The ‘extreme' mould threatening to destroy our heritage

Telegraph

08-05-2025

As if it weren't enough of a challenge to conserve priceless paintings and historical artefacts from environmental damage, there is a sinister new threat to contend with: a strain of mega-mould that has wreaked havoc in Denmark's museums could already have made its way to the UK. While we usually associate mould with damp, this variety actually thrives in dry conditions, creating a fresh headache for art owners and gallery directors.
Camilla Jul Bastholm, head of conservation and collection storage at the National Museum of Denmark, says she first noticed the problem around 10 years ago. 'I was wondering what was going on in the Danish museum repositories,' she recalls. 'I couldn't work out what kind of mould it was because the methods we normally use to detect mould, for example after a building has water damage, don't work with this species.'
It's a completely atypical variety called aspergillus section restricti, explains Bastholm, and it's known as 'extreme' mould because 'for mould, which usually prefers humidity, dry conditions are extreme'. It's also very difficult to spot because it's white and nearly transparent.
But the damage it does is significant. 'The fungi decimate enzymes and assets, which deteriorates an object, and secrete pigments that colour it, so it actually changes its appearance,' says Bastholm. 'Even if you clean the mould off, it can leave marks. And the longer it grows on something, the worse it gets.'
It's a potentially very expensive predicament for museums. One of the storage depositories Bastholm examined had more than 200,000 artefacts. 'Even if only 40 per cent of them have mould, it's still an extensive job to clean them all off. Some may need restoration work depending on the level of damage.' So far the mould has been detected in 12 Danish museums, and there are studies under way at 150 more sites.
Oddly, Bastholm notes, this mould isn't just restricted to organic material like wood or paper, which is normally more vulnerable. 'I've seen it on ceramics, glass and metals too. But it doesn't seem to affect modern materials. For some reason this mould has a taste for cultural heritage.'
That's potentially alarming news for many in the UK, whether gallery heads, custodians of stately homes, churches, libraries or even members of the public who have historic pieces of art or book collections. 'That mould could well be present in Britain already,' says Chris Woods, director of the National Conservation Service. 'Moulds spread around. We're constantly surrounded by spores – we can't escape them.'
Woods is taking a keen interest in the Danish situation. 'We need to be aware of the risks if a mould is emerging that is seriously different to what we're used to and could potentially breach the stable environments in which we keep art and artefacts. I'll definitely be keeping a close eye to see if any British collections might be at risk. Our climate is very similar to Denmark's, so we could learn a lot.'
Bastholm believes this is already an international problem. 'There are case studies from numerous places, like a Chinese museum depository, an Austrian church, a Swedish library and a Leonardo da Vinci self-portrait in Italy. It seems to be the same species.' She thinks we have it in the UK as well, but just haven't detected it yet. 'I've seen photographs and thought, 'Oh, yes, that looks like it.' The reason you haven't found it might be that you tend to find what you're looking for – and no one was looking for this.'
It's essentially impossible to stop the spread of mould, explains Woods, with so many objects (including loans for exhibitions) transported around the world. 'We're alert to the risk of pests but we're not able to track mould spores at a very microscopic level, which are embedded in materials. You could bring back an unusual mould and not even know it.'
Most big museums and galleries have sophisticated systems in place to control the environment for their collections, such as the light, temperature and relative humidity. However, Bastholm notes that the Danish museums where she found mould had all adhered to the International Council of Museums guidelines, and it hadn't prevented this particular mould from thriving.
It's even harder in an uncontrolled environment, points out Woods. 'Think of a historic stately home with furniture, libraries or venues – you can't seal it off. Churches are definitely the trickiest.'
Woods is an advisor to cathedrals such as Lincoln and Salisbury on how to conserve their Magna Carta exemplars. 'They're made of sheep-skin parchment, so they have naturally accumulated mould and bacteria over 800 years. That's why they're kept in tightly sealed frames that maintain extremely stable relative humidity of 40 per cent, which stops the mould from growing and prevents new spores from landing on them.'
That should mean the Magna Carta is safe from this new strain reaching it, but, says Woods, you can't be too careful. 'Mould can create total destruction if it's allowed to. Parchment is such a nutritious source: I've seen it virtually be turned to dust.'
Since few objects have the level of protection that the Magna Carta enjoys, what can the rest of us do to protect our artwork?
The key thing is to be on high alert, says Camilla Hughes-Hunt, managing director of restoration specialists Plowden & Smith. That includes using a data logger to record the temperature and relative humidity of the space where you're storing your pieces, and keeping a close watch on the surfaces. '[This strain] may not smell as strongly, unlike typical moulds, making it harder to detect without close inspection.'
She also recommends good ventilation and air filtration, which helps to reduce spore load, and frequent removal of dust, since these types of mould need it to get enough moisture to survive.
Danielle Burke, lead art historian at the Fine Art Restoration Company, says many people forget to look behind their paintings. 'But mould can come through from the back, and then you don't see it until it's too late and already causing cracking or flaking on the canvas. We become so used to the art in our homes that we stop paying attention to it. We really need to monitor it carefully, especially if something is high value – either monetarily or emotionally, like a family heirloom.'
Once spores have become ingrained in the surface of a painting or piece of furniture 'it becomes increasingly difficult to fully and safely remove the mould and prevent it from recurring,' says Sophie McAloone, conservation manager and easel painting conservator at the Fine Art Restoration Company. 'As the mould develops, it feeds on the organic material in the artwork and can cause irreversible damage.'
McAloone suggests keeping a photographic record of artworks so you can immediately spot early signs, like staining or warping. Let it take hold and you'll pay a steep price. McAloone says paper restoration can be upwards of £900 for badly damaged pieces.
Mould is also a health issue, which is another nightmare for museums, observes Bastholm, since by nature they need to be accessible for exhibitions, education and research. 'But if there's huge fungal growth, it becomes a health hazard. You might need to look at precautions like not letting people near items without wearing a mask.'
It's a crisis in the making – and it could be right under our noses.