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Copper Cookware Rivets

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!

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Tinning Copper Over Fire

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There’s something a bit exhilarating about huddling over a hot fire with some metal in your hands and making it melt.

Our House Copper copper pots are hand tinned here at HC, and while I (Sara) do it now, I couldn’t have learned at all without the beautiful work and constant mentorship of Dan, our original tinner. He was awesome enough to lime up a few of the HC lids at the 2016 tinsmith convergence, show how it is done, and hand over the whole kit for me to try.

First of all, disclaimer! I personally believe it’s the best (affordable) way to line cookware (especially great copper cookware that is spun in .060 – .090 (that’s 1.5mm – 3mm)) because of the molecular bonding occurring between tin and copper with heat, creating a single molecule thick of bronze. Now you can continue, knowing I’m biased, but backed by some science, too!

My first foray into tinning was a thin little number called Galvanizing and Tinning: A Practical Treatise on Coating by W. T. Flanders, published in 1900. While many parts of tinning has remained the same, we’ve obviously modernized in the past 118 years, and learned even more about the science of cookware.

 

How a traditional tinner’s workshop would have been laid out

(image from a book which allows reproductions of the material)

 

 

 

Our copper cookware is formed from pure copper sheets, which need to be deoxidized during the smelting process. This is done using phosphorous. I like this particularly because phosphorous is an element on the pure periodic table, which means we aren’t dousing the cookware in a ton of chemicals even in its infancy. Plus, the phosphorous allows for a better tinning finish.

Back in the 1700 and 1800’s, tinning (and soldering) used to be done using small metal tools, aptly named “coppers” for small pieces. Big pots and kettles were done in a tin shop. Most work in the molten tin was for covering cast iron pieces to prevent rust.

 

Traditional tinsmith coppers, used for soldering tin seams on cookware, etc.

 

 

 

But hot forge tinning was done (and is still done) with either one kettle of tin or more. The tin is maintained at a temperature of about 500F, and the work is cooled off in hot water ideally, and dried in sawdust. This is how it was done for hundreds of years, and how it’s still done today!

A few things happen when dealing with tinning copper, especially copper cookware that has iron handles. The handles themselves are attached with copper rivets, which helps adjust for the different coefficients of thermal expansion between the copper and the iron, but the iron will still pull off the heat from the copper body when heating up the material for tinning.

This means that when you’re starting to tin over fire, beyond what it takes to heat up the limed up copper (cover the cookware with lime ahead of time to help protect it from direct flame), the place where the rivets are mushroomed is actually the coolest part of the piece. That’s what needs a lot of initial heat to compensate for the “cooler” metal attached at that point pulling off the rising temperature.

You’ll apply flux after you’ve heated the copper cookware for a bit. There is ongoing debate in tinsmith circles about the best flux to use, and everyone has particular choices based on the application of the tin. (I’ve been dealing with both Acro Flux and Harris Stay-Clean, for those of you who care!) Once the flux reaches just the right temperature (usually it starts to almost ‘brown’), you start running your bar of tin, spreading, then wiping, and then dousing in water before cooling it in a big box of sawdust.

Once it cools completely it can be cleaned and polished and the handles are treated. Some people prefer butcher’s wax, but we’ve experimented with using organic flaxseed oil, sometimes mixed with traditional, old fashioned stove blacking (the kind used on pioneer potbelly stoves), which is basically powdered charcoal.

And you’ve got a tin-lined copper pot. Easy as that!

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Copper Cookware Interior Linings

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There’s the ongoing debate (which I suspect will never die) about what interior is better for copper cookware: stainless vs tin vs nickel vs silver. American brand copper cookware is generally made either overseas (ironically) or by small boutique artisans these days, and anyone who cooks in copper has opinions about what they prefer their copper to be lined with. This is my take on it and why, but as always, it’s just my two cents!

First of all, I am completely in the camp that lining a non-ferrous metal (copper) with a ferrous metal (iron) goes against the point of using copper. Copper (and the tin bonded with it) has a thermal conductive speed of over 380W/m K, whereas stainless is around 25W/m K. Plus, if you want something made to last, and won’t slowly rust away, having metals that don’t contain iron is where to place your bets. Copper and bronze ewers used by the Egyptians thousands of years ago and stuck in Nile mud are still found in relatively the same shape and in good condition. Meanwhile, iron cooking pots in Viking digs that date back a mere 900 years are crumbled up and nearly gone to oxidation. Granted, stainless might not oxidize quite that fast (there’s no way to know yet, considering stainless steel has only been around for about 100 years) but you get the picture. I’m all about making something that should last for millennia. (why not?)

 

Anyway, regardless of my opinion, there are four generally used and/or viable metals for lining copper cookware.

The first is silver. This stuff is amazing. It’s the fastest (that means its thermal conductivity is superb, better than copper’s (406 W/m K), and is highly efficient in heating and safe for cooking food. It bonds molecularly with the copper, and lasts a long time if you use wooden or silicon utensils. There’s mainly one problem with silver, and that’s price. Since I know my cookware is already pretty pricey for some people, could you imagine if I coated it with silver? They’d be crazy awesome and super beautiful, but probably not practical. Bloomberg’s luxury list recently popped a silver saucepan, and it’s only you know…few thousand bucks.

