Metal Care


Metal Handling and Storage

Some time ago we ran an essay on the preservation of leather products. Since that time we have been asked for information on metal preservation. This is so good we wouldn't add a thing to it so please excuse that it is seen here reprinted in it=s entirety. Excerpted from the National Park Service Museum Handbook. 


The Curatorial Care of Metal Objects

Deterioration of Metallic Objects

Metallic objects can be easily damaged by improper storage or exhibit conditions. Common problems include poor support, high relative humidity, air pollution, and incompatible storage cabinet or exhibit case materials.

Hardwoods such as oak can produce acidic vapors which can corrode lead and silver. A common observation in older museum cases is the formation of a white lead corrosion on lead artifacts such as minie balls. The adhesives, paints, woods, and textiles used in more recently constructed cases have resulted in similar problems. However, good storage and exhibit case construction affords an opportunity to implement a wide range of prevention conservation procedures that result in the creation of environments which are metal friendly.

The best materials to use for a display or storage case are metals. Metal is far less problematic than organic compounds such as wood.

While metal is a reactive material, in most cases, when rust or corrosion occurs there will be an environmental factor. When caring for metal objects remember: Do not just treat the symptom, but address the cause of the deterioration.

The most common problem facing the collector who has metal object is corrosion. Many factors can contribute to corrosion; the type of metal in the object, how it is exhibited or stored, and the general environment. While you may have no objection to using a commercial brass polish on your new brass bed, most collectors of militaria would shudder at the thought of applying such a product to a 19th century European parade helmet or fine sword details.

In fact, the most commonly observed mistreatment of historic metal objects comes not from nature but from cleaning and neglect. Over-cleaning often results from a desire to have metals bright and shiny, especially brass and silver objects on display. Items such as swords, daggers, firearms, and medals are obvious victims of this well-meaning but misguided attempt at preservation.

Archaeological artifacts are often over-cleaned because of a desire to see what is under the layer of corrosion, or because of the mistaken assumption that corrosion is like a "cancer", and unless every last bit of it is removed, the object will continue to corrode.

Neglect too can often cause irreversible damage to a metal object. Until recently, the green and black corrosion products commonly observed on bronze sculptures were mistakenly believed to be protective by some people.

There are certainly instances in which the corrosion layer is protective, aluminum oxide being one example. Unfortunately, most historic metals do not form protective corrosion layers. Historic objects that have corroded are best left untreated until a metal conservator has had an opportunity to examine them. This is far better than the risks of over-cleaning, and the consequences of higher corrosion rates on freshly exposed metal surfaces. Remember, conditions seldom remain static, so when an object appears to be stable, monitoring and maintenance are essential elements of the collection maintenance program.

A precedence for appearance and a failure to understand the corrosion process has resulted in another form of mistreatment. Objects have been sent to conservators for treatment and then reinstalled under the same conditions which were responsible for the initial damage. Conservation treatments is frequently the wrong option for many objects. It is labor intensive, time consuming, and expensive. Treatment is never a permanent solution to the long term preservation of an object, and even if treatment were, there will never be enough conservators to treat all the objects that need attention. As with other types of museum object materials, practicing preventive conservation is essential to the curatorial care of metal objects.


Preventive Conservation

There are practical methods for the care and maintenance of the majority of metal objects. An object which is in an active state of deterioration can frequently be stabilized by isolating it from harmful agents of deterioration. The first step in proper care is to understand and to control, the extent feasible, the harmful environmental factors (e.g., relative humidity and air pollution). A second, more manageable step is to ensure that proper exhibit cases or storage cabinets are used. Cabinets can be easily conditioned with desiccants such as silica gel, pollutant absorbing material such as activated charcoal paper, and/or vapor phase corrosion inhibitors. The third step focuses on the object itself.


Rules for Handling Metal Objects

All of the general rules for the safe handling of three dimensional museum objects apply to metal objects. Two special concerns for metals are weight and skin contact with bare metal surfaces.

Keep movement and handling to a minimum. Frequent handling increases the risk of eventual damage. One method to reduce the need for frequent handling is to apply protective coatings so that objects will not have to be polished on a regular basis. Objects made of precious metals, such as sterling silver, are often put into drawers or safes at night. This practice greatly increases the chances for damage and wear. If such frequent handling is necessary, prepare special padded trays and carrying boxes.

Whether the object is going a long distance or a few feet, plan each move. Metals can be heavy. The inadvertent placement of a metal object on another object or on a period piece of furniture may result in dents, scratches, or staining.

The site to which the object is to be moved should be prepared before hand. Space should be cleared, and an inner buffer such as mylar or blotting paper put down prior to the move. The route of the movement should be free of obstacles.

A variety of gloves should be kept on hand. Smocks without buttons should be worn when working with metal objects. The smock prevents the scratching of objects by belt buckles and other accessories, especially in those instances when the object is large and likely to be held against the torso during a move.

