The terms “flammable,” “inflammable,” and combustible all describe materials that will burn. Semantics and nebulous definitions of each have led not only to a great deal of confusion over the years but, probably, to several unfortunate accidents. In doing the research for this blog, I discovered that there is no simple and fast definition of any of these terms. I’ll just try to distill what I learned into something that will, at least, promote err on the side of caution.
First off, “flammable” and “inflammable” both mean the same thing! There seems to be a common misconception that “inflammable” indicates something that will not burn. NOT TRUE! The confusion stems from the origin of the words coming from different branches of the language tree. At the same time, something that is “non-flammable” may well burn under the right conditions.
“Combustible” describes things that will burn under the proper conditions. Many “combustible” materials may, in fact, be labeled “non-flammable” especially if they require the continuous addition of heat, oxidizer or some other impetus to stay ablaze. In fact, almost any substance will burn under the proper conditions. Some materials change classification depending on temperature. The “flash point” defines a temperature at which a combustible liquid becomes a flammable liquid. Alcohol, for example, is a combustible liquid at moderately low temperatures. Once the flash point temperature is exceeded, however, there is sufficient flammable vapor (produced by increased evaporation of liquid into gas above the liquid surface due to the increased temperature) that, mixed with air, will ignite with a minimum ignition source like a spark or electrical discharge.
One example of a “combustible” substance is wood. It is difficult, for example, to set a log ablaze in the fireplace with a single match. The job is easily accomplished, however, if an intermediary, more flammable, material like paper or kindling is used. Once the paper or kindling (which is more easily ignited by a match) is burning, it provides sufficient heat to cause the combustion cycle of the log of wood to begin. The heated wood produces flammable vapors which ignite producing additional heat causing the production of more flammable vapors. Once the cycle is begun, it continues on its own.
Another thing that differentiates (in)flammable materials from combustible materials is the degree of difficulty in initiating combustion. An electrical spark will easily ignite a mixture of natural gas and oxygen provided that they are mixed in the proper quantities (see the preceding blog on LFL and UFL). The same spark, however, will probably not ignite a log in the fireplace.
There are materials which, although they are combustible, will not support combustion on their own. In some cases, there is not sufficient heat produced by the combustion of the vapors to provide adequate ongoing vaporization of the material to provide the mix of vapor and air required to continue combustion cycle. The addition of more heat, as from the combustion of more flammable materials in the immediate vicinity, may, however, allow these materials to continue to burn. In other cases, the products of combustion actually extinguish the fire. This self-extinguishing property is sometimes enhanced, especially in plastics, by the addition of ingredients which, when heated, produce vapors that do not support combustion. “Micro-beads” containing water are an example of such an ingredient.
As I stated in the beginning, the terms defining flammability are, seeming, nebulous. The flammability rating of most materials changes depending on their application and the surrounding environment. It is best to err on the side of caution by making every effort to break the “fire triangle.”
– FJF –