Preserving your Coca-Cola artifacts. © 2012 Blaine Martin
While many collectors show no regard for how their collection is maintained, there are many of us who care greatly about the conservation of the items in our care. Since many of these items are over 50 years old and rare or fragile, it only makes sense to take whatever steps we can to preserve these items. Below I have gathered very basic information addressing preservation concerns faced by collectors.
The energy in light reacts with the molecules in objects causing physical and chemical changes. These changes cause fading, darkening, yellowing, embrittlement, stiffening, and a host of other chemical and physical changes. Lithographed Coca-Cola items such as trays, posters, ads and other items such as photographs are particularly sensitive to light damage.
The non-visible light spectrum
Because humans only need the visible portion of the spectrum to see, we can easily limit the amount of energy that contacts objects by excluding UV (ultraviolet) and IR (infrared) radiation. All types of lighting (daylight, fluorescent lamps, incandescent (tungsten), and tungsten-halogen lamps) emit varying degrees of UV radiation.
This radiation is the most damaging to objects. and should be completely eliminated by installing the following filters: UV filtering film for windows or glass on framed objects, UV filtering plexiglass instead of glass, and UV filtered fluorescent tubes or filter sleeves for fluorescent tubes.
The visible light spectrum
The strength of visible light is referred to as illuminance and it is measured in lux. Lux is the amount of light flowing out from a source that reaches and falls on one square meter. It is the light that falls on the object that is measured not the light at the source. 50 lux is generally considered the maximum for items such as lithographs, photographs, and leather. 200 lux is considered the maximum for finished wooden surfaces. 300 lux is considered the maximum for metal and glass items.
As a point of reference – sunlight on an average day represents more than 30,000 lux, TV studios are lit to about 1,000 lux, and a a bright office is about 400 lux. Ambient outdoor light at sunset and sunrise is about 400 lux, while a typical museum setting is lit to about 50 lux, and moonlight is only 1 lux.
Remember that damage from exposure to light is cumulative and cannot be reversed. Even small amounts of light cause damage.
To limit visable light damage we have two options. Number one: Reduce the amount of light falling on an object (a 200-watt light bulb causes twice as much damage as a 100-watt bulb in the same amount of time). And, number two: Reduce the exposure time of an object (a Coke cardboard on display for six months will fade about half as much as it would if left on display for one year).
Visible light can be controlled by the use window coverings such as blinds, shades, curtains, and tinted films or glazing on windows. Close window coverings when the room is not in use. Opaque dust covers, such cotton muslin, can be used to cover particularly light-sensitive objects when you are going to be away for extended periods.
The amount of light from fixtures can be reduced by using colored filters, lowering the wattage of incandescent bulbs, using fewer fixtures, using flood light bulbs instead of spots, and turning off lights when people are not present. Incandescent lights produce very little UV and should always be used instead of fluorescent lights.
At higher temperatures, chemical reactions increase, and chemical reactions cause deterioration. As a rule of thumb, most chemical reactions double in rate with each increase of 10°C (18°F). In areas where personal comfort is a factor, the recommended temperature level is 64-68° F, and should not exceed 75° F. Abrupt or fluctuating changes in temperature can cause materials to expand and contract rapidly, setting up destructive stresses in the object. Temperature is also a primary factor in determining relative humidity levels.
Relative humidity is important because water plays a role in various chemical and physical forms of deterioration. We use relative humidity to describe how saturated the air is with water vapor. “50% RH” means that the air being measured has 50% of the total amount of water vapor it could hold at a specific temperature. It is important to understand that the temperature of the air determines how much moisture the air can hold. Warmer air can hold more water vapor. These RH percentages are considered ideal for the following materials: Paper 45% - 55%, Photographs 30% - 40%, Wood or leather 45% - 60%, Metals <35%, Glass 40% - 60%.
High humidity: When relative humidity is too high, chemical reactions may increase, just as when temperature is elevated. High humidity can cause chemical deterioration to proceed more quickly. Metal corrosion or fading of dyes and paper buckling are examples. Mold growth and insect activity is more likely as RH rises above 65%.
Low humidity: Very low RH levels cause shrinkage and cracking of wood or leather; and drying up of paper and adhesives.
Variable humidity: Changes in RH cause artifacts to continuously expand and contract causing slow deterioration. The crackling of paint on older Coca-Cola trays is a good example of this. Ideally, fluctuations should not exceed ±5% from a set point, each month. Humidifiers or dehumidifiers and tight, well constructed display cases can help control humidity fluctuations in a home. It is quite normal for a home to have higher RH in the summer and lower in the winter. These variations should be slow and gradual over several weeks.
Acid Content of Paper
Acids are the primary cause of paper deterioration because they cause paper to become weak, brittle, and stained.
The source of acids in paper include:
– materials used in the paper making process, especially after 1850
– residual bleaching chemicals, air pollutants
– direct contact with acidic materials (file folders, adhesives, mat boards,wood, unstable plastic sheeting)
– exposure to acidic vapors from closed document boxes or wooden storage drawers
Acids migrate from acidic materials such as frames or incorrect matting materials to objects of reduced or no acid. Generally, the higher the humidity level the faster the migration.
The concentration of acid is measured on the pH Scale, with numbers ranging from 0 to 14. The number 7.0 on this scale is neutral. All numbers below 7.0 indicate an acidic condition. All numbers above 7.0 indicate a basic or alkaline condition. The pH scale is logarithmic—a paper object with a pH 3.0 contains 10 times as much acid as an object with a pH 4.0, and 100 times as much acid as paper with a pH 5.0. A desirable pH range for paper is 6.5 to 8.5.
Acid-free and archival are general names applied to a variety of plastic and paper products designed for use in proximity to museum objects. When used for the sleeves, boxes, and folders that store paper objects, or in framing for display, these products must be free of acid, lignin, alum, and sulfur.
Acid free is a general term indicating that the product is free of acids, or has a pH of approximately 7.0. This term is often used incorrectly to describe materials that contain alkaline buffers.
Alkaline-buffered products contain an alkaline compound (such as calcium carbonate) designed to neutralize any acids that are present, or retard the evolution of acids in the future. Alkaline-buffered products are often used to interleave between paper objects that might transfer acids to surrounding objects, and generally are in the pH 8.5 range.
Unbuffered products have no alkaline reserve. Generally, they are a neutral pH (7.0) or acid-free material.
100% rag paper products are made exclusively of cotton fibers. Although of good quality, 100% rag paper products do not meet the standards of permanence necessary for the long-term storage of museum collections.
Lignin-free and pure alpha-cellulose are terms used to describe some manufactured storage papers, boxes and cardboards. These are made from wood pulp fibers that remain after the lignin is chemically removed.
pH Neutral applies to materials that have a pH of around 7.0. This term applies to both paper and plastic materials.
Inert is most often applied to materials like polyester film and acrylic sheets like Plexiglas. It means that the material will not react chemically with your paper objects All plastic materials used for collection storage must be chemically inert. Acceptable plastics are free of powders, coatings, plasticizers, and other additives. These acceptable plastics include – stable polyester, polyethylene, polypropylene (archival polyester film is the housing material most commonly used for paper collections. It is one of the most dimensionally stable and chemically inert plastics available.). Polyvinyl chloride (vinyl, PVC), polystyrene5 are NOT acceptable for use.
Most of the information in this article was gathered from various publications provided by the U.S. Department of the Interior's National Park Service Museum Management Program
For more detailed information about preservation and archival matting and storage — check out these informative links—
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