What Causes Varnish and Sludge?
It might first be a good idea to begin by drawing a distinction between varnish and sludge. As will be explained, sludge and varnish can form directly or through a sequence of intermediate steps. Therefore, at the risk of oversimplification, we will refer to varnish as a tough, adherent oxide or carbonaceous material that coats internal machine surfaces. Hot surfaces and/or time will often cure varnish to a hard/brittle consistency. In contrast, sludge, which is sometimes a precursor to varnish, is soft and sticky and can move about the system until finally coming to rest at sump bottoms, troughs, strainers, filters, and narrow fluid passages. Other common words for varnish and sludge include deposits, lacquer, tars, pigments, gums, and resins. The recognition and control of varnish and sludge needs to begin with an understanding of how these materials emerge from the oil. Figure 1 shows four common formation mechanisms. However, there are numerous other causes that will not be explore