In addition to cotton gloves, (These are the white gloves always used by curators in museums when handling relics or artifacts) polyethylene gloves are useful when polishing or waxing a metal object. If there is concern that the metal surface may be too slippery for cotton gloves, or that a rough metal surface may snag the cotton glove, place the object in a padded box or tray for movement.

If it is necessary to pick up an object for inspection, hold it over a table or some other piece of furniture, and not over the floor. Also, have on had a jewelers cloth for uncoated gold, silver, copper alloys, and highly polished steels. If an object has to be handled or is inadvertently touched, any finger prints can be easily buffed away with the jewelers cloth. These cloths usually contain a fine abrasive.

The advantage of a jewelers cloth is that it permits localized polishing, without the mess of liquid or paste polishes. Some polishing cloths contain no abrasive and rely on the stiffness of the weave for their polishing effect. Any clean, soft cloth may be used to buff an object after a move or handling. Jewelers cloths are available at may jewelry stores.


Environmental Conditions

When discussing the environmental conditions it is important to note that the lower the relative humidity, the better. Steel will not rust and brass will not tarnish below 15% RH. The best temperature to store metal objects is between 60 and 80 degrees F. In salt air environments, the best thing to do to prevent active corrosion is to house the items in spaces that do not allow RH levels to exceed 35%. As far as temperature goes, it is best to avoid low temps for most metal objects. This is because low temps usually result in higher levels of relative humidity, and the possibility for condensation to form upon metal surfaces. Low temps should be avoided for tin alloys. Another environmental threat is air pollution. Polluted urban air and coastal environments are among the most severe areas. Dirt and dust may contain chemical compounds that will react with metals and trap moisture close to the metal surface. Keep your items dusted and protected if possible. A common pollutant is cigarette smoking. NEVER allow smoking, eating, or drinking around valuable artifacts.


Storage Techniques

Obviously, the best place to house metal objects is in an indoor environment to reduce the possibility of corrosion. (This is a major problem for museums such as The Aberdeen Proving Ground Museum. They have an outstanding collection of rare armored vehicles but do not have an indoor facility to protect them. If that doesn't change soon, we WILL lose those rare and valuable vehicles.) Never store metal objects directly on the floor, or in close proximity to exterior walls. Avoid storing metal objects in attics and basements. These areas are conducive to condensation.

One factor which is usually not a concern for metal objects is lighting. Most metals are not affected by visible light, ultra violet radiation or infrared radiation. However, paint, and other coating systems may be affected. Unlike cloth which is susceptible to moths and other pests, metals are normally immune to these attacks. However, droppings of mice, and even insects can cause pitting and corroding of a metal object's surface.

The best storage medium for metal objects is steel shelving. It is preferred over wood because it is stronger and does not emit harmful vapors. Line shelves with an inert, nonabsorbent material such as expanded polyethylene. Loosely drape over shelves, clear polyethylene to protect metal objects from water leaks, dust, and air pollution. Insure objects are placed so that they do not bump into each other when picked up or moved.

One method you can use to limit the amount of humidity is to purchase and run a dehumidifier. Another method is by using Silica gel packets. They are used to reduce and buffer the relative humidity in a closed space. (If you use Silica Gel packs, keep an eye on them as they can become saturated and then become a source of moisture.)

You can also place artifacts inside clear polyethylene bags to create a clean micro-environment plus it is easy to monitor the conditions inside the bag. In wintertime, never allow the temperature to fall below freezing in your storage space. Try to keep stored items in the center of your storage room. Again, keep your artifacts away from exterior walls, fireplaces, and entry halls. Insure you monitor your facility during peak condensation times which is the transition period between winter and summer. Make sure you dust the stored items from time to time as dust is abrasive and can cause chemical pollutants.


Exhibit Techniques

An exhibit case present the ideal means for object preservation and display because it is generally limited in size, is usually sealed to some degree, and can be modified to contain a variety of moisture controlling materials. Case controls can be either passive or active. A passive display case uses trays of silica gel, pollution control agents, and other corrosion inhibitors to protect artifacts. Active cases usually employ small mechanical systems which may be built into the case such as dehumidifiers or filters. Electronic sensors can be concealed in the case to control temperature and moisture.

Lighting is an important consideration. Some fixtures such as fluorescent light ballast's or transformers may generate ozone. Ozone will cause corrosion on metal surfaces. Light fixtures should not be installed directly in an exhibit case.

Housekeeping procedures are an important element in the preventive conservation program. Many cleaning products contain ammonia, weak acids, solvents, waxes, and fats which may have an adverse effect on metal objects. Use caution when using spray air fresheners and other cleaning products. Try to keep items in cases which prevent dust and dirt from entering them. If steps need to be taken to clean an object consultation with an expert in restoration is advised.


Like we said, this is from the professionals that have in their hands some of the most important artifacts the country possesses. It should be good enough for our knives and bayonets too!