Spark Testing

Spark Testing is used to identify different types of steel. Carvers are always unearthing tools at flea markets or reviving tools passed down through generations. Sometimes it is just a tool that has been over sharpened and it was your favorite. Whatever the case, these are tools that you would like to be able to rework. In order to do that you must know the steel. This is the only way to know how to properly work the steel. Deferent alloys will require different heats and methods for heat treating and using the wrong ones will only make the matter worse. We are constantly being asked to heat treat existing tools and unfortunately this is not as easy as it once was. Spark testing used to be an easy, quick and accurate way to determine the type of steel. Then you could proceed with proper heat treating. In today's world of steels there are virtually thousands of different alloy combinations used in industry and this makes spark testing all the more difficult. Even the manufacturer will change the type of steel from batch to batch. This makes it virtually impossible to properly identify the type of steel. Therefore, spark testing is not used as it once was, but it can be used to get a rough classification.
Sparks will fly off steel when put to a grinding wheel and each kind of steel produces a different color and volume of sparks. The volume of sparks depends on the carbon content. Wrought iron, which has very low carbon produces very few sparks. High carbon produces many sparks. Here are a few examples of what to look for when spark testing.

Wrought iron
long yellow streaks, becoming leaf like in shape before expiring



Mild Steel
more variety in streak length with smaller leaves and some sparking



Medium Carbon Steel
almost no leaf, some forking great variety of streak length, sparking nearer the whee

High Carbon Steel
no leaf, bushy spark pattern, forking and sparking starting very close to the wheel, less bright than medium-carbon steel

High Speed Steel
Faint red streaks forking at the tip


Stainless Steel
Bright yellow streaks with a small leaf

Spark testing requires some practice to correctly classify the steel. The pressure used in grinding all the pieces should be the same. The lighting should always be the same. This is not unlike heat treating when you are learning the colors. It is a technique that its better to be shown than to read about, for once you see the colors it easier to work from there. Keep in mind the lighting for one shop will be different from another and this can produce entirely different colors.
Now this may help you determine if the tool is worth pursuing. You will find that of the thousands of alloy steels available, only a few are really of interest to wood workers. We use a very simple steel that has very little alloy content and this is because we are looking for the longest lasting edge. Our steel has only a trace amount of vanadium for this aids in the forging process and provides the steel with a finer grain. This is very desirable for our tools. Most woodcarving tools will be a high carbon steel.
When we are asked to rework other tools we look at them on a piece by piece bases. It is easier to work with older pieces, as the alloys are more predictable. More often than not it is not worth the extra time and money involved to reheat treat existing tools unless you are sure of what you have.