The sini
A few weeks ago, I made a typical Greek phyllo dough pie, filled with a mixture of greens. I assembled and baked it inside in what is known as sini, the traditional, round baking tray for Epirus pies, characteristic of its short walls.
In that particular instance, I had another reason to get super excited because I used for baking the gastra, which is the traditional, wood-fired, outdoor oven.
Of course, the pie, like everything you bake in such ovens, came out great and I’m planning to keep using the tray plus the oven much more in the future.
In that case, and so as to be safe for long-term use, I was suggested to take the sini to a professional metallurgist for inspection and if needed replenish its surface by applying a particular technique, which in Greek is called ganoma (γάνωμα) or kalaysma (καλάισμα).
Copper cookware
Sini is made of copper (Cu), which was the first metal to be worked by human hands and has historic importance in the kitchen because many old and traditional cookware are made of it. Even nowadays you can find copper pots in houses and restaurants.
Copper has the ability to conduct heat quickly and evenly but when removed from fire loses that heat just as fast. That responsiveness to heat makes it an ideal material for cookware when delicate food, like fish, needs to be cooked, and particularly for cooking using gas as a heat source.
However, the other side of the coin is that copper is very reactive and when it comes in contact with acidic foods (e.g. tomatoes, vinegar) it oxidizes and leaches into the food and then into our body. Ingestion of copper in small amounts isn’t a health issue but in higher amounts, for instance, when eating frequently food cooked using copper pots, can have toxic effects (1).
For that reason, copper pots are lined with other metals that are inert and unreactive to acids or other food constituents. Such a metal that typically is used to line copper pots is tin (Sn). Interestingly, the word tin in Turkish is kalay, which gives rise to the aforementioned kalaysma technique (literally means to apply tin on something).
Except from being inert, tin has also non-stick properties that prevent food from sticking onto the pan, and in that sense, makes cooking and cleaning much easier. The downside is that it has a relatively low melting point (230 °C), hence the tin-lined copper pots should never preheated without the food inside, and also, it’s not a good idea to be used at very high cooking temperatures. Additionally, tin is somehow a soft metal and it can be scrubbed off the pot, exposing the underlying copper, if abrasive materials are used for cleaning. With proper care though, a tin-lined copper pot can last for many years before it shows signs of deterioration and the necessity for applying a fresh tin layer.
Now, to re-tin a copper pot the craftsman has first to expose the copper layer by removing the deteriorated, old tin layer. Then, the pan is heated, the new tin is melted into it and subsequently wiped all over the surface to form an even coating layer.
But what about when you want to cook some food and you preheat your freshly tinned pan, is it possible that the tin will melt again running like water all over the pan’s surface? It sounds ridiculous but that’s a valid question taking into account tin’s aforementioned low melting point.
Basically, you don’t really need to worry and the reason is an interesting chemistry that takes place between the two metals (Cu and Sn).
The intermetallic layer
Tin has the ability to bond and thus stick very well to copper. Thus, it’s pretty difficult to get the tin completely off the pot by simply heating the latter. That’s because strong physical force is required to scrape off a metal layer that lies underneath the pure tin. That metal layer, known as the intermetallic layer, is made up of both copper and tin. Simply speaking, the intermetallic layer is a sort of glue between the two pure metals and it has the consistency of ceramic rather than metal (2). It’s formed as soon as melted tin is layered on copper pots through the formation of bonds between the two metals. It’s consisted of two compounds, namely Cu3Sn and Cu6Sn5. As mentioned, it’s hard, rigid, and very difficult to separate from the pot bottom of pure copper. To illustrate better this, look at the following picture that shows through a cross-section of your freshly-tinned copper pot the different metal layers that are formed.
The intermetallic layer (orange and yellow layers) is dynamic and it continues to grow and change consistency because tin and copper keep reacting and bonding each other. In the first stage, pure tin bonds with copper in a precise atom ratio (3:1, 6:5) to form thus Cu3Sn and Cu6Sn5. Then, given time and temperature, tin is gradually converted to the more chemically stable form of Cu3Sn (3). Obviously, the more you cook, the faster this conversion will take place.
At some point, after maybe years, all pure tin is gone, leaving behind just the intermetallic layer, which makes the surface of your pot have a matte grey appearance with spots of different shades.
This is maybe the time to re-tin your pot!
References
(1) Royer A, Sharman T. Copper Toxicity. 2023 Mar 27. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 32491388