The second interior option is nickel. In fact, many old and vintage pieces are wiped with or plated with nickel. Many times they can be refurbished with some good cleaning and a new coat of tin, and this is probably best due to the amount of nickel allergies out there. The nickel doesn’t leach into food the way, say, lead would, but it still would be touching the food and having a slight chemical reaction with it, so if you (or your dinner guests) had any type of nickel aversion, you probably don’t want to be cooking with it. And who wants to be on the line for that type of issue? No maker I know…

Then there’s stainless, which many people like because they say it’s easier to work with. I have yet to receive an answer on why exactly that is. If you’ve got a copper sauce pan that’s 2.5mm but lined with stainless, my guess is you’re still probably hand washing (correct me if I’m wrong and you have a magical dish soap for your washer that doesn’t result in pitting your copper?!) There is some ease in that stainless doesn’t scratch like tin, so you can scrape away at the stainless lining with wire and metal. Bear in mind that stainless is sticky (so clean up is harder) instead of non-stick like tin, and eventually it may pop apart due to the huge range in thermal coefficients between stainless and copper. Or your cookware isn’t pure copper, so it may stick to the stainless without much issue.

Clearly, I’m in love with tin lining. For me, molecularly, thermally, and purely – tin-lined copper takes the cake. Because of the electron exchange that happens when tin and copper are heated together (we’re talking over 500F), the molecular bond allows the thermal conductivity properties (ie: fast!) of the copper to transfer seamlessly through the crystal structure of the tin, allowing the copper to actually work the way it is supposed to when used as copper cookware. Plus, it’s non-stick and you need far lower temperatures to achieve the same thing, so it’s green/energy efficient plus always can be re-tinned over the decades, meaning it’s renewable and sustainable and won’t end up in a landfill. Tin has its downsides, of course. With daily use and proper care, it can slowly wear down. After about 12 -15 years your copper cookware may need re-tinning, which has to be done by hand.

 

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Copper Cookware: Vintage copper cookware seams

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COPPER COOKWARE SEAMS: HOW DO THEY DO THAT?

Before CNC machines and even hand-held lathes to make copper pots, we made ‘em out of copper sheets. This meant that we were stuck using small sheets sent over (and heavily taxed) from England (even though the copper itself was mined in America) and riveting, braising, seaming and pressing those sheets together in order to create nearly all copper cookware.

Everything that was made with copper was usually required to be waterproof. From boilers to cups to coffee pots to washpans – everything held some sort of liquid. By the 1700 – 1800’s, tin and coppersmiths knew to line the cooking wares with tin in order so the copper and heat wouldn’t combine for an icky combination.

But before anything could be tinned, or considered finished at all, the sheets of copper would need to be cut, fit and joined together with seams which were then either soldered or braised together.

Because there’s few places to list these seams, and there’s buzz about vintage copper cookware out there, I thought I’d put it out in writing in case anyone wants to delve into it. I’ve learned this at the tinsmith and coppersmith apprenticeship I’m so lucky to have at Backwoods Tin!

 

The Lap Seam

This very easy connection is simply when the very ends of the metal sheet (which you’re going to bend or curve) are clipped to your seam/burr allowances. The two metal ends are then placed next to one another, overlapping slightly, and then usually soldered together.

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Example of a lap seam exterior (on a tin mug).

The Crimp Seam

Make your hand into a C shape. Do it with the other hand. Now join the “C’s”. There’s your crimp seam. The copper sheet ends are bent into V shapes that fit together and then are pressed together. Another way to make a crimp seam along the base of a copper pot is to splay the bottom of the copper into a 90” burr, and fit a burred base around the burred bottom. Hammering down the exterior base burr over the top of the copper body burred base creates another version of the crimp seam.

(this can also become a double crimp seam if you take the finished seam and then push it up against the side of the vessel, or fold it over once more)

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Crimp Seam example along the base of a vintage sheet metal copper pot.

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Exterior View of a Crimp Seam on a Copper Pot

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The interior view of a crimp seam on copper cookware.

The Cramp Seam

A lot of very old vintage copper cookware has this particular method along the base. It has been called “dovetail seam” which is one way to describe it, as it certainly looks like splayed versions of the square dovetailing done on woodworking. However, in smith-talk, that’s a cramp seam. Those were insanely hard to do. Not only was the cutting very difficult, but also matching the copper together and then braising it to essentially melt the copper together, was incredibly painstaking. It’s one of the reasons they are rare – they were harder and more expensive to make so a plethora wasn’t made. They are beautiful…but tricky!

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Example of a Cramp Seam on a copper cookware base.

So there you are! Three kinds of seams, all of which can be found on American copper cookware made the old way – sometimes more than one is used, depending on the copper cookware made. Either way, they all helped make those canteens, cook pots and beer mugs waterproof, which was the end goal after all.

{If you want to purchase the modern versions of pure metal American copper cookware, I’d be thrilled to share my copper obsession!}