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Firstly, 'Fire', according to numerous comments and answers here[here][1] is a 'process', in which case, the answer to the question will be 'no', since plasma is a state of matter. It would be unfair to leave it there by blaming the semantics, and given the abundant references to 'flame' region, I am going to assume that that is what the question meant to ask. I am also assuming that proving a candle flame constitutes plasma is enough to sufficiently answer the question.

From some papers (a quick google search gave me [2][2,[3]3]) that flames have ionised content and that they are electrically conductive. My suspicion was that not all flames are conductive, but [3] includes the statement:

It has been known for a long time that flames possess a high electrical conductivity and can be distorted by an electric field.

Sources [4] and [5],[5] and numerous other sources, including a video on youtubeYouTube [6] claim that a candle flame is ionised and that's what causes the flame to be affected by electric field.

Francis F Chen's book[10]book [10] includes an exercise (you can temporarily check it here: http://socrates.berkeley.edu/~phylabs/adv/ReprintsPDF/HAL%20Reprints/06%20-%20Intro%20to%20Plasma%20Physics_CH1.pdf) on page 12 that connotes a typical flame being plasma. This claim is repeated in [4] and [5] (refers to candle flame).

I understand that the Plasma coalition paper[8]paper [8] says that the temperature of a candle is too low for much ionisation to occur, but technically, the experiments cited above[2][4][6]above [2,4,6], demonstrating the significant effect of flames in an electric field, coupled with the theoretical predictions[3][10]predictions [3,10] seem to imply that the flame is indeed a plasma. Even by the condition stated by the Plasma Coalition [11] itself!

_[2]:$\ \ $ [1] Electrical Properties of Flames: Burner Flames in Longitudinal Electric Fields Hartwellhttp://physics.stackexchange.com/questions/9708/is-fire-matter-or-energy, Physics Stack Exchange.

$\ \ $ [2] Electrical Properties of Flames: Burner Flames in Longitudinal Electric Fields. Hartwell F. Calcot and Robert N. Pease, Princeton University, Princeton, N. 1Ind. Eng. Chem. 43 no. 12, pp 2726–2731 (1951).

_[3]: Mechanisms for the formation of ions in flames,$\ \ $ [3] Mechanisms for the formation of ions in flames. H.F. Calcote 1957. Combust. Flame. 1 no. 4, pp. 385–403 (1957).

_[4]:$\ \ $ [4] Waves in Dusty Space Plasmas,. Frank Verheest (Kluwer Academic, 2000, The Netherlands).

_[5]:$\ \ $ [5] Sun, Earth and Sky,. Kenneth R. Lang (Springer, 2006, Berlin).

_[6]: What's in a candle flame, Veritasium$\ \ $ [6] http://www.youtube.com/watch?v=a7_8Gc_Llr8What's in a candle flame, Veritasium YouTube Channel.

_[7]:$\ \ $ [7] http://www.plasmacoalition.org/about.htmAbout the Coalition for Plasma Science.

_[8]:$\ \ $ [8] http://www.plasmacoalition.org/plasma_writeups/flame.pdfAbout Plasmas. Coalition of Plasma Science, 2008.

_[9]:$\ \ $ [9] http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_04.pdfPlasma State of Matter. Lecture notes for PX384 Electrodynamics at Warwick University, chapter IV. Erwin Verwichte, 2013.

_[10]:$\ $ [10] Introduction to Plasma Physics and Controlled Fusion,. Francis Chen. Available here for the moment.

_[11]: What is Plasma$\ $ [11] http://www.plasmacoalition.org/what.htmWhat is Plasma?. Coalition for Plasma Science, 2000.

Firstly, 'Fire', according to numerous comments and answers here is a 'process', in which case, the answer to the question will be 'no', since plasma is a state of matter. It would be unfair to leave it there by blaming the semantics, and given the abundant references to 'flame' region, I am going to assume that that is what the question meant to ask. I am also assuming that proving a candle flame constitutes plasma is enough to sufficiently answer the question.

