Medieval Methods vs. Modern Methods: Soldering

Much like sewing, not a whole lot has changed in the last few millennia. Soldering has been a technique in use since early on in the process of metalworking. Give or take 5,000 years. And metal, like fabric, can be made into hundreds of thousands of different things, but the methods of making those things are fairly limited, and defined by the material's properties. Metal can melt. Metal can bend. Metal can harden and soften. Metal is reactive to some acids. Metal can be mechanically fastened to other materials. And metal can be melted and "fused" onto other metals or itself (welding), or a metal with a lower melting point can be used like glue to fuse two other pieces of metal. Which is soldering.

By its nature, metal has to be clean and free from oxidation in order for it to be properly soldered. Flux is what we use to prevent oxidation, and there are several different chemical forms of flux, with the simplest being a borax cone that gets combined with distilled water in an unglazed ceramic dish. Borax and its chemical properties with respect to metal were known and can be documented in medieval metalsmithing practices. Interestingly enough, while Theophilus of Edessa mentions the use of borax in "On Divers Arts" (12th century), he separately provides a recipe for flux to be used in soldering silver. It involves the resin left at the bottom of wine bottles. He also provides a recipe for soldering gold, using lye, salt, and copper, but mentions that it can be used to solder both gold and silver. Cellini, in the 16th century, simply indicates borax.

Heat must be used to melt, weld, fuse, or solder metal. Modernly, most people use torches, with working pieces placed upon heat-reflective fire safe surfaces. I, and many people, use a simple charcoal block. There are other options available, including ceramic honeycomb, vermiculite, or magnesia blocks. Now, specifically, I work on a dense charcoal block made specifically for this use, that sits on a vermiculite block, that sits on my desk. The vermiculite block is bigger, and acts as a secondary, heat-safe workspace for things to be set upon safely, and to act as a safety if sparks fly. Charcoal doesn't act as a heat sink. It reflects heat back on your piece, and it can, by its own nature, ignite and smolder, so working with it on a secondary heat-safe surface is important. Some people choose to douse or quench their charcoal blocks after use.

That said, charcoal is indispensible to the discussion about medieval metalwork. Charcoal fires can get as hot as 2110'F. In a forge, with oxygen added to it via bellows, a coal fire can get as hot as 3590'F. Charcoal fires are how medieval metalworking was accomplished. For reference, the melting point of silver is 1,763'F, the melting point of gold is 1,948'F, and the melting point of steel is 2500'F.

Theophilus describes building a forge on a bed of sand with charcoal, and then for a soldering process, placing the pieces to be soldered on the charcoal, supporting them with more coal if needs be, and then building up charcoal walls around the piece, so that it can be heated evenly all the way around. Gaps were left so that the piece could be observed through heating. The coal fire was fed oxygen with bellows until it became hot enough for the solder to flow, and then the charcoal was removed and the piece was taken off the heat.

Ultimately, while a few of the tools have gotten better, and the visual aids and artificial lighting we have modernly make fine detailed work easier, the actual processes of metalworking have been virtually unchanged for millennia, which is pretty danged cool, if you ask me.

Soldering Continued

My previous post went over the absolute basics and vocabulary of soldering. But it didn't discuss how to actually solder two pieces of metal together. Or even what soldering particularly is. Let's discuss that a bit more.

Solder, as previously discussed, is a metal (let's keep talking about silver, but this can apply to gold and lead, too) that is alloyed together with certain precise percentages of copper and zinc (and sometimes tin). The finished alloy has a lower melting point than pure silver. When solder is placed at the joint of two pieces of clean metal that have been prevented from oxidizing, and then those pieces of metal are heated to the melting point of the solder, the solder will then melt and "flow" through capillary action into the joint it is closest to, and once it re-hardens, it will create a physical join between those two pieces.

Let's unpack that a bit.

• The metal pieces must both be at the correct temperature for the solder to melt and flow. It's not enough for the solder to be that temperature.
• The solder won't fill gaps between pieces. If there's a gap, what solder is infinitely more likely to do is to just flow over whichever piece it's closest to, making a mess, and not joining it to anything.
• The solution to this is to make sure that your pieces are touching.
• The solder loves heat, and it will flow towards heat. Heating the side of your piece that is away from the solder can be useful, so you don't burn your solder.
• Flux keeps your metal from oxidizing. If you do not thoroughly clean and flux your pieces prior to soldering, the solder will not flow.
• "Clean" means free from dirt, debris, and oil, including oil transferred via skin. "Cleaning" metal often means going over it with a piece of fine grain sandpaper.
• Not only do you have to get the solder to melt and flow, you must also remove the heat from the piece after the solder has flowed, to allow it to re-harden. You can overheat a piece.

