Spark Testing: Identifying the Carbon Contant of Steel

Some of you might have noticed that it’s been four weeks now, and I’ve talked a lot about yarn, but not about iron. You might have been wondering "What gives?" This blog is called “Of Iron and Yarn” not “Of Yarn”.

Well, the fact is, its been about a thousand and one degrees here in Texas. Don't even ask me about the humidity levels! Seeing as how our shop is not air conditioned, I wasn’t willing to go turn on the forge. Fortunately, the temperature has started to cool off, and I came up with something that doesn’t require the forge.

Spark Testing

Spark testing is a way of seeing how much carbon is in a piece of steel. While it’s not precise, it will tell you if its high carbon, low carbon, no carbon. Spark testing is best used when you’ve found something in your scrap pile, and you think you know what it is, but need extra verification. For example, my Dad was going to give a friend of ours a piece of metal called cru-forge-v. But he couldn’t quite remember if he had the right piece of steel. So, he spark tested it and voila! It was, in fact, cru-forge-v.

Why do we care about carbon content you ask? Because it affects the properties of the steel, which in turn affects the outcome of your project.

Low Carbon Steel

A low level of spark with little secondary branching indicates low carbon steel. As you can see, the primary branches are very long.  This type of steel is flexible and soft. For bladesmithing it is not a great choice as it doesn’t hold a great edge, it is more likely to bend than break. For blacksmithing, it is often used for decorative work as it moves well under the hammer.

High Carbon Steel

Hight carbon steel has a much higher level of spark and secondary branching.  This type of steel can be hardened, so it is good for holding an edge. This can be good choice for bladesmithing, but if improperly tempered, knives made from this steel are prone to breaking. Blacksmiths usually use high carbon steel for tool making due to its strength.

Mid Carbon Steel

Mid carbon steel is somewhere between high and low carbon. It has a spark level like high carbon, but it branches more like low carbon.  Functionally, it is great for impact tools like punches, or large chopping blades like machetes'. It holds a good edge and provides the required toughness to withstand repetitive strikes. As such, it can be used for both bladesmithing and blacksmithing.

Stainless Steel

Stainless steel has a high spark level, but the primary branches are quite short, and there is almost no secondary branching. Stainless steel is great for stock removal knife making, but is ridiculously difficult to forge. It is difficult to move once red hot (called red hard) and is prone to crumbling. Also, prolonged exposure to high heat in atmospheric forge can cause havoc on the steels structure.  Due to the difficulty in forging stainless steel, it is not used at all  in blacksmithing.

Titanium

If you find anything that looks like this DON’T FORGE IT! Anything that does not follow the typical long, yellow spark routine is potentially poisonous when the fumes are breathed in. Unless you know exactly what it is and whether or not its toxic then don't forge it.

What is pictured here is titanium, although it is not poisonous it is a good example. It has a clearly defined spark that looks a bit like Christmas lights - small and extremely bright. Technically, you can forge, but it is very expensive so most people don't. Titanium can be a good choice for bolsters and guards on a knife.

Wrought Iron

Wrought iron has a fairly high level of spark,  the primary branches are of medium length,.   There is  no secondary branching with wrought iron, but the sparks are more of a golden color then yellow. True wrought iron has hardly any carbon. It is the king for forgeability, but please, I beg of you, don’t make a knife out of it. You can’t harden it.  Of note: There is only one producer of wrought iron in the world.  Located in England, they only produce one batch each year.  Most things called wrought iron now days is actually just mild steel that is called wrought iron out of tradition. 

                                                                   

After looking at all this you might ask "why does it spark like that?" You see, sparks are little flakes of steel that are burning due to friction. But its not just the iron that’s burning, it’s the carbon and other elements as well. The carbon is what causes the spark pattern to branch out, so the higher the carbon, the larger the branching.

Keep in mind that other elements that can affect the spark pattern as well.  On occasion, you will get a piece that has so many other elements, it drowns out the carbon, making it appear to have no carbon. That is why its usually best to always work with known steel, but what’s the fun in that?

Again, spark testing is best used when you already have an idea of kind of steel you have. Its a good idea to create a spark testing kit: a box filled with marked, known metals. It is used to compare the spark of unmarked steel against a piece in your box. However, remember that it is only an indicator of what material it might be. Steels that show a similar spark pattern may require significantly different heat treating.  That being said, it’s a cheap, handy tool to use when trying to identify mystery metal!