A definition of Grease: Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil. ( https://en.wikipedia.org/wiki/Grease_%28lubricant%29).
Many Grease formulas actually contain 80%-95% of the base oil. It is the base oil that provides the lubrication in grease. (https://www.efficientplantmag.com/2014/02/whats-in-a-lubricant-characteristics-of-grease/)
Many Greases are listed as having a metal “complex” as the thickening agent. To give grease a wider temperature application range and enhanced properties, a second thickener known as a “complexing” agent is added to the mix. This agent is a salt, usually of the same metal hydroxide used to originally thicken the grease. If lithium is used as the alkaline agent, lithium salts are added to the mix to create lithium complex grease. (https://www.efficientplantmag.com/2014/02/whats-in-a-lubricant-characteristics-of-grease/)
Making grease is similar to making soap: Both products rely on a chemical reaction to take place between oil, fat or fatty acids (often present in the oil or added) and an alkali base material (referred to as the “thickener”) to form soap-like material. This reaction is known as “saponification.” Grease uses a variety of metal hydroxide alkaline to make and define the grease type. For example, aluminum hydroxide makes aluminum grease, lithium hydroxide makes lithium grease, and calcium hydroxide makes calcium grease. (https://www.efficientplantmag.com/2014/02/whats-in-a-lubricant-characteristics-of-grease/)
A possible problem arises because of all the different metal hydroxides in various grease formulas, you have to know that all these different metal hydroxides do not always mix well and in virtually every case where that happens the grease in a joint winds up having different characteristics that either of the 2 greases that are mixing in the joint. (please see our Grease Compatibility page for more on this at this URL:
This page should be consulted when buying grease to help you avoid mixing incompatible grease types. There are also recommendations in our page for what to do if you mix incompatible greases by mistake. If you are unsure what kind of grease is in your grease joints, and if you are working on general automotive or home machines, the most likely grease is lithium grease and other lithium greases are always compatible. If you are running heavy equipment or are greasing joints in extreme environments, you really need to check before putting new grease in those joints. You could easily save yourself a lot of time and some very expensive repairs with a little research and erring on the side of caution.
Speaking of extreme environments, this is a good time to talk about additives to greases. Additives give grease the ability to withstand extremes of pressure, temperature, humidity, provide corrosion resistance, and even make a joint waterproof (yes, there are greases designed to work underwater!). All grease containers should have the metal hydroxide type used as a thickener, all additives, and recommended uses, as well as not recommended uses. If you are looking at a grease where this information is not easily obtainable, do not buy the product, you could be in for some real problems going down that path.
Finally, there is no standardized color scheme for grease, and it comes in all the colors of the rainbow. The grease color is a decision that each manufacturer makes and outside that manufacturer’s line of greases has no bearing on the performance characteristics of any other manufacturer’s similarly colored grease.
Much of the information presented here is paraphrased from https://en.wikipedia.org/wiki/Grease_%28lubricant%29. This article is good reading if you have an interest and there are points in the article that I did not deem to be part of this page, so enjoy your reading!