Ignition sensitivity of powders to electro-static discharge (ESD) stimulation is commonly tested to assess the safety of powder handling. However, the mechanisms of powder ignition by electric spark remain unclear. This paper represents an experimental study in which the spark parameters are quantified and the ESD ignition of a metal powder is investigated. A spherical powder of Mg, for which thermal ignition kinetics was described in the literature, was used in experiments. The experimental setup was built based on a commercially available apparatus for ESD ignition sensitivity testing. Additional diagnostics enabled measurements of electrical current, voltage, and spark emission in real time. The spark duration was of the order of a few μs. The spark current and voltage were always observed to have significant AC components. The electrical impedance of the spark discharge was determined experimentally using the recorded current traces and assuming that the spark could be represented as a series LRC circuit. The optical emission was filtered to separate the signals produced by the spark plasma and by the heated and igniting powder. The radiation signal produced by the igniting powder was always delayed after the spark. The delay time decreased from about 2.5 to 0.5 ms as the spark energy increased from 10 to 60 mJ; the delay remained nearly constant when the spark energy continued to increase to over 100 mJ.