The not-so-silent killer.


Much like cholesterol in the Human Cardio vascular system, varnish slowly starts to build and causes often irreparable damage.

Fluids of all types can suffer from contamination, oxidisation and degradation, but when can this become a terminal problem for machinery?

The answer is simple, when vital fluids degrade and have a negative effect on the machinery they serve.

Lubricants can start to degrade in a number of ways, some of the common factors that can start the degradation process include:

  • Heat
  • Moisture
  • Contaminants
  • Aeration
  • Wear Metals


What is oxidation?

Much like an apple, with its skin intact, it will last for a while before it starts to turn bad.

Heat can start the decomposition process quicker, as can contamination from other fruits, parasites and insects.

However, once the apple is cut, or bitten and its flesh is exposed to the air, you will notice

that the white flesh will start to turn brown in a short space of time.

This is a natural process called Oxidation and much like Apples, this process affect’s oil.

Oil is a natural product, and as such can be influenced by external influences.


Oxygen + Oil = Acid & Sludge


When oil molecules come into contact with oxygen under favourable conditions, they react to form acids and oil insoluble

by-products that form sludge and surface deposits such as varnish.

There are a number of factors that can promote oxidation and these include:

  • Heat
  • Water
  • Water debris (metals)
  • Aeration
  • Sludge

Sludge and Varnish can appear in different forms in different applications. The image on the right shows varnish manifesting

as a gold coloured glaze, whilst the image on the left shows not only the gold glazing, but the build up on dark coloured sludge


Effects of Oxidation

  • Acids can cause corrosion on metal surfaces
  • Sludge can form deposits that may block oil passages or cause valves to stick
  • Oxidation causes the lubricants viscosity to increase (except esters)
  • Oxidation by-products will drastically shorten the life of new oil additions
  • Degrades lubricant performance



A diesel engine runs in a way of Compression Ignition, rather than Spark Ignition as seen in petrol engines. Diesel, as a fuel,

has properties more akin to lubrication oil than petrol. The diesel engine will draw air in, compress this creating extremely

high temperatures before injecting the diesel fuel causing the explosion and subsequent power, before the exhaust stroke.

Micro Dieseling is a very similar process that happens within lubricant oil in high pressure situations.

In the example of a pump, the fluid entering the pump on the low pressure side will allow the air particles to be released

within the oil. These will create tiny air particles suspended within the oil. Once the oil makes its way through the pump, it is

subject to very high pressure. This compresses the oil and air suspended within it to such an extent that it causes a reaction

and tiny explosion. This subsequently causes degradation of the oil surrounding these tiny bubbles of air by extreme temperature

exposure. As the oil heats, the heat is dispersed throughout the oil content and machinery. This is evenly distributed and with the

possible use of oil coolers or other devices, maintains the temperature of the oil at a reasonably stable, yet warm, temperature.

Varnish deposits are suspended within the warm oil and move round the system.


Varnish Deposition

Varnish deposition usually follows a three stage process.

    1. Oil degradation leads to dissolved sludge within   the fluid which will slowly be moved round the   system.
    2. Oil quickly becomes saturated and the suspended sludge makes the oil appearance dark and cloudy.
    3. Oil comes into contact with cool spots around the system. This causes the suspended and  dissolved sludge to adhere to  machine surfaces.


Effect of Temperature & Moisture

As the oil is cooled, varnish changes state from a sticky substance to a hard lacquer type substance. At 70 degrees Celsius,

the varnish is clear and soluble. At 20 degrees Celsius, the varnish becomes hard and moves around as a solid within the oil.

The cooling process will allow the varnish to adhere to other particulate or surfaces.

Moisture also has an interesting effect on varnish. With a higher water content, entrained water will keep the varnish soluble

(if kept warm). If the water is removed by use of an oil drier or similar, this will cause the varnish to rapidly become a solid.

Some installation will see sudden rapid failure if the oil is dried with a high varnish content in the system.

    • Oxidised oil contains resinous material that comes out of solutions when oil is at rest.
    • This resinous material is sticky and acts like ‘flypaper’ inside valves.
    • The force required to activate a servo valve increased with dwell time when oil is degraded.
    • Excess force in conjunction with small particles can cause valves to lock and induce electrical failures.
    • 70-90% of hydraulic valve failures are contamination related.


