If you are going to use some type of display case to display or store your coins, It is important that any materials in intimate contact with your coins are safe and will cause no harm to the coins. The material must be acid-free and stable. The question is; how do you know if a given material is safe?
One of the most common tests used to determine this is what is known as the “Oddy Test.” The Oddy Test is named after Andrew Oddy, keeper of conservation at the British Museum. Oddy developed this test several decades ago.
Although the test has been modified somewhat over the years, the test has become and still remains, a conservation standard. The Oddy Test is basically an accelerated corrosion test, meaning that it determines in a short time whether a given material will be corrosive over a longer period of time.
The Oddy Test is designed to help one choose appropriate materials in advance. The idea is to be able to determine if a material is corrosive before it comes into contact with your coin collection.
The Oddy Test takes 28 days to complete. The test involves placing a small sample of the material being tested in an enclosed space with a metal coupon. The metal coupon represents your coin. If the metal coupon is corroded at the end of the test, the material is not considered safe to use.
Alternatively, if the metal coupon remains uncorroded, the material can be considered safe to use. The metal coupon is suspended in the container and is not usually in direct contact with the material being tested.
Direct contact with the material is not necessary, as the materials being tested are corrosive because they give off vapors such as acids, sulfur compounds, formaldehyde, chloride and nitrogen oxides.
A glass tube, jar, or flask is used for the test. The container should be small to ensure an accurate result. The container is sealed either with a ground glass stopper or a lid made from a non-corrosive material. It is important to hold the test in a non-corrosive container so as to not skew the results of the test.
The metal coupon used is cut from a thin Anala R foil. Anala R foil is extremely pure, being 99.5 percent pure or better. The type of foil used in the test should reflect the material of your coin collection.
For example, copper foil is used for copper alloyed coins, silver foil for silver coins, and lead for lead coins and other white metals.
The metal coupon is suspended in the container on a thin piece of nylon monofilament. Preparation of the metal coupon is absolutely crucial. The surface of the metal coupon should be lightly scratched by using a glass bristle brush and is then degreased in acetone. The container being used should also be very clean and free from contaminants.
The process of corrosion is accelerated in several different ways.: First, the relative humidity within the container is elevated by adding a piece of cotton batting soaked with distilled water. Second the container is placed in a warm area to elevate the temperature. A higher temperature helps the material being tested to emit its corrosive vapors at a quicker rate than would normally occur.
The container is sometimes placed in an oven set at 140 degrees Fahrenheit while another option is to place the container in or on a window sill. A control sample—a metal coupon suspended in a container with damp cotton batting and no test material—should also be run. This allows you to determine if corrosion is happening simply due to the presence of humidity itself or due to the material being tested.
After 28 days, the metal coupons are removed from the containers, and examined under magnification and compared with the control sample. If the metal coupons show corrosion, then the material being tested is considered unsuitable to use. The Oddy Test is often used in conjunction with other tests. The tests depend on the time available and the makeup of the artifacts.