🛢️ When MPC ΔL is High and the Patch Turns Black (Soot Formation & High-Temperature Degradation in Turbine Oils) 1. First – What Does ΔL Really Mean? In the ASTM D7843 MPC test, the L value represents lightness (white → black): Black coloration is not typical varnish — it indicates carbonaceous degradation products. Black patches are commonly linked to soot formation … Continue reading 🛢️ When MPC ΔL is High and the Patch Turns Black