This study is about the destruction of fly ashes from waste incinerators (F.S). It was performed in a laboratory sized plasma reactor where these F.S were treated by a transferred arc stabilized by an argon flow. The concentration of the different evaporated elements, especially heavy metals, in the exhaust gases was followed on line by an ICP analysis system. The analysis of the inert glass formed after plasma treatment was made by X-rays fluorescence which allowed measuring the conversion efficiency of the process. For an argon plasma, the type of the lead or zinc anion salts, the arc current, the arc length, and the additives to reduce the viscosity of the slag were the parameters of this study.