What is pot metal? It’s a catch-all phrase that covers casting parts out of nonferrous metals. Usually when the phrase is used, it refers to pieces of unknown or possibly questionable manufacture. But there are sophisticated processes, such as diecasting, that fall into the category of pot metal.
Pot metal originated from the practice of gathering all available nonferrous metals and melting them into a pot for casting. Any process that involves casting nonferrous metals is technically pot metal, though the phrase usually refers to a more haphazard composition. Outside of prototyping, it is still used for making toys, trinkets, and cheap jewelry.
Generically, “pot metal” refers to any mixture of nonferrous metal. In the 19th century, it was usually a mix of primarily copper and lead, with smaller amounts of metals like tin, cadmium, zinc, antimony, magnesium and aluminum. Modern pot metal is usually a mixture of primarily zinc and other metals, but the formulations are nonspecific.
Before plastics became common, pot metal toys were the equivalent of cheap plastic toys today. Plastics have largely displaced pot metal in toys in recent times. But pot metal remains in use in applications where plastic is impractical. Cheap jewelry, for instance, is made of pot metal. The locknut on kitchen sinks is usually made of pot metal too, since it won’t rust.
How pot metal works
You can think of pot metal as an early form of recycling. It happened in factories, but also could happen at a smaller scale, even in a home workshop. Workers would gather up whatever available nonferrous metals they could find, then melt them down for casting parts. This could even involve sweeping the floor to gather up metal dust, and emptying pockets in search of candy or gum wrappers. Some small quantities of iron could incidentally end up in the mix, but too much iron would raise the melting temperature too high. Mixing in larger quantities of lower melting-point metals, such as lead, tin, and zinc, would lower the melting temperature to a more usable level.
The exact composition didn’t matter much. Mixtures of metal tend to melt at lower temperatures than pure metals. You’ve observed this if you’re familiar with soldering. The solder melts at a lower point than the copper or tin you’re soldering to. Your soldering iron can’t melt the copper, but it will melt the solder. And if some solder seems to melt easier than others, you’re not imagining things. Lead-free solders melt at a higher temperature than lead-bearing solders. And solder made up of 63% lead/37% tin melts at a lower point than the more common 60% lead/40% tin composition.
With pot metal, what mattered most was gathering enough material, and getting the melting point low enough. Throw in enough material and it would get there eventually, pretty much at random.
Advantages of pot metal
The advantage of pot metal is that it’s very cheap, because it involves material that otherwise would have been wasted. In days of old, it was impossible to beat the cost of sweeping the floor and emptying pockets. It eliminates the need for any sorting or refining, since impurities rise to the top as it melts. They could pour off the impurities, called dross, then start casting parts. It’s very efficient, as long as the requirements for the finished product aren’t very strict. The locknut on a kitchen sink is a good example. It needs to hold a thread, not rust, and hold up to big sets of pliers. Whatever shade of gray it ends up being is fine, it doesn’t need to be any specific color. And if the finish is a bit rough or uneven, no one will notice or care.
Since it’s cheap and easy to mix up and work with, it’s good for prototyping and experiments to test concepts. Then if it works out, the item can be manufactured with a better material.
Disadvantages of pot metal
Sometimes cheap random metal is good enough for the job. But newer manufacturing methods that allow for a finer level of detail, such as diecasting, have more specific composition requirements. Introducing trace amounts of lead into the mix can cause these alloys to break apart in a relatively short length of time, sometimes only 3-4 years. Suddenly, the practice of sweeping the floor to get more material for the mix goes from being smart and resourceful to being destructive.
The unstable cooling point of the material can trap air pockets in the casting, leading to fragility or brittleness. When price is the main consideration, that may be okay. But when durability and repeatability are important, more sophisticated methods, such as diecasting, or even cast iron, are more appropriate.