Copper pots and bowls all have handles. They are usually either of brass, iron or steel and occasionally copper. But have you ever looked at how those handles are attached?
If you go into the kitchen now and look at those handles, you’ll also see rivets, at least where cookware is concerned (or, if you’re like me before I became educated in kitchen tools, I called them ‘nails’ or ‘screws’).
The interior of that pot or bowl plus the type of handle material will determine the type of rivet used. Rivets, as small as they are, are a necessary, integral part of creating long-lasting cookware. Choose the wrong rivet, and your pot will either come apart, handles will wiggle free, copper will disfigure, or the interior coating won’t stick. There’s a good amount of physics and science behind rivet choice, much of it having to do with thermal coefficients, molecular bonds, and even rivet length. Because it’s such an overlooked, but significant piece of the cookware puzzle, I thought I’d break it down, so that you too can understand exactly what you’re looking for when deciding if a piece of cookware is put together well.
Thanks to some generous mentorship and a healthy drop of research (even going back to grade and high school general chemical element textbooks), rivets have become both easier to understand, and, at the same time, complex little organisms in their own right.
Today, rivets start out as huge coils of metal – stainless, brass, aluminum, copper, or other types of steel, all with varying types of alloys – in various thickness or diameter. American rivet makers have electronic equipment now – rows of automated machinery made anywhere from early 20th century America to overseas in the East or Italy and beyond. Our rivet maker loves the one made in Vermont in 1917, swearing that it’s held up far better than other imports.
Rivets also have their own idiosyncrasies that require specific tools to be made and fit to the machine that is pulling the wire, to create whatever head (round, truss, flat, etc) is required, as well as the length of the rivet, and the finishing of the end (straight vs chamfered). Some rivets are tubular, or semi-tubular to a certain depth within the rivet instead of a solid shank, shouldered (a smaller diameter on the end of the shank), self-piercing, countersunk, collar, or brake. Again, our rivet maker does a lot of the tool making himself – essentially making a mold or jig that allows a machine to pump out thousands and millions of rivets. (This is one of those lost arts!)
The wire is then fed into the machine, where the head is formed, and any finishing to the bottom of the rivet happens right before the rivet is released into a bin. Many rivets are tubular or semi-tubular, unlike the ones used in kitchenware, which we spec as solid shank to manage the pressure of the rivet gun through the multiple layers of metal. Solid rivets are by far the strongest type made, and annealing can be done to make a rivet more durable or ductile depending on the needs of its final applications and use.
In kitchenware, the final application of the rivet is taken into account also with understanding the type of metal that the rivet will be joining. Certain types of metal do not bond, or have enough thermal expansion (heat elasticity) to be the right material used to be the connector of two joints. That’s where metallurgy and chemistry come into rivet decisions – a choice that should be made based on the longevity of the materials working together vs the least expensive manufacturing option, though obviously economics come into play, too!
Kitchenware rivets are usually holding together two dissimilar metals – either in molecular make-up or in terms of shape. Even a stainless steel pot, with stainless steel handles, will not be created as one entire piece. The body component will be spun on a CNC from sheet metal alone, and the handles created elsewhere. There is also a likelihood that the handles are made of a slightly different alloy of steel than the body. A manufacturer will want a cookware body to be higher in thermal conductivity than the handles so that the cooking surface heats as evenly and quickly as possible but the handles don’t necessarily heat as quickly. Therefore, even though you have a full steel pot, you have two disparate types of steel you’re going to connect together. You wouldn’t use a pure copper rivet at this juncture – the copper would heat and cool so quickly that the rivets themselves would not be able to stand up to the slower heating that surrounds it in the form of the steel, likely either cracking or failing completely. Even though you may have two types of steel alloys, one would use a similar metal with a similar expansion rate, in this case, a steel rivet.
In terms of copper cookware, which we make here at House Copper, we need to take the tin lining into account, just as, say, Mauviel does with their stainless steel lined copper cookware. For instance, a copper pot that has a steel interior is, first, not 100% pure copper because it needs to have a lower coefficient of thermal expansion to match the slower expansion rate of the stainless interior, so they add a few alloys in small percentages to the copper make-up, like Falk did, creating a copper alloy usually molecularly made of 99.58% copper, .40% aluminum, .02% boron. Sometimes they will replace the aluminum with zirconium or titanium. Still, the small additions of the other types of metals is all that is needed to slow the coefficient of thermal expansion so that the copper can (semi-mechanically) bond better with the stainless chromium/nickel steel as the zirconium, in particular, acts as an inhibitor – meaning the copper crystals, even with re-heating and cooling to form a pot, do not change as dramatically, creating a far more lasting bond than other types of stainless steel lined copper pots.
Our pots are lined with tin. The tin interior exchanges electrons with copper when the two are heated, creating a solid molecular bond instead of a mechanical one as discussed above. But we do like to add handles to our copper cookware, of course, and those we don’t want to make out of copper. It would heat too quickly and be too weak to hold up to the weight of a full pot, let alone be too malleable. We use pure iron handles, but they’re poured with a ductile grade treated with heat for extra machinability and elasticity (meaning the iron nodules can elongate with heating instead of cracking). This means a rivet needs to work with both the iron (which heats slow), the copper (which heats fast) and the tin (which also heats fast). The rivet must therefore absorb a lot of heat and move quickly to compensate for the slower moving heat of the handles. Both aluminum and copper rivets have this capability. However, tin does not react/bond to aluminum well, leaving our only option to be a strong, solid shank copper rivet.
Without the right rivet…your pot will fail in multiple places both in performance and during manufacturing, or at the very least, deform with use. A good manufacturer will want to know the provenance of the rivet to make sure it is made of the proper alloy/molecular make-up required for performance as well as overall quality control. Use a bad rivet, and the entire piece of cookware can quickly become junk.
Hence, a rivet really holds it all together!