There are various methods and terms used to measure the flammability and combustibility of substances. Two of these are flash point and ignition temperature. Despite their similarities, they are very different terms.
Flash point
The flash point is the lowest temperature at which a substance evaporates into a gas that can be ignited by introducing an external source of ignition. There are two main methods of measuring flash point - open cup and closed cup.
The open cup flash point test occurs when a substance is placed in a container that is open to the outside atmosphere. Its temperature is then gradually increased, and an ignition source is passed over its top at regular intervals. Once a substance "flashes" or ignites, it has reached its flash point.
The closed cup flash point test is carried out in a closed container, and the ignition source is introduced into the container. As a result, the substance is not exposed to elements outside the container, which could interfere with test results. This, in turn, results in a lower flash point because the heat is trapped inside. Because it has a lower melting point, it also has a safer flash point and is widely available, so it is more generally accepted.
ignition temperature
Unlike flash point, ignition temperature does not require an ignition source. In other words, the ignition temperature is the lowest temperature at which a volatile substance will evaporate into a gas and ignite without the aid of any external flame or ignition source. As a result, the ignition temperature is of course higher than the flash point.
Typically, the ignition temperature is measured by placing the substance in a half-liter container and placing it in a temperature-controlled oven. The current standard procedure for such testing is outlined in ASTM E659.
However, it should be emphasized that neither of these terms pertains to an essentially invariant property of the substance. Rather, they are empirical results obtained through laboratory testing, which may vary depending on the equipment used, method chosen and usage environment.
