What is the Spark Test?

Sparks are sometimes the flashiest parts of a workshop. But what is happening when you make metal sparks, do all metals produce sparks and what can these sparks tell us? Using the spark test can reveal characteristics of ferrous metals, and can be used as a quick an inexpensive method to identify different metals.

Sparks are most prominently associated with the high heat produced from welding or grinding steel, either with an angle, pedestal, or surface grinder.

The grinding wheel or disk is grabbing tiny bits of the steel and ripping them off of the surface, then throwing them aside. If they’re grabbed with enough energy, the little bit of metal can become hot enough to glow. This is called incandescence, and it’s the process by which light bulbs of the same name, use to generate light. Most steels will also react with oxygen in the air and it will glow as it burns.

But not all of the little bits flying off a grinding operation will glow. Some of the chunks are too large and cool down too fast to glow. As they fly through the air, the more surface area the piece has, the faster it will cool down. So smaller pieces are more likely to glow than larger pieces.

When a piece of metal gets scraped off its parent piece, there is new metal being revealed to the air. Metal that was part of the original block that has never come in contact with air before. As a bit of metal flies through the air, one side of it was the outside of the original piece, and the other side undergoes oxidation. The process of oxidation is exothermic too, so that heats up the specimen further.

The Spark Test

A common and fundamental test to metal workers is the spark test. Each different type of metal will have a different characteristic spark pattern. You can take a sample of the unknown metal and grind it to see what it’s sparks look like. An experienced metal worker will be able to identify the type of metal just from the sparks it makes. But why do different metals spark differently? And what kinds of different sparks are possible?

The main attributes a spark has are:

• Spark color
• Spark length
• Spark forking.

Spark Color

The spark color is directly related to its temperature and its temperature is related to how much energy is in the metal chunk.

If your grinding wheel is spinning with a certain energy, it will use some of that energy to remove a tiny speck of steel from a larger test piece. This energy is removed from the system and so it has less remaining energy to send that speck flying into the air. From this we can see that harder metals, ones which require more energy to separate, will produce sparks with smaller energies, and a duller, redder color. While steels that are softer, like mild steel, will produce sparks that are white.

Spark Length

The spark length also depends on the amount of energy in the spark and how fast it cools off in air. Sparks with high surface area will cool faster than sparks of the same mass with lesser surface area. Furthermore it would seem that an increase in carbon content also shortens the length of the spark.

Spark Forking

Sometimes sparks will split in midair, like a firework, causing a magnificent display. Sometimes a single spark can fork multiple times. The oxidation of steel on the surface of the metal can produce carbon dioxide as a byproduct. If this happens inside the spark, the pressure will increase to the point where the entire mass explodes. This formation of carbon dioxide is related to an unequal distribution of carbon in the original steel. Cementite or Fe3C is thought to be the culprit here.

So now we know that when you rip off a chunk of metal with enough energy, it will glow as it travels through the air. Does it work on all metals? Not really.

While almost all solid objects will glow red hot at a certain temperature, some metals don’t spark very easily, or at all! Some people like to say that only ferrous metals will spark, but that isn’t true either, because titanium makes the most brilliant sparks of any metal.

So why won’t a piece of copper, brass, or aluminum spark?

For these metals it is mostly attributed to their softness and their thermal conductivity. Heat dissipates through these metals and back into the air so fast that there is not enough time for incandescence. Among metals, silver, copper, gold, and aluminum have some of the highest thermal conductivities, as opposed to iron and steels which have relatively low comparative thermal conductivity. Furthermore, due to their softness, larger chunks are stripped off by the grinder which require far more energy to heat up to the incandescence temperature.