A thin, hard, lustrous, oil-insoluble deposit, composed primarily of organic residue, and most readily definable by color intensity. It is not easily removed by wiping with a clean, dry, soft, lint-free wiping material and is resistant to saturated solvents. Its color may vary, but it usually appears in grey, brown or amber hues. Source: ASTM D7843-18



Typically,lubricants degrade in service due to chemical, thermal, mechanical stress which accelerates the reaction of oil oxidation and the varnish formation begins with oxidation.

Phase 1: Oxidation

-Chemical: Many chemical reactions occur as the oil ages. Oxidation of oil leads to numerous decomposition products, including insoluble particulates and acids. Heat and the presence of metal particulars (Iron, Copper) accelerate the process. Additionally, highly aerated oils are far more susceptible to Oxidation.

-Thermal: When air bubbles become entrained in the oil, severe failure of the oil may occur due to the conditions known as PID (Pressure-induced Dieseling) Or PTG (Pressure-induced Thermal Degradation). The localized temperatures exceeds 538℃ when air bubbles are collapsed under high pressure, which also lead to thermal degradation.

-Mechanical: “Shearing” occurs when oil molecules are torn apart as they flow between moving mechanical surfaces.

Phase 2: Polymerization

Polymerization occurs as the oxidation products & additive reactions combine and create long-chain molecules with higher molecular weight. These molecules are polarized. The rate of molecular polymerization depends on temperature and the concentration of by-products of oxidation.

Phase 3: Solubility

It indicates the capability to dissolve the molecules within the solution that is affected directly by temperature. As by-products of oxidation are continuously created, the fluid is close to saturation point.


The process responsible for the deposition of particulate varnish is reversible. In most cases, once varnish forms, they can be reabsorbed into the fluid and broke down if the solubility of the lubricant increases.

Phase 4: Precipitation

The fluid can’t dissolve new polymerized molecules upon saturation point is reached or fluid passes through the cool zones (The solubility decreases when temperature drops). As additional oxidative products cannot be held in solution, they precipitate out and form soft particles(sludge/varnish).

Phase 5: Agglomeration

The insoluble soft particles are easy to agglomerate each other and form larger polarized particles with higher molecular weight.

Phase 6: Varnish formed

Metals are more polar than these polarized particles so that they easily accumulate on the metal surface (cool zones, fine clearance, low flow) where a sticky layer(Varnish) is formed and attract more particles adhere to it. That’s how varnish formed

Varnish Harzds

Sticking and seizing valves

◆ Overheated bearings

◆ Decreased effectiveness of heat exchangers

◆ Increased wear on critical components and valves

◆ Shortened lifetime of Machinery, lubricant, filters and seals


Due to the costly consequence of varnish presence, You have to monitor the condition of varnish potential in your lubricating system. The most widely adopted techniques is Membrane Patch Colorimetry (MPC ASTM7843). This test method extracts insoluble contaminants from a sample of in-service turbine oil onto a patch (with a 0.45µm membrane) and the color of the membrane patch is analyzed by a spectrophotometer. The results are reported as ΔE value.



Model Soluble Varnish Insoluble Varnish Water

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