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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: 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!}