What is Spark Testing? – A Complete Guide

What is Spark Testing?

Spark testing provides a simple method for basically identifying ferrous metals. The actual process is quite simple; you take a sample piece, generally any leftover scrap piece, and hold it against a grinding wheel. The resulting sparks provide information. 

You can compare these sparks with reference charts, or with sparks from a known metal composition (preferably, a sample with a similar shape) to have a good idea of which ferrous material you are testing.

This method is quite useful when sorting ferrous metals in order to differentiate quickly. If the sparks are the same, the metals are likely the same. If they spark differently, then you can safely assume they are different metals.

One of the main reasons spark testing is so popular is that it is easy. It takes very little time, we do not need any complex equipment or special preparation of the sample (scrap pieces are perfectly acceptable), and the costs to perform the test are extremely low.  Of course, spark testing is not perfect or conclusive. 

If you want to accurately identify a material, you must employ chemical analysis, as spark testing will never identify a material, as it does not provide a specific and conclusive answer. Also, you must remember that the grinding process will leave some sort of small mark on the sample itself.  

In general, we will find spark testing primarily used or performed in tool rooms, machine shops, heat treating shops, and therefore in foundries or wherever a person may need to sort or identify ferrous materials or try to identify other ferrous materials quickly.

How to Do a Spark Testing?

A bench grinder is usually used to create sparks, however, there may be times when a portable grinder is a better option. Regardless of tool choice, it is critical that the grinding wheel surface speed is sufficient ideally a minimum of 23 mps (or about 4500 sfpm).

For best speed we would want it to be in the 38 to 58 mps (7500-11500 sfpm) range.

For the wheel we want to choose a harder and coarse wheel so materials like aluminum oxide or carborundum are typical choices.

For the testing, it helps to be in an area with little direct bright lighting; we would like to be dark around the wheel and work area so to make the sparks stand out.

The test piece is then brought gently to make contact with the wheel just enough to generate sparks for viewing.

While viewing the sparks, we need to observe several features: the color, volume, character or behavior of the spark being generated, and length. Note that spark length will vary based on where the pressure is applied to the wheel.

Due to this, comparisons of lengths may be misleading unless the same pressure is applied across samples. Finally, wheel maintenance is important. Dressing the wheel frequently is important to eliminate any metallic build-up that could affect results.

Compressed air method

An unusual method of obtaining sparks is to pre-heat the sample to the hot red state and then blow it with compressed air. In the introduction of the compressed air, there is sufficient oxygen to ignite the sample material, which produces the sparks.

In many cases, this method represents better repeatability than traditional grinding techniques. Because the compressed air provides the same class each time an average number of sparks is produced, the sparks produced are the same length based on the sample. Hence, the spark length becomes a more consistent variable for comparative purposes.

Automated spark testing

Automated spark testing is used to reduce reliance on operator skill and judgment, creating a more consistent and reliable process.

Instead of observing the spark pattern directly like a human would, the system uses metrological techniques (e.g. spectroscopy and spectrometry) to analyze the spark parameters.

With instruments now able to distinguish between two materials that may be indistinguishable to the human eye based on the spark characteristics, the analysis can rely on two spectrometric signals instead of human judgment.

Spark characteristics

• Wrought Iron: Wrought iron spits sparks that travel in straight lines. The ends of the sparks, however, will spread out, appearing stretched out like a leaf uncurling.
• Mild Steel: The sparks from mild steel look similar to wrought iron, but if you look closely, mild steel tends to show you some forked sparks, and the length is not consistent. The sparks should be bright white and clean in color.
• Medium-carbon steel: If you are grinding medium-carbon steel, the sparks will show more pronounced forking with various lengths of sparks. The quantity of sparks will increase noticeably right near the wheel.
• High-carbon steel: High-carbon steel is easy to identify; you will quickly see fuzzy sparks with pronounced forking starting at the wheel. The sparks from high-carbon steel are not as bright as those from medium-carbon steel.
• Manganese steel: Manganese steel throws off medium-length sparks, which usually double, fork before extinguishing.
• High-speed steel: You will see a faint red spark on high-speed steel, but only at the tip. The spark is not as thick, but when you are actively looking for it, it will show as a recognizable spark.
• 300-series stainless steel: The spark patterns produced by 300-series stainless steel are not as thick as carbon steel spark patterns and do not fork; they appear from orange to straw and do not have a bright white element like the others.
• 310-series stainless steel: The spark pattern produced by the 310-series continues to be less in terms of thickness and size than the 300-series, as the color tones red to orange and has no forked characteristics.
• 400-series stainless steel: The sparks from 400-series stainless steel have a similar pattern to the spark from 300-series stainless steel, if not slightly longer, and have a tendency to fork at the ends.
• Cast iron: The length of the spark with cast iron is extremely short, as the rising sparks appear right at the wheel.
• Nickel and cobalt high-temperature alloys: If you see these alloys, your spark will consist of thin, very short sparks. They will be short, dark red sparks and do not have bent, forked characteristics.
• Cemented carbide: Cemented carbide produces dark red sparks all under 3 inches long, and it does not express forked characteristics.
• Titanium: Titanium is not a ferrous metal, but it will give you a fantastic show of uniquely white sparks that are almost blinding and beautiful to view, which makes them distinguished from any type of steel or iron alloy.

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