As Knole currently lacks environmental control in the majority of our show rooms, mould is a big problem for us. The levels of relative humidity (RH) are often above 65% and some rooms have been known to reach 100% RH. Relative humidity at these levels creates the perfect conditions for mould growth.
To grow it also needs warm temperatures of between 10 and 35°C and prefers dark spaces. The mould also requires a food source. This could be the mouldy object itself, particularly when made of organic materials such as paper, leather, wood or textiles. In some cases, the food source is organic dirt on the surface of an object.
Our paintings seem to suffer the most. Some of textiles have mould too. We have also recently discovered some new dry rot in the stair well of the Great Stairs.
Mould fact file:
Mould is the common name given to the visible growth of fungi on materials and is usually seen as fluffy white or coloured spots on a surface. They are a complex group of organisms which are neither plant nor animal, but which get their food from living or dead material.
How does it grow?
Mould begins life as a spore (comparable to seeds in the plant kingdom). Spores are minuscule and are ever present in the air around us. In very humid air, the concentration of spores is much higher.
Because of their tiny size, they are carried by air currents and only settle on surfaces in very still air. They may stay dormant for long periods of time, waiting for favourable conditions to germinate.
Once germinated, the mould produces string-like filaments known as hyphae. This is the growing stage of the mould. When many hyphae come together, the mass is known as mycelium, the visible part of the mould which can produce and release spores into the air to start new colonies.
What damage can be caused?
As well as having adverse affects on human health, particularly those who suffer from allergies or asthma, mould can be very damaging to a historic collection.
As it grows, the mould emits enzymes that then break down the biochemical bonds of the material on which it is growing to provide a soluble food source. The growing hyphae can also physically break the material apart so can cause a material to weaken and become brittle.
The chemical changes caused by the growing mould can also affect the chemicals in dyes used in textiles, altering the appearance of colours.
How can mould be controlled?
Long-term prevention of mould and control of outbreaks is only possible through effective environmental control. Heating controlled by humidistats that maintains the RH below 65% will keep the environment of the show rooms drier and less appealing to mould.
The use of fungicides or fumigants to treat affected objects is not recommended as it may only offer temporary results. The chemicals used in such products are often toxic and have the potential to cause further damage to objects. Here at Knole, full environmental control is not yet possible, but there are things that we can do to help.
– Keeping objects as clean as possible and keeping dust brought in to the show rooms to a minimum through regular and thorough house keeping.
– Improving ventilation and ensuring a good air flow.
– Regularly inspecting objects and materials, including small areas with limited ventilation eg. behind paintings, back of book shelves and inside furniture.
– Regularly monitoring RH levels.
– Keeping the fabric of the building in good condition and watertight where possible to reduce damp caused by leaks in the roof or gaps in windows.
– Keeping materials off the floor and away from outside walls.
– Using covers that allow for air flow when storing items.