Mineral oil, a clear and odorless substance, is a refined by-product of petroleum distillation, making it a liquid hydrocarbon. It is a combustible liquid, but it is not flammable in the way that gasoline or alcohol are. At room temperatures, mineral oil will not ignite if exposed to a spark or open flame because it requires significant heat to release ignitable vapors. This resistance is due to its chemical composition and extensive refining process.
Chemical Properties and Resistance to Ignition
Mineral oil’s resistance to ignition is built into its molecular structure, which consists primarily of saturated hydrocarbons. These long, stable chains make the substance chemically inert, meaning it is largely unreactive under normal conditions. Refining crude oil removes the lighter, more volatile compounds that would otherwise lower the ignition temperature.
This stability means the liquid itself does not burn; rather, the vapors it releases when heated are what catch fire. Because the process removes easily vaporized molecules, a great deal of external heat energy is required to break down the heavier molecules and produce sufficient combustible vapor. Mineral oil is classified as having a minimal fire hazard, as it must be preheated significantly before ignition can occur. Its non-volatile nature allows it to be stored and handled safely without the fire risks associated with other petroleum products.
Defining Flash Point and Fire Point
Understanding the fire risk requires knowing two distinct thermal measurements: the flash point and the fire point. The flash point is the lowest temperature at which a liquid produces enough vapor to form a momentary ignitable mixture with air near the liquid’s surface when an external ignition source is applied. This brief event is not a sustained fire, but an instantaneous flash.
For common grades of mineral oil, the flash point is high, typically falling in the range of 135°C to 155°C (approximately 275°F to 311°F). This high temperature threshold indicates that mineral oil is not a fire hazard under ambient conditions. Reaching this temperature usually requires the oil to be heated in an industrial setting or exposed to an intense, pre-existing fire.
The fire point is a higher temperature than the flash point, defined as the temperature at which the substance produces enough vapor to sustain combustion for at least five seconds after ignition. This is when a true, self-sustaining fire begins. Mineral oil’s fire point is often about 15°C to 20°C (27°F to 36°F) higher than its flash point.
This distinction is important because while the oil may flash briefly at the lower temperature, sustained combustion only happens once the liquid reaches the higher fire point. These high temperature requirements emphasize that mineral oil is a combustible liquid that requires substantial external heat to burn, unlike highly flammable liquids that ignite easily at or below room temperature.
Safety Measures and Handling Mineral Oil
Given mineral oil’s high ignition thresholds, safety measures focus on preventing it from reaching those temperatures. Containers should be stored in a cool, dry, and well-ventilated area, away from potential sources of heat, sparks, or open flames. Keeping containers tightly closed when not in use also helps prevent contamination that could introduce more volatile, lower-flash-point substances.
If a spill occurs, absorb the liquid with inert materials like sand or vermiculite, and then dispose of the contaminated material according to local regulations. Although the oil itself resists ignition, oil-soaked materials can still pose a fire risk if exposed to high heat.
In the event that a mineral oil fire does start, it is classified as a Class B fire, involving flammable liquids. The correct extinguishing agents include dry chemical powders, carbon dioxide, or foam. Avoid using a direct stream of water on a hot oil fire, as water is denser than oil and can sink, instantly turning to steam and causing the burning oil to froth and splatter, which spreads the fire.