Your heat source needs to be able to heat the entirety of both pieces you're joining to the correct melting point. For small bits of jewelry, this can be done with a handheld butane torch, which is what I use. For larger pieces, you can use a full size torch head on a propane tank (attached with a hose, for maneuverability), or MAPP gas canister. MAPP burns a bit hotter than propane, and propane burns hotter than butane. You can also get an oxy-propane or oxy-acetylene torch, which infuses the fuel with straight O2, from an oxygen tank. Both of these will burn very hot, and are frankly overkill for tiny jewelry projects, but absolutely perfect if you want to weld steel. They also work perfectly well for jewelry, and professional jewelers use them. For starting out, a solid little handheld butane micr-torch will solder silver just fine, though.

And last thing I want to touch on: the amount of solder you need. It's probably less than you think. Think about it this way. You want enough physical material to fill the joint of your two pieces that are touching, where they are touching. The joint should be able to be measured in micrometers, because your pieces are touching, but it might be several millimeters or even centimeters long. For reference, and to give you an idea, I used 4 pieces of wire solder, each cut about 1mm long, to attach this 8mm diameter bezel to the back plate. You can see three of them in this photo below. This amount of solder made a solid joint all the way around the bezel.

So the actual process for soldering is this:

Step 1. Clean your pieces of metal.

Step 2. Apply flux to your pieces of metal where you want them to join.

Step 3. Place solder as required for your joint.

Step 4. Gently heat your piece. The flux may bubble up if you're using borax. That's normal. The solder may jump around. Pause, re-set your solder where it belongs, and continue heating.

Step 5. The flux will change colors. That's normal. The solder will start to gently glow, and then melt, and then flow. Make sure your piece is evenly heated, and that your joint is fully, evenly heated, to draw the solder into it.

Step 6. Remove the heat and allow the piece to sit for ~30 seconds.

Step 7. Pickle the piece. The time your piece sits in the pickle solution will depend on how strong your solution is mixed, and whether it's warm or cold. Let the piece stay in until it looks clean. It will have a white cast on it, that's normal.

Step 8. Using copper tongs, remove the piece from the pickle, and place it in the neutralizing bath. It doesn't need to sit in this bath, just a quick dip.

Step 9. Dry the piece and check to see if the joint is solid.

Step 10. Assuming the joint is solid, proceed with any filing, sanding, and polishing necessary.

Intro to Soldering

This is essentially a class handout, but the information is a good overview and introduction to basic modern soldering. My tool list is a little tongue in cheek. Essentially, you will absolutely definitely want some of the tools in the "More Tools..." list. Your actual project and interests will dictate which ones. Also, I'm positive that I'm missing tools and chemicals that you may want. These are just the ones that are most common. This guide is meant to be the starting point, not the be-all end-all. Finally, I want to note that this is heavily, heavily skewed toward soldering silver, and soldering jewelry.

Tool List:

Basic necessities for soldering:

Clean metal pieces to be soldered together
Solder
Flux
Pickle solution (in a container)
Neutralizer (in a container)
Copper tweezers
Stainless steel tweezers
Torch
Soldering surface (charcoal, ceramic, etc.)
Small paintbrush for the flux

More tools you may want:
A jewelry saw and blades
Bench pin
Third hand with tweezers
Wire cutters
Round nose pliers
Stepped barrel pliers
Chain nose pliers
Steel ring mandrel
Steel bracelet mandrel
Tripod
Small steel anvil
Nylon, plastic, or rawhide hammer
Steel hammer (ball peen or chasing work nicely)
Solder pick
Files
Different grades of sandpaper (200, 400, 800, 1000, 2000, 3000)
Polishing compound
Rags
Nylon bristle brushes 

Brass brush
Dremel or flex shaft
Magnifier

You can find all tools necessary (and then some) at riogrande.com or ottofrei.com. Some of the above tools can be found at Harbor Freight, Lowe’s, Home Depot, or other hardware stores. Some tools may be found at hobby or specialty stores, but the average Joann’s, Michael’s, or Hobby Lobby is going to only be a good source for things like pliers and cutters.

