Pewter is a soft, malleable metal alloy primarily composed of tin, used to create decorative items and tableware. While the typical oxidation process results in a soft, dark gray layer, a green discoloration can appear under specific circumstances. Understanding the chemical makeup of modern pewter helps explain why this material usually retains its color and sheen.
The Composition of Modern Pewter
Modern pewter is an alloy that is overwhelmingly tin-based, generally containing between 85% and 99% tin (Sn) by weight. Secondary metals are added as hardeners to give the naturally soft tin more strength and durability. A common contemporary alloy mixture includes 91% tin, 7.5% antimony (Sb), and 1.5% copper (Cu).
The addition of antimony and copper increases the alloy’s structural integrity, allowing it to hold a shape and maintain a brighter, more silver-like appearance. This modern formulation is also lead-free, unlike historical pewter, which often contained up to 40% lead. Older, lead-containing pewter tarnished more quickly and was unsafe for use with food and drink.
Standard Tarnish vs. Patina
Pewter undergoes a slow oxidation process that creates a protective surface film known as a patina. Instead of a rapidly forming, dark sulfide layer, this patina is a uniform, dull layer of tin oxide that develops over years.
The color of this natural patina is typically a mellow, dark gray or charcoal color, not green. This deepening of color is often considered desirable, giving the piece an antique appearance that enhances its contours and design. Since this oxidation layer protects the underlying metal, many collectors intentionally preserve the patina and advise against over-polishing.
Why a Green Hue Appears
When a green hue appears on pewter, it is due to the presence of copper within the alloy or an external staining agent. The small percentage of copper added for strength is susceptible to verdigris, a blue-green patina. Verdigris is chemically composed of copper carbonate or copper acetate, forming when copper reacts with air, moisture, and certain chemicals.
If the pewter is exposed to high humidity, moisture, or acidic substances, the copper content may react, leading to localized green spots. External factors are secondary causes of green staining, not the pewter itself corroding in that color. Contact with acidic foods, such as citrus juices, can cause a chemical reaction that pits and stains the surface. Exposure to environmental contaminants or certain household cleaning products can also leave a visible green residue.
Cleaning and Maintaining Pewter
Pewter maintenance involves hand-washing the item with mild dish soap and warm water. It is important to avoid abrasive scrubbers or harsh chemicals like bleach and ammonia, as these can easily scratch the soft metal or strip the patina. After cleaning, always dry the piece thoroughly with a soft cloth to prevent water spots and minimize exposure to moisture.
To address green discoloration from verdigris or external staining, a gentle, targeted approach is necessary. A paste made from distilled white vinegar and flour can be used to polish the surface and remove some corrosion, though it should be used sparingly on polished finishes. For localized green spots, a gentle scrub with a soft-bristled brush dampened with a mild soap or a water and vinegar mixture can help break down the copper corrosion. Preventing future greening involves avoiding high-humidity storage and immediately washing the pewter after contact with acidic foods.