Most of us know that healthy lubricants are generally clear and bright. However, as oils age they can lose their luster, and many become dark and opaque. These optical changes are often important symptoms of impending problems that, if occur prematurely, merit further analysis and corrective action. However, in other cases they can be benign or simply the result of the oil's normal aging process. So how do we know the difference, especially without having to perform complex laboratory analysis? Like much in the field of oil analysis, answering such questions depends first on better understanding the nature of the problem.
We can score an oil visually the same way gemologists grade diamonds; by color and clarity. An oil's luster or brightness is influenced by both of these optical features. Color is affected by the presence of light-absorbing impurities suspended in the oil. Totally pure oil is white; meaning it is transparent to light (little to no light is absorbed). Impurities that absorb light at select frequencies give oil its unique color or hue and are referred to as color bodies or chromophores. Color can be determined using spectrophotometers or by comparator scales (visual spectroscopy). Several color scales are available including the one referenced in ASTM D1500.
A Loss of Clarity
Some impurities don't change an oil's color but rather rob it of its clarity. This could be caused by the infusion of light-blocking oil suspensions, like soot. In other cases the problem may be the co-mingling of immiscible fluids such as air or water in oil. This can make the oil become turbid due to the diffusion of light. The emulsification of two fluids with different refractive indexes will certainly cause a hazy or cloudy appearance. Those of us who have experienced fogged-up eye glasses on a humid day know the optical consequences firsthand (temporary blindness).
Small amounts of water can give an oil a caramel color while more water will give it a milky, Irish-cream appearance. In such case, there is no need to send the sample to a lab to determine the problem or assess the amount of water present. The need for corrective action is immediate.
The Causes of Cloudiness
There are many ways an oil can lose its shine. We've already mentioned soot, air and water (liquid and vapor) as common causes of cloudiness. The following list includes a few more:
Undissolved Additives. For various reasons, a properly formulated oil may lose additive solubility. This is referred to as additive drop out. Cold temperature can cause this in certain oils. The result is a slight cloudiness and even some precipitation (settling of the additive insolubles).
Dead Additives. The normal decomposition of certain additives during service can lead to white, flocky appearance of the oil.
Salt. If saltwater has contaminated an oil, it is possible that the water has evaporated. This would leave the salt and other minerals still embedded in the oil which can result in a dull, hazy appearance depending on concentration.
Wax. Some mineral oil can form a waxy precipitant at cold temperatures. This wax can rise along the oil's upper surface and/or may be simply a hazy suspension within the body of the oil.
Other Insolubles. A hazy or milky appearance can result from mixed incompatible oils, glycol contamination, grease suspensions and a host of other impurities. Many of these contaminants will affect both the color and clarity of the oil.
Analyzing the State of Dispersion
As mentioned, a cloudy oil is due to the presence of insoluble impurities that diffuse or block the transmission of light. In contrast, color change relates to the absorption of specific optical frequencies by pigments within the oil. These pigments or color bodies could be natural to the oil's formulation, caused by contaminants, or due to the formation of degradation products.
For obvious reasons, normally dark oils are difficult if not impossible to analyze optically. Most particle counters use light to count and size particles. It is well known that their performance is greatly impaired when oils become cloudy. A simple way to better understand what is causing the haze is the attempt to remove or dissolve the offending dispersion. For instance, if you subject an oil to a vacuum and the haze disappears, the cause may have been entrained air or vapor. In many cases, dehydrating a wet, milky oil can return it to its original clear and bright appearance.
Heat alone can drive many hazy impurities into solution. Examples include water, additive floc, wax and even some oxide insolubles. You can also attempt to make the oil more turbid by putting it in a refrigerator for a period of time to better understand the influence of temperature. Solvents can be added to aid in the analysis as well. These can be used to either coagulate or dissolve target impurities. Toluene will solubilize many oxide impurities including degraded additives. Combining isopropyl alcohol with toluene enables a broader range of solubility, including the breaking of an oil/water emulsion. Haze can often be cleared up by simply mixing the used oil with new oil or performing a set of serial dilutions using select solvents until the oil regains its clarity.
If the cause of the haze is a solid or semisolid suspension, then it might be removed for inspection by patch testing. Once on the patch or membrane, the sediment can be analyzed microscopically or by infrared or XRF spectroscopy. Wax, additive floc and many organic solids can be analyzed in this manner. You can also use your membranes as sieves by doing step-down patch testing. The procedure involves using progressively finer membranes to analyze the filtrates. The residue on each membrane can then be analyzed qualitatively and quantitatively (for example, gravimetrically).
In the field, a simple laser pointer can sometimes be useful in assessing the transmission of light. Pass the laser beam through a sample of oil in a clear bottle while holding a white piece of paper on the other side. Examine how diffused or well-defined the projected light appears on the paper. Compare this to new oil or the used oil at different temperatures (both hot and cold).
Restoring the Twinkle in Your Oil
Once you've analyzed the offending cause, you hopefully have a strategy for remediation. Maybe you need to polish your oil with finer filtration (beware that these filters may plug rapidly at first). In other cases you may have to bed-in your additives first by heating the oil before returning it to normal operating temperature. If water is the problem, then dehydrators may reclaim it to a serviceable condition. All of these methods can be experimented first in the laboratory. However, it may be found that your oil is beyond repair and an oil change is prescribed. Remember, what causes haze or cloudiness can frequently lead to deposits and sludge which can impede part movement, oil flow and heat transfer - all serious risks to machine reliability.