From some papers (a quick google search gave me [2],[3]) that flames have ionised content and that they are electrically conductive. My suspicion was that not all flames are conductive, but [3] includes the statement:

It has been known for a long time that flames possess a high electrical conductivity and can be distorted by an electric field

[4],[5] and numerous other sources, including a video on youtube [6] claim that a candle flame is ionised and that's what causes the flame to be affected by electric field.

Francis F Chen's book[10] includes an exercise (you can temporarily check it here: http://socrates.berkeley.edu/~phylabs/adv/ReprintsPDF/HAL%20Reprints/06%20-%20Intro%20to%20Plasma%20Physics_CH1.pdf) on page 12 that connotes a typical flame being plasma. This claim is repeated in [4] and [5] (refers to candle flame).

I understand that Plasma coalition paper[8] says that the temperature of a candle is too low for much ionisation to occur, but technically, the experiments cited above[2][4][6], demonstrating the significant effect of flames in an electric field, coupled with the theoretical predictions[3][10] seem to imply that the flame is indeed a plasma. Even by the condition stated by Plasma Coalition [11] itself!

_[2]: Electrical Properties of Flames: Burner Flames in Longitudinal Electric Fields Hartwell F. Calcot and Robert N. Pease, Princeton University, Princeton, N. 1

_[3]: Mechanisms for the formation of ions in flames, H.F. Calcote 1957

_[4]: Waves in Dusty Space Plasmas, Frank Verheest

_[5]: Sun, Earth and Sky, Kenneth R. Lang

_[6]: What's in a candle flame, Veritasium http://www.youtube.com/watch?v=a7_8Gc_Llr8

_[7]: http://www.plasmacoalition.org/about.htm

_[8]: http://www.plasmacoalition.org/plasma_writeups/flame.pdf

_[9]: http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_04.pdf

_[10]: Introduction to Plasma Physics and Controlled Fusion, Francis Chen

_[11]: What is Plasma http://www.plasmacoalition.org/what.htm

Firstly, 'Fire', according to numerous comments and answers [here][1] is a 'process', in which case, the answer to the question will be 'no', since plasma is a state of matter. It would be unfair to leave it there by blaming the semantics, and given the abundant references to 'flame' region, I am going to assume that that is what the question meant to ask. I am also assuming that proving a candle flame constitutes plasma is enough to sufficiently answer the question.

From some papers (a quick google search gave me [2,3]) that flames have ionised content and that they are electrically conductive. My suspicion was that not all flames are conductive, but [3] includes the statement:

It has been known for a long time that flames possess a high electrical conductivity and can be distorted by an electric field.

Sources [4] and [5], and numerous other sources, including a video on YouTube [6] claim that a candle flame is ionised and that's what causes the flame to be affected by electric field.

Francis F Chen's book [10] includes an exercise on page 12 that connotes a typical flame being plasma. This claim is repeated in [4] and [5] (refers to candle flame).

I understand that the Plasma coalition paper [8] says that the temperature of a candle is too low for much ionisation to occur, but technically, the experiments cited above [2,4,6], demonstrating the significant effect of flames in an electric field, coupled with the theoretical predictions [3,10] seem to imply that the flame is indeed a plasma. Even by the condition stated by the Plasma Coalition [11] itself!

$\ \ $ [1] http://physics.stackexchange.com/questions/9708/is-fire-matter-or-energy, Physics Stack Exchange.

$\ \ $ [2] Electrical Properties of Flames: Burner Flames in Longitudinal Electric Fields. Hartwell F. Calcot and Robert N. Pease. Ind. Eng. Chem. 43 no. 12, pp 2726–2731 (1951).

$\ \ $ [3] Mechanisms for the formation of ions in flames. H.F. Calcote. Combust. Flame. 1 no. 4, pp. 385–403 (1957).

$\ \ $ [4] Waves in Dusty Space Plasmas. Frank Verheest (Kluwer Academic, 2000, The Netherlands).

$\ \ $ [5] Sun, Earth and Sky. Kenneth R. Lang (Springer, 2006, Berlin).

