# Autoignition temperature

The autoignition temperature or kindling point of a substance is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure or oxygen concentration increases. It is usually applied to a combustible fuel mixture.

• The ignition temperature of a substance is the least temperature at which the substance starts combustion.
• Substances which spontaneously ignite in a normal atmosphere at naturally ambient temperatures are termed pyrophoric.

Autoignition temperatures of liquid chemicals are typically measured using a 500-millilitre (18 imp fl oz; 17 US fl oz) flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.[1]

When measured for plastics, autoignition temperature can be also measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service. The main testing standard for this is ASTM G72.[2]

## Autoignition equation

The time ${\displaystyle t_{\text{ig}}}$ it takes for a material to reach its autoignition temperature ${\displaystyle T_{\text{ig}}}$ when exposed to a heat flux ${\displaystyle q''}$ is given by the following equation:[3]

${\displaystyle t_{\text{ig}}={\frac {\pi }{4}}k\rho c\left[{\frac {T_{\text{ig}}-T_{0}}{q''}}\right]^{2},}$

where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, ${\displaystyle T_{0}}$ is the initial temperature of the material (or the temperature of the bulk material).

## Autoignition point of selected substances

Temperatures vary widely in the literature and should only be used as estimates. Factors that may cause variation include partial pressure of oxygen, altitude, humidity, and amount of time required for ignition. Generally the autoignition temperature for hydrocarbon/air mixtures decreases with increasing molecular mass and increasing chain length. The autoignition temperature is also higher for branched-chain hydrocarbons than for straight-chain hydrocarbons.[4]

SubstanceAutoignition[D]Note
Barium550 °C (1,022 °F)550±90[5][C]
Bismuth735 °C (1,355 °F)735±20[5][C]
Butane405 °C (761 °F)[6]
Calcium790 °C (1,450 °F)790±10[5][C]
Carbon disulfide90 °C (194 °F)[7]
Diesel or Jet A-1210 °C (410 °F)[8]
Diethyl ether160 °C (320 °F)[9]
Ethanol365 °C (689 °F)[7]
Gasoline (Petrol)247–280 °C (477–536 °F)[7]
Hydrogen536 °C (997 °F)[10]
Iron1,315 °C (2,399 °F)1315±20[5][C]
Leather / parchment200–212 °C (392–414 °F)[8][11]
Magnesium635 °C (1,175 °F)635±5[5][B][C]
Magnesium473 °C (883 °F)[7][B]
Molybdenum780 °C (1,440 °F)780±5[5][C]
Paper218–246 °C (424–475 °F)[8][12]
Phosphorus,white34 °C (93 °F)[7][A][B]
Silane21 °C (70 °F)[7] or below
Strontium1,075 °C (1,967 °F)1075±120[5][C]
Tin940 °C (1,720 °F)940±25[5][C]
Triethylborane−20 °C (−4 °F)[7]
 A On contact with an organic substance, melts otherwise. B There are two distinct results in the published literature. Both are separately listed in this table. C At 1 atm. The ignition temperature depends on the air pressure. D Under standard conditions for pressure.

## References

1. E659 – 78 (Reapproved 2000), "Standard Test Method for Autoignition Temperature of Liquid Chemicals", ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959.
2. S. Grynko, "Material Properties Explained" (2012), ISBN 1-4700-7991-7, p. 46.
3. Principles of Fire Behavior. ISBN 0-8273-7732-0. 1998.
4. Zabetakis, M. G. (1965), Flammability characteristics of combustible gases and vapours, U.S. Department of Mines, Bulletin 627.
5. Laurendeau, N. M.; Glassman, I. (1971-04-01). "Ignition Temperatures of Metals in Oxygen Atmospheres". Combustion Science and Technology. 3 (2): 77–82. doi:10.1080/00102207108952274.
6. "Butane - Safety Properties". Wolfram|Alpha.
7. Fuels and Chemicals - Autoignition Temperatures, engineeringtoolbox.com
8. Cafe, Tony. "PHYSICAL CONSTANTS FOR INVESTIGATORS". tcforensic.com.au. TC Forensic P/L. Retrieved 11 February 2015.
9. "Diethyl Ether - Safety Properties". Wolfram|Alpha.
10. "Hydrogen - Safety Properties". Wolfram|Alpha.
11. "Flammability and flame retardancy of leather". leathermag.com. Leather International / Global Trade Media. Retrieved 11 February 2015.
12. Tony Cafe. "Physical Constants for Investigators". Journal of Australian Fire Investigators. (Reproduced from "Firepoint" magazine)