Is Wood Ash Flammable?

This article will answer the question: “Is Wood Ash Flammable?”. We will also provide more helpful information regarding the safety of domestic fires.

Is Wood Ash Flammable?

If it is cold and there’s no ember on it, it’s not flammable. That white-grey powdery stuff left in a fireplace, bonfire, or fire pit of any kind will never become a fire hazard unless it’s hot. It contains everything the wood had, except for the flammable part. 

What is wood ash?

Wood ash is the powdery residue left in the combustion of wood.

It’s mainly composed of inorganic compounds and other non-flammable elements present in the wood.

Wood ash is everything that wood is, except the combustible and volatile part. Its composition can vary according to the source of wood, but calcium is one of the most common residues.

Wood ash can be used as a fertilizer, especially as a source of potassium, calcium, and neutralization for the soil. It can be composted, and used in soap-making, pottery, and even in cooking.

An introduction to fire

When did the first fire ever happen?

It might sound odd, but fire is unique to Planet Earth, as far as we know. There was a time on our planet that nothing burned, because there was no fuel and oxygen to do so.

Fire is something so familiar that’s really hard to imagine our lives without it. Even if you don’t light up a fireplace normally, you surely use lots of things that required fire to be formulated.

Cement, gasoline, oil, porcelain, metals, jewels, and food. All these are examples of things we only have thanks to fire.  

When we think about early fires we usually imagine our monkey ancestors seeing a bushfire for the first time, or rubbing two pieces of wood together until ignition. 

These events were surely the first time our species had dealt with fire, but the very first ones are much older.

A dish requires ingredients to be crafted, and so does fire. 

Fire has three basic ingredients: fuel, oxygen, and heat. Without all of them, a fire can’t sustain itself and will eventually extinguish. Likewise, the first fire ever occurred on earth needed a source of oxygen and fuel. Heat can happen in many ways.

Oxygen is not normally available on the surface of a planet, even though it is the third more common atom in the universe. This is because oxygen can react chemically with nearly anything, so the oxygen atoms were all in a solid form of oxides somewhere.

Oxygen today composes 23.1% of our atmosphere, but around 2.5 billion years ago, there was none. Early sulfur-based life forms started metabolizing sulfur dioxide (SO2), releasing oxygen into the atmosphere.

For the next 2 billion years, Earth’s atmosphere started to inundate with oxygen, which became known as the Great Oxygenation Event. This was the kick-off for life as we know it.

Ok so, half a billion years ago land plants started to thrive, thanks to oxygen. They “quickly” started getting more diverse. Only 100 million years ago, plants were massive enough on earth so there was fuel to be burned.

We can say that the first fire happened about 400 million years ago, in some primordial dried biome, probably due to a climate event like thunder. 

For comparison, the first Human only appeared about 393 million years later. The first campfire, was 399 million years later. Until 1 million years ago, fire appearance was merely spurious. 

Then, a small group of hominids captured it and developed ways to kindle fire on demand, and suddenly became capable of changing entire landscapes for farming. Back then, the limitations on fire were only due to the nature of the fuel and ignition sources.

Nowadays, we face no limit on the nature of the three fire ingredients, but we must impose a limit ourselves because our atmosphere can’t intake more combustion byproducts.

Wood chemistry

The specific chemical composition of wood varies according to the tree, and so does wood ash.

Overall, wood weight is composed of 50% carbon, 42% oxygen, 6% hydrogen, 1% nitrogen, and 1% of other elements such as calcium, potassium, magnesium, iron, sodium, and manganese.

Almost all carbon and hydrogen (the atoms that are consumed during combustion to create carbon dioxide and water, the major byproducts of a burning) vanish from the remains of a fireplace. 

Whatever is left in the form of wood ashes can’t burn, because it already has. The major constituent of the ashes is calcium carbonate (CaCO3, the main mineral found in limestone) and calcium oxide (CaO, known as quicklime).

Hence why it’s been used as a fertilizer for centuries. But make no mistake: even though wood ash is a fertilizer, burning the vegetation of a piece of land doesn’t necessarily make it fertilized. Other important compounds such as nitrogen are lost in a fire.

Wood ash flammability

Wood ash is made essentially by the part of the wood that can‘t burn, and that ain’t volatile. 

When a fire takes place, the fuel will be burned until exhaustion, unless something stops the flames. Wood ashes, the white-grey powders left after a fire has extinguished on its own, are the remainings of whatever couldn’t burn.

The fire triangle. It represents the three ingredients of fire.


Wood ash presents no flammability itself, but there could be ember as well or another unburned piece of wood that might still be combustible. But even these aren’t considered fire hazards as long as they’re could.

There’s another important matter: the wood ash temperature. Wood ashes can still radiate heat up to 12h after the fire has been extinguished, depending on how big the fireplace was. 

Wood ash disposal and hazards

If you’re going to dispose of wood ash, you must always treat all ashes and coals as hot ashes. We can never assume they had had enough time to cool down. 

Ember is nothing but a slow-burning fire. It can still liberate toxic gases such as carbon monoxide, and should never be stored indoors. Even after the ash cools down, it can still become airborne and irritate our lungs.

Unsafe disposal of what people believed to be “cold ashes” has been the site of many devastating fires. We can never assume that ashes had already cooled on their own.

Here are five tips from the Montgomery County government to be safe when dealing with wood ashes.



Wood ash is not flammable and is not a fire hazard. But if it is still hot, especially if there’s burning ember beneath, it can be an immense danger. In fact, it has been at the center of many domestic fires.

Frequently Asked Questions (FAQS): Is Wood Ash Flammable?

is wood ash good for the garden?

Yes. It can be used as garden fertilizer, boost compose piles, prevent frost damage from damaging your plants, can control the soil acidity, can prevent calcium deficiency in tomatoes, and can act as snails and slugs repellant, among many other things.

But keep in mind that the total content of wood ash is unknown, it varies from wood to wood, and may contain metals that can contaminate the soil in the long term if you use too much. Check out more about it here.

Does wood ash increase or decrease pH?

Wood ashes contain calcium compounds that are the same as limestone and quicklime, often used to increase de pH (reduce acidity).

can wood ash kill weeds?

Most weeds and mosses require a pH between acid and neutral. Wood ashes enhance the basic pH in the soil, so they can help you get rid of weeds. But it’s not a quick response.


Pyne, Stephen J. Fire: a brief history. University of Washington Press, 2019.,carbohydrates%20and%2C%20yes%2C%20oxygen.

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