How to identify a varnish problem

It is imperative to monitor lubricant health at regular intervals in systems. This allows you to see any developing trends

with changes in the oil as well as any unexpected spikes. Not only will this assist you with preventative maintenance,

it will tell the story of your fluids health before it has failed. If you do not regularly sample your oil, the degradation

by- products will go unmonitored and may drastically reduce the life of the fluid and subsequently the machinery.


There are a number of factors that affect oil life. These include but are not limited to;

    • Operating Temperature
    • Lubricant Quality
    • Contamination of all kinds
    • Aeration
    • Sump Size
    • Leakage
    • New Oil storage conditions or shelf time


There are a number of tests to asses oil condition, Delta Xero is a global partner of Polaris Laboratories, and as such we

are able to arrange Oil Analysis across a number of tests and disciplines. Some of these tests include;

    • Viscosity
    • Elemental Analysis (To check additive levels)
    • FTIR (To check oxidation, nitration and additive levels
    • AN Acid Number
    • BN Base Number
    • RPVOT oxidative stability
    • Ruler
    • QSA
    • Appearance and smell.



Put in simple terms, Viscosity is the ‘thickness’ of a fluid. For example, syrup is thicker than water, and as such would have a

higher viscosity.

We measure viscosity in Centistoke (cSt) for our labs and product lines. Viscosity of a fluid can often change due to the

influence of heat, but it is also affected by degradation.

Viscosity will only change when an oil has less than 5-10% of its remaining useful life. By the time varnish has formed to

impact viscosity, system performance including sluggish operation will already have occurred.

Viscosity alone should never be used to diagnose base oil health.

The following image is of oil samples obtained before during and after filtration with a Delta Xero Unit. You can clearly see

the difference in the oil samples, as well as the MPC patch samples. If you are noticing a change in the colour of the oil it is

strongly advised that further tests are completed. MPC Patch tests are performed by taking an oil sample and using a vacuum

device to draw the oil sample through the patch of varying density.

Contamination and particulate is left on the top of the patch and is able to be viewed under microscope and with the naked eye.

It is worthwhile to note that the best way to monitor your oil health is with a mixture of tests – not a single test.

When Delta Xero conducts oil analysis on your behalf, we complete a full ferrography report including MPC.


Removal Options

Simply changing the fluid once a Varnish problem is established will not cure the issue. Introducing new fluid into a system

which has varnish deposits within it, will act like a cleaning fluid. The new oil will remove the varnish from the inside of tanks,

sumps, pipework or valves and redistribute it around the system with often worsened build up in areas.

Vacuum dehydration units will have a ‘drying’ effect on varnish which will shorten the time it takes for the varnish to adhere

to surfaces inside the equipment thus increasing build-up.

Dried varnish is still removeable with the correct filtration, but instead will take considerably longer.


The most financially economic and system effective way of resolving this is to fit an offline Delta Xero filtration device.


The device will start initially by cleaning the fluid itself. Once the fluid has been cleaned, this clean fluid will start to strip varnish,

particulate and sludge from the system. This fluid continues to be filtered, and the varnish is effectively removed from the system

into the filtration cartridge.

It is important to regularly sample the oil for complete analysis in order to develop the trend and thus, results. It is normal to see an

increase in MPC or particulate count after the installation and initial drop.


Ongoing Maintenance

Removing varnish is only the first step in ensuring that your equipment and fluid stay healthy and in service for as long as possible.

Our top tips for reducing the risk of varnish:

    1. Ensure that the physical equipment is in good order. This includes all hoses and connections. Check for leaks or damage regularly.
    2. Ensure any breathers have the correct filters fitted and that these are in good clean order.
    3. If you have coolers fitted, ensure these are operating effectively. If these are water coolers, check for leaks – either way!
    4. Invest in regular oil analysis. These carry a minimal cost for peace of mind.
    5. Fit an Offline Filtration device – and follow the advice of the manufacturer with regards to filter changes.
    6. Finally, attend to maintenance issues when required, but ideally implement predictive maintenance with the assistance of filtration.

Please get in touch with Micron Eagle to assess your requirements