is treated. Where collections remain in place, conservation advice and an assessment of material compatibility come before any decision on treatment.

Can the collection stay in place?

Sometimes. It should not be assumed either way. Moving a whole collection brings handling, cost and risk of its own, and leaving it where it is has its own. The answer depends on the material, the extent and distribution of contamination, how the store is laid out, access needs and conservation advice. If you are unsure, speak to us before anyone decides how the collection should be moved or treated.

What happens at an assessment

The aim is to learn enough to make informed decisions about what happens next. That usually means looking at the apparent extent and distribution of contamination, the condition of the surrounding environment and building, where conservation input is needed and what scope of work, if any, is appropriate.

Our assessment is by visual inspection. If laboratory sampling would answer a particular question, where the results would answer a defined question and change the proposed response, we can point you to independent accredited laboratories, because that work is theirs to do.

Visual inspection can show apparent contamination patterns and signs that need further investigation. It cannot reliably establish whether mould is viable, find every concealed source or measure airborne exposure. Where a specific uncertainty would change the response, we can discuss further investigation with the relevant specialist.

Where we find moisture, ventilation or building issues, we document them for the estates team, building owner or relevant contractor. If remediation follows, we prepare a proposal around the collection, the building, how it is used and the scope agreed. For public-sector clients that proposal can set out responsibilities, conservation requirements and validation requirements clearly enough to support internal approval and procurement. An assessment does not commit you to remediation.

Working with conservators, estates teams and facilities-management companies

Archive mould rarely belongs to one department. The archivist looks after the collection. A conservator decides what is appropriate for sensitive material. Estates may be responsible for moisture, ventilation or the building fabric. Health and safety may have their own requirements. We work within that structure, directly or alongside your existing conservators, estates teams and facilities-management providers. We can also support facilities-management companies and specialist contractors who need archive-sensitive mould assessment and remediation as part of a wider project.

Cleaning is not the end of the problem

Remediation deals with contamination within its agreed scope. It cannot make up for unresolved moisture or building defects, and nobody should promise that mould will never return. If the underlying conditions remain, cleaning alone may only address what is visible today. Where an assessment finds ingress, ventilation problems, condensation or damaged fabric, these need to be dealt with by whoever is responsible for the building. Longer-term protection usually involves monitoring and inspection as well.

Request a Confidential Archive Mould Assessment

If you have found mould in an archive, record store or heritage collection, you may not yet know whether the problem is confined to a few items, extends into the surrounding environment or is being driven by conditions elsewhere in the building. That is what the assessment is designed to establish.

We can look at the collection environment, the pattern of contamination and the building conditions, work with your conservator where needed and develop a remediation protocol that suits the site.

When you get in touch it helps to tell us where the collection is, what type of material is affected, roughly how much, whether it has happened before, whether there has been a leak or flood and whether a conservator is already involved. Photographs help.

We are based in Kent and regularly support clients across London and the South East. For larger, specialist or multi-day archive and heritage projects we can work further across the UK by arrangement. If you are elsewhere, still get in touch, and we will agree project logistics in advance.

Request a Confidential Archive Mould Assessment

Prefer to talk it through? Call 01622 395534 or email info@aridomsanex.co.uk.

Frequently asked questions

1. What should I do if I find mould in an archive?

Avoid unnecessary handling of affected material and write down what you have found, with photographs. Before anyone decides on treatment, consider whether the problem extends beyond the items you can see, and whether specialist or conservation assessment is appropriate.

2. How can I tell whether mould in an archive is active?

There are visual and environmental clues, but appearance alone does not always give certainty, especially where historic staining, foxing or dirt is present. With valuable material, treatment decisions should not rest on appearance alone.

3. Can mould damage historic documents?

Yes. Mould can affect organic collection materials and, depending on the circumstances and how long it has been present, can contribute to staining and deterioration. How serious the damage is depends on the material and its condition.

4. Can mould return after professional remediation?

Yes. Remediation deals with contamination within an agreed scope. If moisture, ventilation or other conditions remain favourable to growth, recurrence is possible.

5. Can the collection stay in place during remediation?

In some cases. In others, material may need to be moved or isolated. The decision should be made for the particular collection, contamination and environment, not assumed in advance.

6. Does archive mould remediation need a conservator?

It depends on the collection and what is proposed. Where historic or sensitive material needs conservation judgement, we work alongside your conservator or an appropriate conservation specialist.

7. Can an archive stay open during remediation?

Potentially. It depends on the contamination, the building, the collection and access needs, which is why operational requirements form part of the assessment.

8. Do you test the air or identify the mould species?

Our assessment is by visual inspection, which has limits. Where laboratory sampling would answer a defined question, independent accredited laboratories carry it out and we can point you to them.

9. Does dry fogging solve archive mould problems?

No. Automated dry fogging may be one part of an environmental decontamination protocol where an assessment supports it. It does not replace physical remediation, conservation input or correction of moisture and building problems.

10. Can you work with our conservator or estates team?

Yes. We work alongside conservators, archivists, estates teams, facilities managers and other specialists, with responsibilities agreed for each project.

11. Do you work outside London and the South East?

Yes, for suitable projects. Kent, London and the South East are where we work most often, and larger or specialist archive and heritage projects can be undertaken elsewhere in the UK by arrangement.

12. Does killing mould make the material safe?

No. Killing mould does not physically remove it, its fragments or staining, and non-viable material can still be allergenic. Whether affected material is safe to handle is a separate judgement that treatment alone does not settle.

13. Can mould stains be removed from historic documents?

Staining and viable contamination are different problems. Mould can leave stains after it is no longer active, and whether any stain can or should be treated is a conservation decision for the material concerned.

 

Guidance and sources

Treatment decisions for valuable or conservation-sensitive collections should reflect the individual material, the site and appropriate professional advice. This article is general information and does not replace a conservator's advice or a COSHH risk assessment.