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Copper the Element

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I must have known copper was something amazing when I used it as one of the colors to my wedding décor back in 2006. That was before using metallic at weddings was considered fashionable, so I felt far ahead of the trend, using copper, as it were.

But what makes copper so fantastic on a pure, elemental level? What makes it such a perfect conductor of heat, or metal that bonds beautifully?

First, let’s start with the copper on the periodic table and it’s material science.

Copper’s atomic number on the table is 29, and it’s symbol is Cu (which I never understand, as there’s no “u” in the word copper…it’s like having a US state abbreviation that doesn’t completely match). The atomic mass of copper is 63.546 u + 0.003 u. The melting point of copper is 1,984 F (or 1,085 C), and the thermal conductivity rate is 386 W/m K. Copper’s coefficient of thermal expansion is 17 per degree C x10^-6. Pure copper rates dead soft on the Rockwell C hardness scale, and is under the “non-ferrous” metal heading, meaning it does not contain any molecules of iron.

Whew. So, if that information helped you out, you’re welcome. I feel smarter just writing it all out in one place!

Copper, compared to other metals, is not highly reactive. That means it doesn’t react to other natural elements the same way iron does, for example. Attacks of oxygen and hydrogen (or water, for that matter) are usually futile – copper needs to be heated to at least 300 C to change it’s molecular make-up and become copper oxide. Iron, on the other hand, just needs to be exposed to air to make iron oxide (aka rust).

Copper can change/bond to other metals with the exchange of electrons. Elements are constantly forming covalent bonds between other elemental atoms (when an element may share electrons with other atom) or losing electrons to become positively charged. When that happens, the lost electrons move to another element, which is then negatively charged (that middle school science class coming back to you yet?), creating an electric (like a magnet) attraction between the two atoms, which is called an ionic bond.

Most metal elements/atoms lose electrons when they form the ionic bonds with other elements. However, copper is unique as it can form two ionic bonds. That is to say, once electrons are exchanged and the atom becomes less stable, it can combine with other elements (such as oxygen, for example) in two ways instead of one. This means deep molecular change can occur at a faster and higher rate when copper comes into contact with other elements. Take, for example, an item sitting outside in the rain. It’s a brass item (containing copper) and as it rains, the oxygen and carbon dioxide create a copper carbonate as the copper reacts with the rain in multiple ways. The brass item is covered with the greenish copper carbonate, thereby protecting the brass item from further corrosion.

For all the numbers above, copper certainly doesn’t come out of Mother Earth so pure and beautiful. We have to mine it out, and it comes out as copper ore, which usually contains only 1% of metal, so the ore needs to be floated. The refineries will pulverize the ore, mix it with water, and then pass it through water-filled tanks.The chemicals used in the water produces foam, which traps the copper minerals on the surface so they can be skimmed off, leaving the remaining ore. This is the part where the type of chemicals (or lack thereof) can determine how a copper is deoxidized, or whether it might turn into an alloy of copper instead of remaining pure. The finished “product” of this process is now about 25 – 35% copper, which is sent to be smelted.

Smelting uses high temperatures to finish purifying the copper. The first stage removes more copper from the ore by heating it with oxygen gas. From there, the “blister” copper goes through a fire refining and electrorefining stage, which results in a 99.99% pure copper.

When you have pure copper, the bonding abilities of those electrons are at a very high peak. Copper is conducting heat at nearly the perfect level of 386, and it is able to bond with silver or tin easily (depending on the chemicals/elements used to extract the copper from the ore – certain ones actually hinder the copper’s bonding ability), creating a molecular bond that lasts, at least in cookware, for a good chunk of time.

And that, all that science put into one place, is probably (now that I know way more about copper than I did at my wedding) what makes copper cookware, to me, so incredibly cool (and, of course, beautiful).

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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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How to Make a Copper Bowl

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copper bowl, copper cookware, pure copper, pure copper bowl, thick copper bowl, original copper bowl, american copper bowlIt’s been a bit of a journey to get these copper bowls into existence.

First, there was the process of the design. My dear friend and colleague Julia is an amazing product designer, and we came up with the volume and tweaked the bead (the curved edge on the top) before sending our drawings to the fabricators.

At the fabricator’s in Ohio, we all sat around and played with the notion of how to spin this bowl.

What type of steel should we use for the tool? Anodized? Softer? If we start with a softer tool, can we harden it later if a ton of people fall in love with these bowls and we need hundreds of thousands (I can dream, I know)? What about adding a stainless steel wire under the bead rim? Can we do that? How?

In the end, a tool was made, and bowls were spun in .090 (that’s 3mm thick copper!) which is just strong and hard enough to support the bead rim without needing the wire underneath (whew!) and a whole tool just for that application.

copper bowl, vintage copper, vintage copper bowl, pure copper, real copper, real copper bowl, unlined copper bowl, heavy copper bowl, thick copper, thick copper bowl, bowl, baking, chef, pastryThese bowls are 5qt capacity and not only are they extremely heavy duty, but they are solid copper to boot. I feel like I need to stress this because there are other copper bowls out there. But there’s a huge difference. Most of them are made in China or India (a very very few are made in Europe) and they are actually either lightweight aluminum plated with a very thin layer of copper or it is a lighter gauge of copper sheet. You can tell because the copper actually starts to chip away with use or bends easily.

If you go cheap, it’s not 100% pure copper, so you’re definitely not getting the full chemical response when whipping those egg whites or preparing the pastries.

So, we have vintage reproduction 5qt pure copper bowls (CDA 122)! Ready for you, forever.

But the best part is the handles. That’s where the story really comes to life. And you know we’re all about the story here at HC!

Not only was it important that the handle for the bowls be made in America, but I wanted copper handles. Apparently (and not surprisingly) this was also hard to come by.

Enter the annual tinsmith convergence and connections via my master tinsmith Bob, and suddenly one of my new acquaintances said: “Sure! I can do that!”

This is Tom Miller, a three-tour US Navy Vietnam Veteran and retired police officer/CSI. He works in stained glass, wood, tin, copper and even ice and sugar when the mood hits.

Another Midwesterner (he lives in Michigan – right next door to my Wisconsin abode), Tom has just lost his wife of almost 40 years and has raised his six children (whew!). When he’s not hammering on my copper handles, he’s operating a masonry restoration company.

Tom jumped in. He made the copper pieces efficiently and opened the lines of communication with the fabricators so they could figure out the tweaks directly. With a few prototypes and some small changes, suddenly an order of 30 handles was filled and these bowls were set to hit the market right as everyone is thinking about Christmas gifts (hint hint!).

It is wonderful to collaborate with another craftsman. Working with him matches our philosophy of staying with small artisans and family owned/operated American companies. I hope tons of orders come in so that Tom can keep making these lovely handles for us – and so you can have a piece of hand-worked artistry in your hands yet again as you mix up some cookies.

PS – one of the coolest things about Tom (among other cool things): check out Tom’s awesome rescue of a newborn infant during the Vietnam War!