What is SILVER SOLDER?

• Silver solder is silver that’s been alloyed with copper and zinc to lower its melting point. It comes in Hard, Medium, Easy, and Extra-Easy. Hard has the most silver content and the highest melting point, while Extra-Easy not only has the least silver content, it’s also got a bit of tin, and the lowest melting point. Professional jewelers do not use Extra-Easy. When performing multiple soldering operations, it can be helpful to use hard for the first step, medium for the second step, and easy for the third step. That makes it less likely that you will melt or disrupt an earlier joint when doing later work.

What is FLUX?

• Flux is a liquid or paste that keeps the metal surface from oxidizing during the heating process. If the metal oxidizes, the solder will not flow properly, and the joint will either not be made at all, or will be made badly. There are many different kinds of flux, as well as period recipes for flux. Personally, I use a borax cone and distilled water in an unglazed ceramic dish. Borax was known and used in metalwork in the medieval period. Because it’s more liquid than paste flux, it’s also easy to see if your surface is clean enough. The borax/water will bead up on the surface if it’s still dirty or oily.

What is PICKLE?

• Pickle is an acidic water solution that is used to clean the excess flux and oxidation from your piece after you’ve soldered it. Alum is a safe pickle (that’s made in using edible pickles, too!) that you can find in the grocery store, near the spices. Citric acid is another pickling agent that is environmentally friendly, and relatively safe to handle. I use sodium bisulfate. It’s a dry powder chemical I purchase from Rio Grande. Either way, your pickle solution will work better warmed. It can be in a glass jar on a candle warmer, or in a mini crockpot on low. It can also be used at room temperature, it may just take longer. If you are using sodium bisulfate, you MUST use copper tweezers, and do NOT touch the liquid with your hands. It can be irritating to the skin. Exercise caution when using chemicals and fire. There are other chemical pickling agents on the market, but these are commonly found.

What is a NEUTRALIZER?

• If you’re using sodium bisulfate, your piece must be neutralized before you can handle it. Simple baking soda will neutralize it. Create a separate solution of baking soda and distilled water and keep that near your soldering station. This liquid can also be used for quenching. Even if you’re using alum or citric acid, I would still recommend rinsing the piece in clean distilled water before moving on with it.

Soldering Basics:

Silver solder comes in several forms:

Wire - looks like regular metal wire, and should be labeled immediately. Can be cut to desired lengths with wire cutters. I find it helps to go over the end with some fine grit sandpaper just before cutting.

Sheet - looks like metal sheet and should be labeled immediately. The idea with sheet is to cut a little fringe at the end, and then cut pieces off the fringe as needed. You can also go over the end with some fine grit sandpaper before cutting the fringe.

Pallions - Pre-cut solder chips. These can be as small as .5mm x 1mm

Paste - Solder filings and paste flux pre-mixed together in a squeeze tube. You do not need to use a separate flux for this. Paste is favored by some who do filigree.

Metals being soldered must be CLEAN. Free of dirt, debris, and oil, including oils transferred from skin. A bit of fine sandpaper over the joint cleans it nicely. Alternately, dip it in your pickle solution for a minute, then neutralize, only touching it with tweezers.

Metals being soldered together must TOUCH each other. They must have good, contacting surface area. The solder won’t fill gaps. A lot of period examples of open filigree from the Byzantines feature granules at joints. My theory is that these granules provide surface area and contact between elements, ensuring a good join is made.

Silver solder will join brass, copper, mild steel, and stainless steel, as well as silver. It will also join any of these metals to each other. Gold solder should be used for working on gold, though easy silver solder can be used to join silver and gold together.

The pieces you’re joining together must both be at the same temperature, the temperature required to melt and flow the solder, in order for the solder to flow. The heat doesn’t directly melt the solder, the metal does. And the solder gets pulled toward the hottest thing. Joining a tiny piece of metal to a more massive piece of metal, therefore, is a bit tricky. It takes longer to heat up the bigger piece, and you risk melting the smaller piece. Sweat soldering can be a useful technique for this kind of task, and is something I’ll go more in depth about in a separate post. However, it's also what the tripod is for. When soldering a bezel to a back plate, or sometimes filigree or granulation to a back plate, where the pieces on top are smaller and lighter than your base, you elevate the piece and apply heat to the bottom. You heat the back plate first, and the heat transfers up to the smaller, more delicate pieces, so they heat more evenly.