Today we’re going to be talking about an agent that I’ve mentioned a few times over the series, and I’ve kept promising you we’d get to: Relative Humidity! But again, like temperature, humidity is not a thing we can escape, so we’re focussing on incorrect relative humidity.
Incorrect temperature and incorrect relative humidity work hand in hand as agents of deterioration because they affect each other, and co-habitate. They’re a team and you can’t split them up. So when you’re tackling one, you can’t forget the other.
Let’s start off with the basics and talk about what relative humidity actually is. Relative Humidity or (RH) is a measure of the amount of water vapour in the air. The humidity is the amount of moisture in the air- so air can be very humid and wet, or not so humid, and dry. We call it relative humidity, because it is dependent on the temperature.
For example, when warm air is cooled down, the relative humidity rises. Think about morning dew on your window – the air was warmed throughout the day and overnight after the sunset, it cooled and the water in the air began to condense. When things are heated, they expand and when they are cooled, they contract, the same goes for water.
Then when cold air starts to heat up, the relative humidity drops. That’s why things get so dry in winter and we need to use humidifiers to bring back a normal amount of moisture in the air.
We can perceive this change in a bunch of ways, and when we talk about percentage of RH, we’re talking about the level of saturation that air at a particular temperature can hold.
I said earlier, we’re concerned about incorrect relative humidity because different collections and materials have different sensitivities to levels of humidity. Because of this, it can be difficult for a museum or archaeological storage facility to find the perfect RH for all objects. The best we can do is try to get the relative humidity stable, and at a decent range for everything in the collection. Luckily, the range that causes minimal damage is a lot wider than we previously thought, so it doesn’t have to be such an exact, panicky science.
There are 4 main categories of relative humidity when we’re talking cultural heritage objects:
Damp, which is over 75% humidity
RH above or below a critical value for a specific object
RH above 0%
Damp causes different types of damage like the speeding up of metals corrosion or mould growth. Damp environments mean there is more water in the air, and we discussed in this video here how water could deteriorate objects. The higher the relative humidity, the faster the deterioration happens, so just like with temperature, and reduction in the humidity in the air can really help prolong the life of an artefact.
A relative humidity above or below a critical value can also be dangerous. For example, salts will either crystalize or deliquesce which means essentially forming a salt solution from absorbing the moisture in the air. If salts are in this water vapour, they can land on vulnerable materials like metal or pottery. In this solution, the salts can really corrode metals, especially iron.
If salts are introduced into pottery, if the relative humidity drops, then the salts will recrystallize. When salts recrystallize, they grow in size, and usually there isn’t’ enough space for this crystal and a lot of pressure starts to happen and they can start pushing out of the surface and taking object material with them.
Unstable glass is also very vulnerable to this critical value. Unstable glass can begin to what we call ‘sweat’ which looks like the glass is actually sweating, with all this goopy watery beads. This happens when certain compounds in the class deliquesce. This happens when the relative humidity is above 55%, but again only for unstable glass, so glass that wasn’t manufactured in with materials or techniques that would make it stable.
When we talk about Relative humidity above 0%, we’re really talking about when any water vapour level is incorrect. This is mostly for acidic paper or old acetate or nitrate films that have a lot of chemicals inside of them and decay very rapidly. The decay that these materials suffer is something called acid hydrolysis. This decay requires moisture to be in the air. Any level of moisture, so the presence of any water vapour will spark a reaction, which is very scary.
Of course a 0% relative humidity is near impossible, so decay is imminent. So in order to preserve the object for as long as possible, we go by the rule of thumb where if the relative humidity is halved, we can decrease the rate of decay in half as well.
Relative humidity fluctuations is quite a dangerous one, because a change in the humidity will change the moisture content in organic materials, which will then cause them to change in size. This doesn’t matter too too much if the material is free to expand and contract on its own. But if there’s anything blocking this expansion and contraction like a museum support, another inorganic part of an object, or even if the object is really bulky, the expanding parts might be crushed or the shrinking parts might crack or break.
Another thing to keep in mind with relative humidity fluctuations is the repetitive stress the object undergoes when dealing with all of this. This constant expansion and shrinking can cause something called fatigue cracking.
So those are all the ways incorrect relative humidity can affect an object. Now let’s talk about where this can all come from. Incorrect relative humidity can be the result of something as simple as the local climate your objects are in. If you are in a part of the world that is super wet and rainy like Amsterdam, then you’ll have to deal with damp problems. If you live somewhere super dry, it doesn’t actually mean that there’s a low relative humidity, it just means that it doesn’t rain very often. You can live in a hot, humid place…. Just look at Florida. It’s usually that high relative humidities are intensified by wet climates and low relative humidities are brought on by cold climates because people like to turn heaters on when it is cold outside.
Other things that can affect relative humidity are faulty heating and cooling systems, bad drainage, basements that suffer from rising damp or condensation on cold surfaces, and micro-climates. Micro-climates are created when things are placed near exterior walls, just above a cold damp floor, or just anywhere that has a certain tendency to be affected by something we can’t completely control and that is a tiny space. They create these small areas that create special relative humidities that you won’t find in other parts of the room.
So how do we protect our collections from falling into the hands of an incorrect relative humidity?
The best ways are to avoid damp as much as humanly possible- ESPECIALLY in museums! Relative humidity control is the best long-term approach while focussing on the reduction of it to materials that are most vulnerable. Keep to the status quo, I guess you can say and keep it as low as possible. As materials will adjust to the humidity it is in, avoid fluctuations at all cost and just stabilise it at much as you can. Low and stable wins the race!
Make sure your collection storage and exhibition areas are well sealed from any unwanted outdoor fluctuations, and regularly monitor your RH using a handy dandy device like this, or a much lower tech paper like this: We also use bags of silica gel in cases and storage in order to absorb excess moisture in the air. The best thing about silica gel, is that you can buy it in packs that are calibrated to a specific relative humidity, and then you can reuse them by drying them out in a low temperature oven!
Use humidifiers and de-humidifiers in areas of the museum you need them most, especially if you have one space that requires a very low relative humidity. These are also very useful in winter and when seasons are changing, because the atmosphere will be fluctuating like crazy.
Remember, the damages caused by incorrect relative humidity are ones that you can’t really come back from. Mould damage is forever, and you can’t un-corrode metal.
If you are a conservator or you want to be a conservator, you will definitely be hearing the term RH anywhere and everywhere so you really need to make sure you become familiar and aware with what it is and how it can affect cultural heritage objects.
Looking to find out more?