The Spark Test and Spark Testing Metals


Ahhh, the allusive spark test. This indispensable tool/test in sorting metals/alloys has been used for at least the past few centuries, along with the magnet test. With the standardization of metals, it can be quite easy to distinguish many types of alloys just by looking at the stream of sparks they throw off (with a little practice of course)… Oh, and before starting the spark test, please be sure you have a large enough sample of metal. If it is so thin that it just melts, then you will not get an accurate spark stream.

The first thing you want to do, is find a grinder of some kind. A bench grinder should be used, as opposed to an angle grinder or the like. Plus, I would rather hold the sample than hold the grinder, and that way I can get a nice steady stream of sparks due to control.I tried writing out good description of the sparks of metal, but the truth is, this info graphic is 100x better than a written description. Here is this amazing pictorial I found in a very old magazine.

The Spark Comparison: The look of a spark is only useful if you know what it should look like! That is why above the grinding wheel in my shop you will find a small fishing tackle box with small samples of different metals. In each compartment, I have different metals/alloys that I have come across in the past few years. (including a little Titanium). All of these metals produce very different types of sparks that any shopman should become comfortable identifying. Ideally, everybody should have a small utility box of some sort in their shop or garage filled with samples of different types of labeled metals.When you come across an alloy you need to identify, do a Magnet Test. That will narrow down many of the alloys in your box. Then, cross reference your sample's sparks with the sparks of your known metals.

Note worth characteristics: Process

A bench grinder is usually used to create the sparks, but sometimes this is not convenient, so a portable grinder is used. In either case, the grinding wheel must have adequate surface velocity, at least 23 m/s (4500 surface feet per minute (sfpm)), but should be between 38 and 58 m/s (7500–11,500 sfpm). The wheel should be coarse and hard, therefore aluminium oxide or carborundum often are employed. The test area should be in an area where there is no bright light shining directly into the observer's eyes. Moreover, the grinding wheel and surrounding area should be dark so that the sparks can be observed clearly. The test sample is then touched lightly to the grinding wheel to produce the sparks.[1][2]

The important spark characteristics are color, volume, nature of the spark, and length. Note that the length is dependent on the amount of pressure applied to the grinding wheel, so this can be a poor comparison tool if the pressure is not exactly the same for the samples. Also, the grinding wheel must be dressed frequently to remove metallic build-up.[1][2]


Compressed air method

Another less common method for creating sparks is heating up the sample to red heat and then applying compressed air to the sample. The compressed air supplies enough oxygen to ignite the sample and give off sparks. This method is more accurate than using a grinder because it will always give off sparks of the same length for the same sample. The compressed air applies in essence the same "pressure" each time. This makes observations of the spark length a much more reliable characteristic for comparison.[4]


Automated spark testing

Automated spark testing has been developed to remove the reliance upon operator skill and experience, thereby increasing reliability. The system relies upon spectroscopy, spectrometry, and other methods to "observe" the spark pattern. It has been found that this system can determine the difference between two materials that give off

Spark characteristics

Wrought iron sparks flow out in straight lines. The tails of the sparks widen out near the end, similar to a leaf.[1][5]

Mild steel sparks are similar to wrought iron's, except they will have tiny forks and their lengths will vary more. The sparks will be white in color.[1][5]

Medium-carbon steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel.[5]

High-carbon steel has a bushy spark pattern (lots of forking) that starts at the grinding wheel. The sparks are not as bright as the medium-carbon steel ones.[5]

Manganese steel has medium length sparks that fork twice before ending.[5]

High-speed steel has a faint red spark that sparks at the tip.[5]

300-series stainless steel sparks are not so dense as the carbon steel sparks, do not fork, and are orange to straw in color.[2]

310-series stainless steel sparks are much shorter and thinner than the 300-series sparks. They are red to orange in color and do not fork.[2]

400-series sparks are similar to 300-series sparks, but are slightly longer and have forks at the ends of the sparks.[2]

Cast iron has very short sparks that begin at the grinding wheel.[1]

Nickel and cobalt high-temperature alloys sparks are thin and very short, they are dark-red in color, and do not fork.[2]

Cemented carbide has sparks under 3 inches, which are dark-red in color and do not fork.[6]

Titanium is a non-ferrous metal, it gives off a great deal of sparks. These sparks are easily distinguishable from ferrous metals, as they are a very brilliant, blinding, white color.[7]





Source: Wikipedia http://en.wikipedia.org/wiki/File:Spark_testing_2.pngEnglish: Diagram of sparks for spark testing various types of steel. Key:
  1. High-carbon steel
  2. Manganese steel
  3. Tungsten steel
  4. Molybdenum steel


  1. Wrought iron
  2. Mild steel
  3. Steel with 0.5 to 0.85% carbon
  4. High-carbon tool steel
  5. High-speed steel
  6. Manganese steel
  7. Mushet steel
  8. Special magnet steel

References




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