Varnish is triggered in various ways. In general, the cycle of varnish starts with something that upsets the stability of the lubricant, usually a thermal event such as excessive heat. This causes the oil to start degrading and impurities to start forming. For a long time, those impurities stay dissolved in the oil and don’t cause any harm. As more and more accumulate, however, they begin to stick together, forming insoluble, suspended submicron particles. Ultimately, these particles become polar, or electrically attracted to metal surfaces. When they start sticking to those surfaces, they become varnish.
Cross-contamination of oils with incompatible additives can also trigger the varnish cycle. If an operator introduces a new oil with a different additive formulation from the existing oil, the additives can react with each other, which upsets the lubricant and starts the process of degradation that ultimately leads to varnish.