Preservation and conservation of historic and artistic objects, artifacts, and structures is deeply enmeshed with the "environment". In conservation we define the environment as the local physical space in intimate association with an artifact or object; this can be mostly air with it's chemistry, moisture, and particulates, or, it can be soil or seawater. It can be a very complex and technical subject but often what is most important are core principles. If the environment around an object is steady and stable for long periods of time the object reaches an equilibrium with it and the condition of the object can be very stable with a low rate of deterioration. In fluctuating or unstable environments damage and deterioration is often greatly accelerated. One of the most dramatic examples of this are marine archaeological artifacts. An excavator may find a wooden artifact in near perfect condition because it was quickly buried in silt which was oxygen-free (anaerobic) and not in seabed areas with worms or shifting currents where artifacts could be disturbed. Over hundreds or thousands of years the artifact has lain in a very stable environmental cocoon. Once the artifact is uncovered and raised above the water it's environment has changed radically. It is surrounded in a "sea" of air which is drier than the waterlogged wood, it is surrounded by oxygen, and most likely surrounded by higher temperatures. The artifact will immediately begin to loose moisture, salts from ocean water will begin to migrate and effloresce, oxygen and higher temperatures will accelerate chemical processes, and mold and fungi may begin to form. Most organics including wood are very sensitive and responsive to moisture - physically swelling or shrinking. So in a matter of days our once pristine wooden artifact may be distorted, splitting, and falling to pieces. A slower but no less significant process happens to historic objects and structures. A wooden chair will respond to a change in environment when the relative humidity changes by either swelling or shrinking. So, when you are told that 50% relative humidity is what is recommended for furniture you have to first and foremost understand the environment where the chair has been - this may be fine for a chair from a moderate and temperate environment like the East Coast of the USA but it would be disasterous for a chair from arid regions like the Southwest with it's far lower relative humidity. Similar things happened 30 years ago when historic houses started putting in museum quality HVAC systems - and the change of relative humidity in the interiors in contrast to the exteriors started causing all kinds of visible damage after a short time. So knowledgable conservators do not follow an absolute published number for RH (though for light levels and exposure we do) - we have to first and foremost keep the core principle of environmental stability in mind - or simply put, keeping it "Relative". What about our pristine marine wooden artifact? First, keep it wet - preferably in sea water at first. If it came from cold water keep it cold. And then it becomes a matter of treatment or intervention. Soaking it in purer and purer water to remove salts and then adding a bulking chemical such as PEG (Polyethylene Gylcol) which, at the right molecular weights, will replace water in the cells of the wood and minimize dimensional shrinking. In some instances using freeze-drying after the PEG treatment to sublime off the water is the best method for a waterlogged artifact - for larger artifacts this is impractical. Then our artifact will have to be slowly acclimated and housed or exhibited in an enviroment that will minimize dimensional change while still inhibiting conditions for mold and fungal growth. And it has to be very closely monitored as there may be chemical changes that don't become apparent until decades later (the conserved ship Vasa is a case in point). Notice that we have to put a lot of energy, time, and expense into the process to preserve something that was once very stable. Often that is what conservation and preservation is all about.
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