Museums say 25% to 65%. link (http://www.collectioncare.org/cci/ccier.html
There is no single relative humidity range that is ideal for all museum objects. Recent work by the Rochester Institute of Technology's Image Permanence Institute shows that lowering the relative humidity (RH) and temperature (T) will greatly increase the life of plastics and other organic materials. Relative humidity (RH) should not fluctuate rapidly. For mixed collections, a non-fluctuating relative humidity above 25% and below 65% is recommended. Many museums have set their relative humidity at 45% and gallery temperatures between 65 and 68 degrees Fahrenheit. Lowering the temperature greatly increases the longevity of collections. However, lower temperatures are hard on visitors to the museum.
Fluctuating relative humidity (RH) causes stress on materials. Rapid humidity fluctuation damages a wider range of museum objects than does temperature change. A change in RH causes dimensional alteration in hygroscopic materials (for example, wood, ivory, skin, and other organic materials), resulting in warping, splitting, and delamination of sensitive materials. Seasonal slow drifts are less harmful to structures and objects than abrupt changes. High RH (above 65%) can cause mold growth and metal corrosion. Low RH (below 25%) can cause embrittlement of hygroscopic materials such as leather and paper.
However, if you are in a wet or dry climate, it may not be possible to maintain the ideal RH level. Try to set your relative humidity level so that it is stable somewhere between 25% and 65%. Above 65% mold will grow, more rapidly as the RH rises. Below 25%, the materials may lose structurally important water. If you cannot achieve even these levels, achieve a reasonable level that does not fluctuate. If this level is above 65%, make sure you have good air circulation and regular inspections for mold growth.
All metal except gold is susceptible to oxidation or corrosion. Prevention of corrosion or oxidation is the primary goal in caring for metal artifacts. Most corrosion is caused by moisture although certain chemicals can also play a role. The oils and acids that occur naturally on skin can be very damaging to metal artifacts. One of the simplest ways to help preserve your artifacts is to store them in a relatively dry environment. Typically metal artifacts should be stored in living areas which are much dryer then sheds garages or basements. Attics are generally too hot for most artifacts.
* If you are unsure what to do seek professional guidance
* Do not attempt to clean or polish metal artifacts with out seeking professional advice
* Do not handle metal artifacts with bare hands, gloves should be used
* Generally speaking commercially available dip type tarnish removers should be avoided
* The use of spray-on lacquer or similar type coatings is seldom advisable
* Although some oxidation or corrosion may be damaging to an artifact, they may also add to the value. Restoring an artifact to its original condition is not always the prudent course of action.
Iron and Steel
This is one of the most common metals and is commonly found in firearms, bayonets and swords. Steel and iron, especially those having a bright polish, are very susceptible to rust. The fine polish of a sword blade can easily be permanently marred by touching the blade with bare hands. Always handle metal artifacts with clean cotton gloves. Steel artifacts may be preserved by keeping them oiled with light oil like 3 in 1 or the metal parts may be protected with a coating of wax, such as Johnson’s paste wax. Care should be taken to coat all areas; you may wish to consult a gun smith to help with disassembly of weapons.
Basic Preservation of Wood Artifacts
Wood is a relatively stable material to preserve. Wooden artifact can be maintained for years provided that some basic care and attention is given to their preservation.
* Store wooden items in your home where they are protected from extremes of temperature and humidity.
* Avoid direct sunlight or bright light which will fade finishes.
* Avoid all temptations to over clean or refinish wood items.
* The use of linseed oil or other oil based products on wood items is not required or recommended.
* If wood items are handled a protective coating of wax like Johnson’s paste wax may be applied.
* When oiling firearms take care not to get oil on the grips or stock.