Fighting fire: from ancient Egypt to Notre-Dame de Paris

Article written in partnership with The Conversation France.
By Rodolphe Sonnier, IMT Mines Alès.
This article was co-authored by Clément Lacoste (IMT Mines Alès), Laurent Ferry (IMT Mines Alès) and Henri Vahabi (Université de Lorraine).

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[dropcap]T[/dropcap]he discovery of fire is often cited as the most important discovery in the history of mankind, given its major impact on the development of the Homo genus. By reducing the amount of energy required to digest food, cooking led to an increase in brain size. Fire seems to have been mastered approximately 400,000 years ago, although evidence of its use much earlier has been found. However, with urbanization, fire has also become a serious problem when it spreads uncontrollably. Examples include the great fire of Rome in the year 64 AD or the recent fire at the Notre Dame de Paris Cathedral.

What is fire?

A fire requires a combination of three elements: a fuel source, an oxidizer and a heat source. This combination of elements is called the fire triangle. These elements interact through a complex process involving physical phenomena, such as heat transfer, and chemical phenomena, such as pyrolysis of the fuel source and combustion of the pyrolysis products.

Technically, a distinction is made between reaction to fire and fire resistance. Reaction to fire involves the combustible materials, which are likely to release heat when they decompose as a result of the temperature and in the presence of an oxidizer (most often the oxygen present in the air). Fire resistance considers an element’s ability to maintain its load-bearing capacity, thermal insulation and smoke and gas tightness properties during a fire. Since wood is a combustible material used as a structural element in buildings, it is considered in light of both of these aspects, which rely on specific standards and a variety tests.

When it comes to fighting fire, there are two strategies which are not mutually exclusive. The first calls for using what are referred to as active systems in the event of a fire: extinguishers, smoke detectors or automatic sprinkler systems. The second consists in using materials that will contribute as little as possible to the propagation of the fire.

Fireproofing

Since many materials, including most plastics and wood, are naturally highly flammable, additives called flame retardants must be incorporated within or on the surface of the flammable material. These flame retardants make it possible to modify the material’s behavior by disrupting the fire triangle.

Their effects are mainly to delay the appearance of flames, slow down flame propagation speed, reduce the heat released and the power of the fire, and limit the opacity and toxicity of the smoke.  All these effects are assessed through standardized reaction to fire tests. They result in classifications that determine the potential use of a material for a given application according to regulations. There is no universal flame retardant. A fireproofing system must be tailored to the material it is intended to protect, in particular by taking into consideration its decomposition process. Furthermore, the choice of a flame retardant is also informed by the process used to manufacture the material and must not have a significant effect on its intended functional properties.

Archeologists place the beginnings of fireproofing in antiquity. Around 400 BC, the Egyptians used minerals to make certain fabrics like cotton or linen fire-resistant. Later,  during the siege of Piraeus (23 BC), alum solutions were used to make the wooden ramparts fire resistant. Yet it was not until 18 June 1735 that Englishman Obadiah Wyld would file the first patent, patent number 551, for a cotton treatment. In the 19^th century, the king of France, Louis XVIII, requested that a solution be found to prevent fires in Parisian theaters which were lit with candles.  Joseph Louis Gay-Lussac filed a patent for the use of a mixture of ammonium phosphate, ammonium chloride and borax to fireproof the curtains in theaters.

Flame retardants

There are several families of flame retardants, which are based on different chemical elements and work in various ways. Historically, halogenated molecules containing chlorine or bromine have been widely used since they are effective even in small quantities. These molecules act by disrupting the combustion reactions that take place within flames, which makes it easier to extinguish them and limits the amount of energy released. This is referred to as flame inhibition. However, the toxic nature of certain halogen compounds has led to a ban on their use. Since it is impossible during recycling to easily distinguish between authorized and prohibited brominated molecules, it is no longer possible to recycle plastics treated with these flame retardants. Moreover, these molecules lead to the formation of opaque, corrosive smoke in the event of a fire. For all these reasons, this family of fire retardants has increasingly come under scrutiny.

It has been replaced largely by phosphorous flame retardants. Given the wide variety of such retardants , they are able to act in a number of ways. But the main mode of action remains facilitating the formation of a residual layer on the surface of the combustible material, protecting the healthy part of the material. The strategy consists in disrupting pyrolysis reactions (decomposition of the material due to the heat) and facilitating the formation of a thermally stable residue rich in carbon, called “char.” Particularly effective systems are called intumescent because the char forms an expanded, insulating, very protective layer. This type of intumescent system is used in protective coatings for metallic components or wood.

We can also mention metallic hydroxides, which are inexpensive but proportionally less effective, meaning they must be incorporated at higher levels (up to 65% in mass in outer sheathes for wires) to produce a significant effect. As the result of the temperature, these particles release water in the form of vapor through endothermic decomposition, thereby contributing to cooling off the material and diluting the fuel in the flame.

There are also other chemistries, based on nitrogen (melamine), boron (zinc borate) or tin (hydroxystannate) for example. Nanotechnologies have also been used in the field of fireproofing for the past fifteen years. Lamelar clay or carbon nanotube type nanoparticles improve the insulating properties of the char formed, even at low levels.  But on their own, they are insufficient for providing overall protection of the material.

And wood?

In general, materials of organic origin (derived from biological organisms) such as oil, wood and coal   all have a composition rich in carbon and hydrogen atoms, which are likely to be oxidized. They are therefore combustible. Wood is a material with a complex structure and an elemental chemical composition made up of carbon (50%), oxygen (44%), and a small amount of hydrogen (6%).

Wood is a low-density material and possesses a natural ability to char, meaning a protective layer of char is formed between healthy wood and flames. When wood is burned, it first loses water, and becomes completely dry at 120 °C. Then, its structure gradually decomposes as the temperature rises. Its components remain relatively stable up to 250 °C, the temperature at which wood starts giving off smoke. At 320 °C, there is enough gas to ignite wood. Pyrolysis takes place mainly up to 500 °C,  after which point only charcoal (char) remains, which can slowly decompose through oxidation. While the char layer slows down the pyrolysis of the underlying healthy wood, its mechanical strength is negligible. As pyrolysis continues, the useful section of a wooden structural element is therefore reduced along with its load-bearing capacity.

The flame retardants used to fireproof wood belong to the families listed above (phosphorous, boron, nitrogen, metallic hydroxides). However, unlike with plastics, it is not possible to incorporate these additives when wood is manufactured.  Fireproofing therefore takes one of two forms: applying a surface coating (paint, varnish) or impregnating the core of the wood, meaning the hollow part – called the lumen – of wood cells through an autoclave process. The process involves filling all the lumens by degassing under vacuum then forcing the flame retardant to penetrate the wood by subjecting it to high pressure. This more complex solution makes it possible to prevent a diminution of the material’s fire resistant property in the event of surface defects.  When a coating deteriorates, it can no longer play its role as a flame retardant, leaving the wood without protection in the event of a fire.

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This article was co-authored by Clément Lacoste (IMT Mines Alès), Laurent Ferry (IMT Mines Alès) and Henri Vahabi (Université de Lorraine).

Rodolphe Sonnier, Mines Alès – Institut Mines-Télécom

The original version of this article (in French) was published on The Conversation and republished under a Creative Commons license. Read the original article.