Good tip.
Spark appearance is also a good way to tell whether an arc'd groove is adequately clean or not. When a gouge is done incorrectly contaminated metal coats the gouge. Once the surface of the area is ground, it can be unidentifiable by appearance. However, a lot of contamination may remain. The spark stream will give a way remaining contaminaation in a mild steel groove immediately. Of course this is no substitute for learning how to run the air arc correctly.
When the type of material is uncertain, you can sometimes use a grinder to perform a spark test to help narrow down what the material may be.
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