$\ \ $ [6] What's in a candle flame, Veritasium YouTube Channel.

$\ \ $ [7] About the Coalition for Plasma Science.

$\ \ $ [8] About Plasmas. Coalition of Plasma Science, 2008.

$\ \ $ [9] Plasma State of Matter. Lecture notes for PX384 Electrodynamics at Warwick University, chapter IV. Erwin Verwichte, 2013.

$\ $ [10] Introduction to Plasma Physics and Controlled Fusion. Francis Chen. Available here for the moment.

$\ $ [11] What is Plasma?. Coalition for Plasma Science, 2000.

    Bounty Ended with 200 reputation awarded by Ben Crowell
1
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Firstly, 'Fire', according to numerous comments and answers here is a 'process', in which case, the answer to the question will be 'no', since plasma is a state of matter. It would be unfair to leave it there by blaming the semantics, and given the abundant references to 'flame' region, I am going to assume that that is what the question meant to ask. I am also assuming that proving a candle flame constitutes plasma is enough to sufficiently answer the question.

From some papers (a quick google search gave me [2],[3]) that flames have ionised content and that they are electrically conductive. My suspicion was that not all flames are conductive, but [3] includes the statement:

It has been known for a long time that flames possess a high electrical conductivity and can be distorted by an electric field

[4],[5] and numerous other sources, including a video on youtube [6] claim that a candle flame is ionised and that's what causes the flame to be affected by electric field.

Now is it plasma?

The 'Plasma Coalition', which is a coalition of many reputed institutes around the world [7], says that ionisation alone is not enough, but enough atoms have to be ionized to significantly affect the electrical characteristics of the gas, in order for it to be called plasma. In one of its documents [8], it expands on this description in great detail.

It actually has a paper dedicated to this question, [8], which says that some flames contain plasma, whilst others don't. It expand further in sufficient detail, claiming that the answer depends on the region, what's being burned, the temperature, etc.

It also acknowledges that the current knowledge about flames is quite limited to conclusively ascertain the charged particle densities at a particle location in the flame, as of 2008.

A wide variety of sources that claim that a flame (like a candle flame) is plasma is referring to the fact that it is ionised.

Francis F Chen's book[10] includes an exercise (you can temporarily check it here: http://socrates.berkeley.edu/~phylabs/adv/ReprintsPDF/HAL%20Reprints/06%20-%20Intro%20to%20Plasma%20Physics_CH1.pdf) on page 12 that connotes a typical flame being plasma. This claim is repeated in [4] and [5] (refers to candle flame).

My Conclusion

I understand that Plasma coalition paper[8] says that the temperature of a candle is too low for much ionisation to occur, but technically, the experiments cited above[2][4][6], demonstrating the significant effect of flames in an electric field, coupled with the theoretical predictions[3][10] seem to imply that the flame is indeed a plasma. Even by the condition stated by Plasma Coalition [11] itself!

I found it interesting that an old paper [3] proposes to explain the excessive amounts of ions formed in hydrocarbon flames by suggesting that it is in part due cumulative excitation or chemi-ionisation. I do not know if it is still relevant today.

_[2]: Electrical Properties of Flames: Burner Flames in Longitudinal Electric Fields Hartwell F. Calcot and Robert N. Pease, Princeton University, Princeton, N. 1

_[3]: Mechanisms for the formation of ions in flames, H.F. Calcote 1957

_[4]: Waves in Dusty Space Plasmas, Frank Verheest

_[5]: Sun, Earth and Sky, Kenneth R. Lang

_[6]: What's in a candle flame, Veritasium http://www.youtube.com/watch?v=a7_8Gc_Llr8

_[7]: http://www.plasmacoalition.org/about.htm

_[8]: http://www.plasmacoalition.org/plasma_writeups/flame.pdf

_[9]: http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_04.pdf

_[10]: Introduction to Plasma Physics and Controlled Fusion, Francis Chen

_[11]: What is Plasma http://www.plasmacoalition.org/what.htm