Oxidation vs. Sulfurization: The Truth Behind the Term and Its Chemistry
In the world of stamp collecting and postal history, various terms describe physical phenomena affecting stamps. One such term, "stamp oxidation," has persisted for decades despite its technical inaccuracies. Collectors use it to refer to the darkening of stamps over time, believing it to be a reaction with oxygen. However, the actual process is more accurately described as sulfurization—a scientific term the experts at Professional Stamp Experts (PSE) have begun to popularize to reflect the true chemical transformation. Currently half of the stamp collecting experts seem happy to use the new term while others, including myself occasionally, use the term “oxidation” to speak to a larger audience who have not yet been educated about this chemical reaction.
An understanding of sulfurization enables collectors to better preserve their stamps by reducing exposure to sulfur-containing materials, particularly certain plastics used in mounts, which I have previously discussed as potentially hazardous without proper care.
The Basics: What Is "Stamp Oxidation"?
The term "stamp oxidation" is commonly used to describe the darkening of metallic ink on older stamps. Certain ink colors, particularly metallic ones, undergo noticeable changes over time, leading to a murkier or darker appearance. While oxidation typically refers to a reaction with oxygen, the chemical reality is more complex.
The Chemistry: Sulfurization, Not Oxidation
Rather than oxidation, the darkening of stamp ink results from sulfurization—a chemical process where sulfur atoms replace oxygen in compounds found in the ink. Many early stamps used metallic inks containing lead-based compounds, such as lead oxide, to enhance brightness. Over time, environmental sulfur or sulfur from stamp mounts interacts with these compounds, replacing oxygen atoms and leading to a darker appearance.
Though sulfurization produces a result visually similar to oxidation, the underlying chemical mechanisms differ. Understanding this distinction is important for accurately describing the aging process of stamps.
Treating Sulfurized Stamps
A common method for restoring stamps affected by sulfurization is to use an oxidizing compound to replace the sulfur atoms with oxygen. Hydrogen peroxide is a well-known oxidizing agent, among stamp experts, that can help reverse this process. Since hydrogen peroxide is essentially water with an extra oxygen atom, it can reintroduce oxygen into the ink compounds, lightening the darkened areas and restoring the stamp closer to its original color.
The process to re-oxidize the stamps involves placing a mesh screen over a solution of hydrogen peroxide and allowing the oxygen-rich fumes to envelop the stamp for several hours or even days until the color is restored. This method gradually reverses the sulfurization process, helping bring the stamp closer to its original appearance without direct application of liquid, which could damage the gum or paper.
Here are some stamps that have been oxidized and treated to restore some of their original color. I borrowed this image from oldtriguy1960 at https://www.stampcommunity.org/topic.asp?TOPIC_ID=63355
The darkening of red and orange inks is typically caused by sulfur compounds in the atmosphere reacting with the ink. Given that sulfur levels in the air have significantly decreased over the past century, any re-darkening of treated stamps is expected to occur at a much slower rate than before. While treatments can improve a stamp’s appearance, complete accuracy in color restoration remains uncertain.
Conclusion: A Misleading Term in the Stamp World
The term "stamp oxidation" is misleading, as the darkening of stamps is caused by sulfurization, not oxidation. The historical misunderstanding of this chemical process has led to widespread mislabeling, but greater awareness can improve preservation practices and appreciation for the science behind stamp aging. While "oxidation" may persist in common usage, "sulfurization" is the more accurate descriptor of the chemical changes affecting stamp inks over time.
