Thursday, September 12, 2013

Soldering: Selecting Your Solder

We've now come to the eponymous part of soldering: solder. When you walk into your local jewelry supply store and you say you'd like to buy some solder they'll say to you, "what kind?"

Huh?

Okay, so maybe you're more enlightened than I was the first time I went shopping for soldering supplies - I do like to jump in and give things a try - but I was a little taken aback by that question.

"Um ... soldering solder?"

If ever a look was given implying I belonged somewhere else - I got it then. So, let me share the knowledge I've gained in the many years since that embarrassing moment.

Let me clarify first, for anyone jumping in to this series, that I'm talking about soldering with a micro torch or jeweler's torch, also called brazing or hard soldering. So, when I say soldering solder, I'm not talking about the tin/lead solders used by stained glass artists and applied with a hot soldering iron, which only creates a superficial bond with the underlying material, not a deep molecular bond.

Solder Density

Solder is usually an alloy of sterling silver and zinc. Sometimes they throw some cadmium in there for the easy or extra easy solder, but fewer and fewer vendors are carrying products with heavy metals and, if you don't have good ventilation in your studio, it should definitely be avoided. The zinc (and cadmium) lower the melting point of the solder. Solder is available with different melting/flow temperatures, called solder density because it depends on the percentage of silver in the alloy, because designers may need to construct a component in several steps and don't want previous joints/seams to unsolder when heat is applied to the new joint.

I will add that it is possible to do several successive joints/seams with the same solder density because as the previous seam heats up some of the zinc melts effectively increasing the flow temperature of the solder in that seam. You need to be aware, however, because the solder begins to pit as the zinc melts, potentially weakening the bond at the seam.

It is also worth saying that the more silver that's in your solder the stronger the bond it will make at the joint/seam. So, start with the highest melting-temperature solder that's practical with your project.

Solder Density   Melting Temp   Flow Temp   silver content   Notes:
extra-hard   1,370°F (743°C)   1,490°F (810°C)   80%  also called I.T. (intense temperature); used in enameling
 
hard   1,365°F (741°C)   1,450°F (788°C)   75%  Highest melting temperature solder used in standard sterling silver soldering
medium   1,275°F (691°C)   1,360°F (738°C)   70% 
easy   1,240°F (671°C)   1,325°F (718°C)   65% 
extra easy   1,145°F* (618°C)   1,205°F (652°C)   56%  has a yellowish color from the zinc; used for repairs
*Temperatures given are based on the sterling silver solders available at Rio Grande.

Let me skip ahead quickly so explain why you can't just heat one joint or seam and avoid any previous ones. First, sterling is a heat conductor, so the heat from the flame flows away from where the torch flame is heating the metal. So, if you were applying heat to a seam, it immediately starts going everywhere else. You may get enough heat at the seam to melt the solder, but it also starts melting out the zinc (increasing solder's melting and flow temperature) and oxidizing the flux and sterling's 7.5% copper. So, to solder a joint/seam, the heat needs to start anywhere else but the joint/seam in question, making all of the others more vulnerable to coming undone. More about heat control in two posts.

Solder Type

solder

Remember here, as I present even more forms of solder, that the choices here give you the power to tailor make the soldering process to suit the special needs of your projects or working style. To that end solder is available in sheet, chips, wire, solder-filled wire, and paste.

Sheet is normally the least expensive and a 1"x2" section cut into 1-2mm square pallions should get your through many, many projects. Chips are just pre-cut pallions. What you should remember about sheet and chips is that solder needs to be clean (like the metal you're using, and the hands you use to touch and move everything) and chips or pre-cut pallions are not fun to clean, so I'd recommend buying sheet and cleaning and cutting it as you go. Buying sheet rather than chips also allows you to cut the precise size of chip you need, which is often smaller than the standard chip size in my experience. A little solder really does go a long way. You can apply chips and pallions next to the joint/seam you are soldering, or you can melt it to the tip of a tungsten pick and, once your piece is to temperature, apply it directly to the joint/seam. This is the more efficient method for production soldering and prevents overheating the solder and melting out the zinc as you heat up the metal of your project. To make this work well, however, you'll need to work on your ability to read temperature and time your placement, so it takes practice.

Solder wire can be used round on the coil or flattened and cut. Some artists like to use solder on the coil in place of a soldering pick, but I'd classify this as an advanced technique requiring a strong ability to judge heat and timing. It's a really easy way to apply way too much solder to a joint/seam. Care needs to be taken not to confuse wire solder with your sterling silver wire. You can also buy sterling silver wire with solder inside to simplify the process of making jump rings and links; for better or worse I will say, based on my experience, that this is very soft wire.

Finally there's solder paste which is a mix of solder and flux and can be applied with a syringe (when you buy it in a syringe) or brush (when you buy it in a jar). Syringe paste solder is my solder of choice. I love that the flux is part of the paste because the process of heating and drying the flux doesn't move the solder like with pallions. I love that the syringe allows you to place the paste precisely where you need in the amount you need. I find that I am much more efficient in my solder application because I end up using less solder than with pallions and it stays where I put it, including underneath a raised joints/seams, which allows me to place my project in a way that maximizes the heat distribution rather than forcing me to place my project in a way that keeps the solder pallions in place (or where I can reach the joint with a tungsten soldering pick).

But, of course, read about what methods other artists use and why they like them and try different kinds of solder and see what works for you.

For those of you getting anxious for more I'll leave you with some links to more Ganoksin articles:
Basic principles of construction and soldering
Some soldering hints and tricks
Why soldering will never be as easy as brewing a pot of coffee
To help you be mindful of potential challenges so you can overcome them.

Thanks for stopping by!

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