MadSci Network: Chemistry

Re: are 'black flames' possible?

Date: Sun Mar 15 22:54:55 2009
Posted By: Michael Kay, President and Consultant AMBRY, Inc., and
Area of science: Chemistry
ID: 1232868557.Ch


Normally during a combustion process, there is little ultraviolet 
emission because most of the changes in the atom involve changes in the 
valence shell of electrons. The energies for changes in this region are 
normally in the visible or infrared spectral regions. 

The references for this are very wide--any college level freshman 
chemistry text will cover these changes. It will also give ionization 
energies in a table at the back of the book. 

In order to reach ultraviolet energies, the electron(s) ionized are 
usually deeper in the electron shell than the valence electrons. Small 
amounts of UV may be available from high temperature reduction of 
fluorine or chlorine.

UV light is usually generated through gas discharge tubes where an 
electric current is passed through a low pressure gas in a quartz tube. 
The mercury gas discharge tube (as well as mercury vapor arc lights used 
in gyms and outdoors) emits a high amount of UV light. There have been 
many accidental burns from mercury arc lights or gas discharge tubes. 

A new method of producing UV is presented in a patent at the following 

(Paste that into your browser, and you will get the document)

The device uses air or oxygen mixed with fluorine or chlorine and 
undergoing the reaction between two high voltage plates. That is not a 
device for home or even general use. 

The mercury vapor gas discharge tube just requires a high voltage 
transformer (a neon sign transformer will work) to maintain the 
discharge, and will produce large amounts of UV radiation.

The industrial mercury vapor lights REQUIRE shielding. There are cases on 
record of severe burns and partial or worse blindness from lights with 
missing or broken UV shields. An example is a news release from OREGON 
Occupational Safety and Health of an accidental overexposure in Salem, OR.

Old Coleman Lanterns using mantles containing thorium and cesium which 
formed oxides when the mantle was ignited after being installed.The light 
emitted by these oxide mantles is shorter in average wavelength than can 
be explained simply by the temperature of the burning gases, so it isn't 
just thermal radiation at the ambient temperature. The mantle's 
unexpected light emission is called candoluminescence and is thought to 
involve non-thermal light emitted as the result of chemical reactions and 
radiative transitions involving the burning gases and the mantle oxides.

You notice that none of the methods produce UV alone; all have a large 
component of visible radiation.

The following is a good introduction to "Black Lights" where a special 
glass is used to absorb visible light:

Black Lights are used extensively in theaters and shows; they produce 
longer wave UV light, and are safer (not totally safe) than bulbs that 
produce shorter wavelength (more energetic) UV A and B.

Be sure to wear safety goggles that absorb UV-A and UV-B rays when doing 
any experimentation involving ultraviolet emissions!

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