Agents of Deterioration: Pollutants

Today we’re talking about something that doesn’t just affect cultural heritage objects, but in fact impacts each and every one of us: Pollutants.

Pollutants is a very large umbrella word for what we’re going to be getting in to because pollutants covers things such as gases, liquids, aerosols, particles, dust… you name it. Anything that can have a chemical reaction with any part of an object.

Under this umbrella term we have three categories:

  • Airborne pollutants

  • Pollutants transferred by contact

  • Intrinsic pollutants.

Airborne pollutants can be anything from the atmosphere, so ozone, hydrogen sulphide, sulphur dioxide, nitrogen dioxide, or even particles like soot and salt that can get in the air. Then we have airborne pollutants that can come from people or machines or anything that will emit gases, which could be organic gases, organic bases, or more sulphurous gases.

There are a bunch of examples of airborne pollutants, so as not to bore you I’ll only be going in to a few- just remember to look into this more if you’re concerned about your object environment. Actually, just remember to look into pollutants more because this one involves a lot of chemistry and it definitely needs more attention if you’re planning to work with archaeological collections or work in a museum.

Acetic acid happens mostly indoors and comes from things like acidic wood display cases. Overtime, they will start to emit this acid, and materials such as lead can be highly susceptible to it. Lead starts to degrade super fast when in contact with acetic acid.

Hydrogen sulphide is easy to identify because it smells like farts. YEEEEP that rotten egg smell is a gas that can be quite the pollutant. This stuff can tarnish silver and gold, and it can even darken lead white pigments in paintings. And guess where a lot of this gas comes from when inside a building…. People. Yup. We gassy as heck. I have now cursed you all with the knowledge that every time you pass wind in a museum, you may be contributing to object degradation. You’re welcome.

Fine particles like dust can also be dangerous to collections. Fine particles can be of various microscopic sizes. The smaller the particle, the harder it is to protect against or get rid of. Other examples of particles could be salts, particles that form crusts, sulphate compounds, all the stuff that floats around in the air. Fine particles can settle on objects and discolour the surface. The more porous the surface, the harder it is to clean out those particles because they like to get in to all the nooks and crannies.

Some particles can be oily or metallic which can also speed up deterioration, because some things don’t react well to these materials. Oily particles are also sticky and even more particles will attach themselves and create even more nastiness to deal with.

Water vapour is also an airborne pollutant and it can cause both physical and chemical damage. Some materials don’t like to be wet, like things made out of cellulose. Even though water vapour isn’t full on water, if there’s enough of it that lingers, it can really start to affect sensitive materials. Then of course there’s the chemical damage that can occur like the formation of mould and bacteria.

Moisture from your breath, when you are speaking ‘moistly’ can send water vapour on to materials and just wreak havoc. That’s why they started limiting the amount of people to see cave paintings in France. Too many people were going in, speaking moistly or breathing heavily and creating a very humid environment that is detrimental to the conservation of those prehistoric works of art.

Oxygen, surprisingly enough is also an airborne pollutant that we can’t do anything about because….. we can’t live without it! So this is one that we just have to live with, but luckily it’s not a key pollutant or one that is extremely dangerous. Oxygen helps with the breakdown of organic materials, and adding oxygen into a chemical process can speed up the degradation process like the rusting of iron or the tarnishing of silver.

Pollutants transferred by contact are introduced by materials with potentially harmful components coming in to contact with an object. These could be things like plasticizers from PVC, sulphur compounds in rubber, staining materials from unsealed wood, old foam, paper clips on paper, tape or other types of adhesives on objects or all the fats on your little fingertips there coming in to contact with an object. Various pollutants will affect various types of objects in various ways, but they mostly cause discolouration or staining.

Intrinsic pollutants are pollutants that come from the object itself. This means, for example, maybe you have a leather book that has a copper or brass inlay or some design on it. Like the book from the never-ending story. The materials that are used in tanning leather can be quite acidic, so it will kick-start the process of metal corrosion on the book decoration. We can also think of some inks like iron gall ink on paper.

There are also things like secondary intrinsic pollutants that fall in to this pollutant category. These are pollutants that are formed in objects during degradation such as acetic acid, which comes out of things like wood and cardboard if they get moist. These types of acids can speed up degradation being caused be other pollutants like oxygen or water vapour.

So how do we protect ourselves from all these pollutants?

With airborne pollutants, you want to determine the “lowest observed adverse effect dose” which essentially means keeping the object in an environment with these pollutants for a calculated amount of time before the pollutants start to have any negative effects on it. It’s another reason why museum collections go on rotation, because storage environments are a lot easier to manage than a public museum space, and we can make properly regulated and monitored microenvironments in storage that can better preserve an object. The key thing to keep in mind is which materials are most vulnerable to each pollutant and then plan accordingly. Keeping objects inside is already a good start in mitigating airborne pollutants so even if your storage area isn’t ideal, it’s still better than nothing.

For contact pollutants, it’s pretty straight forward. Avoid contact with potentially harmful materials and make sure you’re using the correct glues and adhesives in accordance to what you’re treating. Use acid free paper in between your objects, and use products that won’t produce secondary pollutants like glass.

Intrinsic pollutants are tricky because a lot of them are caused by the manufacture or materials used by the artist, so there is not much you can do. There are methods such as the deacidification of books which help, but the main thing is to keep objects cool and dry so as to not speed up the degradation process.

Looking to find out more?

Canadian Conservation Institute- Agents of Deterioration: Pollutants by Jean Tétreault

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