Everything you never knew that you wanted to know about relative humidity
Birmingham Museum Trust’s sites might currently be closed, but for the Conservation team, the work of preserving the collections continues.
Conservation can be broadly split into two disciplines:
- Interventive Conservation – working directly with objects to stabilise or repair deterioration.
- Preventive Conservation – controlling the environment surrounding objects in order to prevent deterioration from occurring in the first place.
With the museum sites closed to the public and most staff, it is important that preventive conservation work continues, in order to safeguard the collections for when everything gets back to normal.
One of the primary environmental factors which preventive conservators are concerned with is ‘Relative Humidity’.
What is relative humidity?
When we talk about ‘humidity’, what we often mean is ‘Absolute Humidity’.
Absolute humidity is the total measure of water vapour in a volume of air (expressed as grams per cubic centimetre). This is impossible to measure practically, because to find it you need to pull all of the water out of the air. Imagine that you want to find the absolute humidity of the air in the room at the moment – you would need to pull all of the air out of the room, and collect all of the water from that air. This is a problem if you also want to be breathing in the room at the same time.
Instead, conservators are concerned with ‘Relative Humidity’.
Relative humidity is the amount of water vapour in a fixed volume of air relative to the amount of water which could be held by that air at a given temperature. It is expressed as a percentage:
- Amount of water in the air ÷ Amount of water which could be held in the air at the current temperature x 100 = %RH
That sounds complicated, but it’s easier to visualise:
If we were to decrease the temperature back to 20°C, then the amount of water in the air relative to the amount of water which could be in the air would increase again.
As the temperature rises, the relative humidity of a sealed body of air will fall.
As the temperature falls, the relative humidity of a sealed body of air will rise.
What can go wrong if we have an inappropriate relative humidity?
High humidity can lead to:
- Corrosion in metals
- Salt efflorescence
- Organics expanding/warping as they absorb moisture from the air
- Mould growth
- Increased risk of pest infestation
Metal Roman grid iron from a damp archaeological environment, which is heavily corroded and leaching salts.
Mouldy kitchen units in a damp store.
Marquetry table top - the inlayed wood has expanded as it absorbed moisture, and popped out of the surrounding surface.
Low humidity causes:
- Organic materials to shrink, crack, and split as they release moisture back into the air
Skin splitting around the eyes of a piece of taxidermy as it shrinks over the former beneath.
Fluctuations in relative humidity will cause organic objects to repeatedly expand and contract, causing warping and cracking. Fluctuating relative humidity is particularly problematic because materials will be in constant flux and unable to reach an equilibrium with their environment.
Most collections, and many individual objects, are composed of a mixture of both organic and inorganic materials.
The ideal relative humidity for metals is low; whereas the ideal relative humidity for organic materials is higher.
Therefore we usually have to compromise, and aim for a stable relative humidity between 40 and 65%.
How do we measure relative humidity?
We have a variety of tools which allow us to measure relative humidity.
Some of these give us spot readings – these can only tell us what is happening at the moment and location that the reading is taken. Other tools allow us to record changes in relative humidity over a period of time. These methods require the gathered data to be collected or downloaded at regular intervals.
More sophisticated monitoring systems allow data to be viewed remotely in real-time, as well as recorded over time. We use a system of telemetric monitors which take readings at pre-set intervals and then transmit that information to a central hub. The data can be viewed from any computer with access to the server, allowing a single person to either see what is happening at any Birmingham Museum site as it happens, or view data for any past period.
Relative humidity may also be affected by factors including the weather - if it is raining outdoors, damp air will enter the museum, and visitors will bring water indoors on wet coats and shoes; and the number of visitors in a gallery, who will all be breathing out moist breath and perspiring.
Relative humidity is generally more stable in an enclosed environment; whether that is a building or a display case. It is also far easier to control the relative humidity in a smaller volume of air.
How do we control relative humidity?
There are a range of ways that we can alter the relative humidity within a gallery.
Some of Birmingham Museum’s sites have conservation heating systems, which automatically alter the temperature in order to raise or lower the relative humidity, based upon the data collected by our environmental monitors.
Desiccant gels such as silica can be used to absorb moisture from the surrounding air and lower relative humidity. They are usually seen in cassette form inside display cases, which act like larger versions of the small sachets of silica that you get in shoe boxes. To condition anything larger than a display case requires an impractically large quantity of desiccant however, so it is not generally used to control the RH in whole galleries.
If we want to raise the relative humidity we can use humidifiers, which work by pumping water from a reservoir to a fan which blows it out into the air as vapour.
Those who have visited Birmingham Museum and Art Gallery’s picture galleries will likely have seen and heard some of our humidifiers humming away at their work.
One of our humidifiers in the galleries at Birmingham Museum and Art Gallery.
Ideally, we aim to reach a point where the museum environment can maintain a stable relative humidity without the need for our direct intervention.
Even while Birmingham Museum Trust’s sites are devoid of visitors and most staff, the conservation team continue to monitor the relative humidity in order to safeguard the collections and ensure that no harm comes to the objects during this time of lockdown.