Machining of various electrically non-conductive materials is possible with Spark Assisted Chemical Engraving (SACE). Even though this technology presents several interesting properties like simplicity, flexibility and the possibility to obtain very smooth machined surfaces, it has one severe weakness: reproducible machining can hardly be achieved. One of the main limiting factors is the unstable gas film around the tool electrode in which the necessary electrical discharges for machining take place. The known facts about this gas film are reviewed and a theoretical model allowing an estimation of its thickness is derived. An experimental method for measuring this thickness using the inspection of the current-voltage characteristics of the process is presented. Several methods to obtain more reproducible machining are proposed. It is demonstrated that decreasing the gas film thickness by changing the wettability of the tool electrode can result in significantly higher machining repeatability.