Fire Behaviour of Ageing Wood

In recent years the scientific literature on wood combustion has seen a considerable number of new important studies. Among these, a 2024 study (Fire Behavior Characteristics and Computational Simulation Research on Historic Wooden Structures) by  Tongshuang Liu and other researchers, gives the opportunity to take a look at the topic of great importance for historical buildings of the aging of wood exposed to fire risk.

According to the authors: ” wooden elements of historic buildings are subject to prolonged exposure to environmental elements such as wind and sunlight, which can alter their combustion properties and, consequently, affect the fire risk associated with these buildings“.

Such condition, which is more critical in historic buildings, should be related to one of the study results, according which wooden structures, particularly pine wood, become more susceptible to fire as they age.

The investigation involved testing both fresh pine wood and aged pine subjected to accelerated aging processes. Samples were analyzed using conical calorimetry to assess parameters such as heat-release rate, smoke production, carbon monoxide, and carbon dioxide emissions. The study aimed to understand how these factors contribute to the spread of fire in wooden buildings.

In particular, a case study modeled after a historic building in Xi’an, China, was conducted using Fire Dynamics Simulator (FDS) software. These simulations demonstrated that aged pine wood not only burns faster but also accelerates the spread of flames, leading to rapid temperature increases and significant smoke production.

The results clearly indicate that aging significantly enhances the flammability of pine wood.

The methodology adopted by the authors can be summarised as follows:

• Fresh pine wood and artificially accelerated aged pine wood were used.
• Combustion properties were evaluated by cone calorimetry, measuring heat release rate (HRR), smoke production, and carbon monoxide (CO) and carbon dioxide (CO2) yields.
• A historic building in Xi’an was modeled with PyroSim, and the experimentally obtained combustion characteristics were input into the model to simulate the development of fires in both new and aged buildings using FDS software.
• Temperature sensors and cross-sections were used for detailed analysis of temperature and smoke spread.

Results of the Study

The main findings of the research concern the impact of ageing and in particular that ageing does not change the patterns of flame and smoke spread, but accelerates fire spread, increases temperatures and smoke emissions, thus increasing the fire risk in historic wooden buildings.

Furthermore, simulations revealed that aged wood has a lower combustion intensity and faster smoke spread, leading to an increased fire risk. The results can be summarised as follows:

• Ageing of wood increases the susceptibility to ignition and the speed of fire spread in historic wooden structures.
• Simulations and laboratory tests confirm that historic buildings with aged wood present a higher fire risk.
• During a fire, temperatures and CO and CO2 concentrations increase more rapidly in the upper areas of the building, suggesting that escape routes at ground level are safer.

The finding of the study (that investigated the combustion characteristics of fresh pine wood and artificially aged pine wood) underscores the critical need for enhanced fire protection measures in wooden buildings, especially those with historical significance. Ultimately, the study highlights the importance of assessing fire risk and implementing fire protection measures in historic wooden buildings, taking into account the impact of wood ageing.

The role of fire retardants and fire resistants paints

Fire retardant and fire resistant paints work through different mechanisms to protect materials from combustion. These mechanisms can be classified mainly into two categories, depending on the type of paint: intumescent and non-intumescent.

Intumescent paints react to heat exposure through thermal degradation that produces an expanded carbonaceous layer. This layer acts as a non-flammable physical and thermal barrier.

The intumescence process is characterized by the formation of a carbon matrix on the surface of the material, which acts as an insulator. This layer reduces the heat transfer from the source to the substrate, protecting it from ignition and structural collapse.

The typical formulation of intumescent paints includes an acid source, which acts as a dehydrating agent, a carbon source, which forms the insulating char and a blowing agent, which releases non-flammable gases to swell the layer.

The synergy between these components and their optimal ratio are critical to the effectiveness of the paint. These paints are commonly used to protect structural steel in civil buildings.

Non-intumescent paints act primarily in the gaseous phase, releasing radicals that act as flame inhibitors. The radicals (such as Cl• and Br•) react with highly reactive species (H• and OH•), forming less reactive or inert molecules, thereby interrupting the exothermic reactions of combustion and reducing the temperature. Non-intumescent paints can also act in the condensed phase, forming a non-bulky (glassy or carbonaceous) protective layer on the surface of the material. This layer acts as an insulator against thermal radiation.

The main compounds in these paints are often halogens (such as bromine and chlorine), phosphorus and inorganic compounds. The use of halogens is effective but can generate toxic emissions, pushing towards the use of alternatives such as phosphorus, nitrogen and silicon.

The general mechanisms of action of protective paints, both intumescent and non-intumescent, aim to delay or prevent ignition of the substrate. They reduce the heat transfer from the source to the substrate and the spread of flame. They also reduce the emission of smoke and toxic gases during combustion, but remember that their effectiveness also depends on the fact that the protective paints must be resistant to abrasion and easy to apply.

Specific benefits of protective paints for historic buildings

Another paper, concerning the potential use of protective paints to historic buildings (authored by Inês Soare and others) highlights that the use of fire-retardant paints can slow the ignition of coated materials, providing more time to react in the event of a fire.
Paints can be formulated to be transparent, preserving the original appearance of historic buildings.
Fire-retardant paints can help maintain the traditional characteristics of structures, for example in timber construction.

Some paints, particularly those based on ammonia, phosphorus or silicon, can cause damage to historic wood, such as defibration, loss of stability or reduction in strength. It is essential to carefully select the paint composition to minimise these risks.

Moreover, fire-retardant paints containing halogens can be harmful to health and the environment, due to the emission of toxic and corrosive gases during combustion. Research is therefore being directed towards more environmentally friendly alternatives.
The application of protective paints on historic textiles can alter the aesthetic appearance, cause metal corrosion and salt migration, so their direct use on textile artworks is not always advisable.

Final considerations

Fireproof paints are a solution that can encourage the reuse and improvement of existing materials.
More research is needed on the application of fireproof paints in media and storage materials in museums and depots, assessing their long-term effectiveness and safety near collections.
In summary, fireproof paints deserve to be considered as one of the tools for the protection of historic buildings, contributing to fire prevention, provided that their limitations are taken into account and the most appropriate formulations and applications are selected.