MATERIALS, PART I — COPPER AND SILVER

by Editorial Staff | May 18, 2020 | Quality & Operating Procedures | 0 comments

In this piece and the next one we cover the most traditional materials still used to make cookware: silver, copper, iron, and earthenware. At least until aluminum became widespread after World War II, practically every vessel for preparing food was made from one of these four materials.

Silver and copper

Silver and copper are metallic elements (symbols and atomic numbers: ₄₇Ag and ₂₉Cu) that humankind has mined and worked since time immemorial. Considered “precious,” together with gold they form the historic metallic triad for coinage and jewelry in every age and place.

Easy to work thanks to high malleability and ductility, resistant to weathering, and excellent conductors (approx. 429 and 390 W m⁻¹ K⁻¹ respectively), silver and copper remain best-in-class for fine heat control, both on the way up (heating) and especially on the way down (cooling)—a crucial trait for quick cooking and sauce-making, particularly when egg is involved.

Practically speaking, silver’s use is limited by the high cost of the metal and workmanship. At present there are no mass-produced lines of solid-silver cookware, only niche artisanal pieces—statistically irrelevant on the global market. As we’ll see later, however, technology has brought silver back into play in combination with its more affordable “cousin,” copper.

Performance and safety compared

Copper and silver share several strengths in physical and mechanical terms, but they could not be more different in food safety: in contact with food, silver does not release compounds toxic to humans; moreover, silver has been known since antiquity for its natural antiseptic properties, especially against fungi and bacteria.

Copper, by contrast, is potentially hazardous because of the high toxicity of compounds released at high temperatures and especially in the presence of acidic ingredients. Even simple exposure to certain environmental conditions of temperature and humidity, when the utensil is unused for a long time, can foster the formation of harmful compounds, such as verdigris.

The risks associated with bare copper in the kitchen—as well as its two main alloys, bronze (copper and tin) and brass (copper and zinc)—have been known since antiquity. Focusing on copper, the traditional solutions are essentially two: meticulous cleaning and tinning.

Meticulous cleaning is reserved for three historical uses of bare copper vessels: water tubs, polenta cauldrons, and pastry polsonetti (small saucepans). In the first case, if the water is low in minerals and the vessel is constantly topped up, the formation of harmful compounds is limited and, in the past, was considered a lesser risk than other non-oxidizing materials, such as hides commonly used for flasks and canteens. Copper can dent, but unlike earthenware—another historically common liquid container—it does not easily shatter.

For the polenta cauldron and the polsonetto, direct contact between food and copper should ideally be avoided, but continuous stirring with various tools (e.g., a metal whisk) would quickly damage any tin lining as well as modern coatings like PTFE.

Cleaning with weak acids: how and why

Before each use, therefore, polenta pots and polsonetti should be inspected to ensure there are no traces of blackening or verdigris. At the slightest suspicion, it is essential to thoroughly clean the interior with an acidic solution. Various household methods are used: plain vinegar or lemon juice are enough to restore copper’s bright, rosy sheen.

The old cooks’ remedy—still valid today—is a thick paste of white vinegar, salt, and coarse cornmeal. The only modern addition we recommend is a common sponge with a mildly abrasive side to spread and rub the paste more easily. Once the interior has been restored to its original lustre, rinse and dry the utensil thoroughly and use it as soon as possible, before the copper surface oxidizes again.

After cooking, remove the polenta, cream, or sauce immediately and clean and dry the utensil carefully again before storing it. We dwell on these details because even occasional negligence can cause significant discomfort to anyone eating food prepared in contact with imperfectly cleaned copper.

All other copper cookware—in other words, the vast majority—must be lined on the inside with a food-safe material. For centuries the only suitable option was tin (₅₀Sn), thanks to its low melting point (just 232 °C), ready availability, and the ability to adhere perfectly to copper via a process known as tinning.

Tinning: how it really works

Technically, tinning is a form of brazing—a solder between dissimilar metals in which only one reaches its melting point. Performed by a master coppersmith (a craftsperson now almost extinct), traditional tinning involves heating the copper utensil over a flame to about 500 °C, then passing a stick of virgin tin along the interior, which melts instantly. To aid pickling, fluxes are added, usually ammonium chloride (NH₄Cl) or zinc chloride (ZnCl₂).

The heating temperature must be high so the tin stays fully molten for several seconds after the piece is removed from the flame for the next step: spreading. Using a cotton rag or, more modernly, a pad of mineral wool, the operator spreads the molten metal to coat the interior with a few microns of tin. Finally, the piece is rapidly quenched in water, solidifying the tin.

When performed as described, tinning ensures a highly food-safe cooking surface. The amount of flux used is generally minimal and, in any case, much of it evaporates or remains trapped beneath the tin layer.

Although chemically nearly inert and resistant to acidic ingredients and salt, tin linings are mechanically sensitive both to scraping with metal tools and to high-temperature cooking, as noted earlier. These are technical limits intrinsic to the metal, not the procedure—which has been essentially unchanged for millennia. This explains the gradual retirement of tinned copper from professional kitchens in favor of the almost-as-capable copper–stainless multilayer constructions, which we will cover in the chapter on multi-ply cookware.

Alternatives: electrolytic, paste—and silver

For completeness, we should mention two alternative tinning techniques which, in our view, underperform the traditional method in both durability and safety: electrolytic tinning and paste tinning.

The former uses an electrolytic bath to slowly deposit a layer of tin on copper, but the layer tends to be thinner and more fragile. Moreover, once worn, electrolytic tinning cannot simply be reapplied over the existing layer: it must be stripped and redone from scratch.

Paste tinning employs a tin alloy with other elements to lower the melting point. This is generally used on thin copper (< 1.5 mm), which could deform if heated to 500 °C. Typically it’s for mass-market goods not intended for professional use. Paste tinning requires less skill and manual dexterity, making it far cheaper; however, it is even more sensitive to abrasion and heat than the traditional method, not to mention food-safety risks from pastes that may contain contaminants such as heavy metals.

Finally, some coppersmiths offer the more expensive option of lining copper cookware with silver (again, via various techniques). The advantages over tin are twofold: although silver is a relatively soft metal, it is still more resistant than tin to mechanical stress; and, crucially, silver melts above 960 °C, so the utensil can be used without issue for sautéing and deep-frying.

Conclusions: the prince of materials

In conclusion—setting aside solid silver due to its prohibitive cost and rarity—copper remains the most suitable metal for preparations that demand perfect heat control. That secures copper a place of honor in fine dining and pastry work. Many enthusiasts also invest in a few prized tinned or silver-lined pieces for the pleasure of classic cooking. Professionals and amateurs alike must accept the functional limits and the need for meticulous maintenance—also for consumer safety.

High specific weight, a delicate lining, copper’s still significant cost, and its incompatibility with induction hobs (copper is not ferromagnetic) are further aspects to weigh before purchasing traditional copper cookware, which nevertheless remains—despite the centuries—the prince of culinary materials.

Carmine F. Milone
Food Technologist