In this article I will endeavor to explain the chemistry of pyrotechnic combustion. Whilst a little bit of the chemistry will be involved, I will try and explain it in simple terms.
Let’s start with the flame. This flame has a certain shape. For practical reasons we often speak of a ‘flame envelope;’ that is the volume of the flame in which all these chemical combustion reactions take place. Inside this flame envelope many different chemical species are created. Some chemical species only exist for a very short time. Others can exist longer. So, we are looking at very short-lived chemical reactions where new chemicals are synthesized; new chemicals are created, but they only live for a short period of time.
The chemicals we need for the flame were not there to begin with, for example let’s take a look at a typical red pyrotechnic flame. The flame requires a very specific chemical species to be created: a molecule called strontium monochloride, and going even further; strontium monochloride needs to be brought to an ‘excited state.’ To start to understand the ‘excited state’ of this molecule imagine that this a string on a guitar if it’s being pulled on, it is then in this excited state. When it falls back to its equilibrium state, it emits a very specific frequency. In our pyrotechnic example it is the wavelengths that we associate with the color red. In reality there are a great number of emission lines in the spectrum visible wavelength, almost like a fingerprint of this chemical.
Besides the color, of course there is the light intensity. Some colored flames are ‘faint’ and some are intensely bright. The latter is usually the result of the fact that some pyrotechnic compositions contain metallic powders, which burn up as a fuel. Frequently used metal powders in colors are magnalium,
magnesium and aluminium. Magnalium is a term that is used to describe an alloy of magnesium and aluminium.
So why it is so hard to create blue? We do not exactly know why, it is a really big problem. This topic is so complicated, it could be the basis for someone’s PhD study. There is a great deal of speculation available in various literature, but there is not a lot of direct scientific experimental evidence. Granted, it is quite an expensive effort to produce experimental data, because you need spectrometers. It is said that the instability of the chemical species that we need for blue is Copper monochloride CuCl. In simple words one could say that it falls apart (due to vibrations inside the molecule) as the temperature becomes too high. This is why it is said that especially with blue the flames cannot get too hot. Which makes sense as it is unusual to see highly luminous blue stars or comets.
Another thing that does not help is the fact that impurities literally destroy the deep blue color. What impurities do we mean? Well, impurities in the form of dissolved salts which are everywhere, as with the crystals that form on your tap water (if you have hard water). In addition the eye is more sensitive to other colors, so that’s why blue can also look weak and pale.
It is said that some of the best colors that were created in the past were made with chemicals that are now forbidden. Chemicals like Paris Green, or better known as copper(II) aceto arsetinite.
Let’s look at some of the ‘easier’ colors to make such as Yellow and Red. The main reason that they are easier to create is that they are closely located to the point where natural impurities draw them to and or where the law of physics draws them to (Plank radiation). Another hard color to create is deep green. Other colors that would need much discussion are mixed colors such as Purple, Pink, Aqua, etc.
Can you make a colored flame by mixing salts to Black Powder? No, it is not possible to create a deep saturated color based on Black Powder, mainly because a ‘BP’ flame already has an orange color. For a good colored flame you need a ‘neutral’ flame, such as an alcohol flame.
Back to the practical reality, it is useful to bear in mind that the final application drives the design of the pyrotechnic composition; in this case a color composition. A colored flash report composition will have a very different formulation to that of a 1 minute Bengal fire.
When did people see the first colored fireworks? This depends on what you would say is a real color. From the earliest days of entertainment fireworks, pyrotechnicians were able to create colors ranging from reddish/gold to a pale white, just be varying the ingredients and so the temperature of the flame. This in essence made good use of the laws of physics, using black body radiation, a little bit like an incandescent light bulb can do when you turn up the current through the wire. Other than the shades of red, yellow and white there was not a lot to be found. But after Berthollet invented the new oxidizer potassium chlorate in 1786, the world of pyrotechnic changed for good. Carefully using potassium chlorate, it became possible to create green and blue. The more pyrotechnicians worked on this, the better the colors became.