Early in our school days, we learned about the  basic states of matter: solid, liquid, and gas.

If you heat a solid enough,  it melts into a liquid. And if you heat that liquid some  more, and it boils off as a gas.

But not everything plays by those rules. Take wood, for example. If you  heat up a wooden log enough,   instead of melting, it catches fire and burns.

And sometimes, that’s a good thing,   like when we get a nice, cozy fire during  winter instead of a wet pile of mush. But why doesn’t wood follow the same  rules as water, or butter, or crayons? Let’s break down why wood doesn’t  melt… except when it does. [♪ INTRO] All molecules everywhere  are vibrating all the time,   and they vibrate faster when  you add energy from heat.

A solid substance becomes a liquid  when its molecules vibrate fast   enough to break out of their orderly arrangement. Those molecules start to slide past each other,  and the solid loses its shape to become a liquid. The temperature at which this  happens is called the melting point.

And the transition between states of matter, like  solid and liquid, is known as a phase change. Even in a mixture of a bunch of different  chemicals, they all have their own melting points. For example, when ice cream melts,  it’s because the water ice is melting.

There’s less going on in the “cream” part. Part of the reason wood doesn’t melt is because  it’s also made up of a mixture of chemicals. These include water, as well as the  plant materials lignin and cellulose.

Some of these substances have  their own melting points. But others, like cellulose, don’t  actually have melting points at all. Cellulose is made of really long  chains of carbon, hydrogen, and oxygen.

The amount of heat needed to disentangle these   chains and melt cellulose is so  high that they hardly ever melt. Instead, the chemical bonds holding the chains  together start to break apart, and the cellulose   breaks down into different, smaller chemicals  before it gets hot enough to melt, per se. The molecules in cellulose still  vibrate when you heat them up,   like when you hold a match or  a lighter under some firewood.

But since it’s so tough for those  molecules to slide past each other,   they end up vibrating themselves to pieces! So, when you heat up wood, giant molecules  like lignin and cellulose start to break   into smaller molecules, including  water, carbon dioxide, formic acid,   acetic acid, methane, methanol,  hydrogen gas, and several others. Some of those new molecules are flammable gases.

As they form, they escape from the  wood like air out of a balloon. On their way out, they catch fire  and burn, generating more heat. And, fun fact: The popping sounds you  sometimes hear in a campfire happen when   some of these gases get trapped inside  a log and then explode out all at once.

Not all of the new chemicals are gases, though. Any that don’t burn off or escape  as smoke end up as ash or charcoal. The heat from the flames breaks down even  more of the large molecules in the wood.

And that releases even more gases,  which burn and generate more heat,   which breaks down more molecules, and  on and on until all the fuel is spent. Because the lignin and cellulose break down  into other chemicals at high temperatures,   burning a log isn’t considered a phase  change in the same way melting is. A phase change like melting has to be reversible.

If you’ve ever stuck melty ice cream  back in the freezer for a while,   you know that it’s still ice cream afterwards. If sometimes a little grainy and ice crystal-y. But while you might be able to re-freeze water  into ice, you can’t un-burn a log. …Right?

Let’s get a little weird for a minute. What if you didn’t have to  melt a whole piece of wood? What if you could melt just one of  the many chemicals that make up a log?

Or maybe just a couple at a time? It turns out, that is a thing we can  do, under the right circumstances. It’s called wood welding, and  scientists and manufacturers   can use it to stick two blocks of wood  together without using glue or nails.

For some time now, construction workers  and furniture makers have been using   glues or even melted plastics to  join two pieces of wood together. But many of those adhesives give off toxic  fumes, and some of them can cause cancer. So researchers have been looking for some safer,  more environmentally-friendly alternatives.

Wood welding takes advantage of the different  compounds that are already inside the wood. The technique uses the lignin at the  surface of the wood as a type of glue. Normally, the lignin in wood is  solid and not very glue-like at all.

To turn it into an adhesive, a  wood welder vibrates one piece   of wood on top of another while applying pressure. At just the right frequency, the heat and  friction from the vibration melts the lignin   at the wood’s surface, as well as a little  bit of another chemical called hemicellulose. The vibrations only need  to last for a few seconds.

That’s long enough to heat the surface  to above lignin’s melting point,   which is about 170 degrees Celsius. And it’s also short enough to keep the  temperature below the point where lignin   starts to break down into those smaller  chemicals, around 280 degrees Celsius. The welding process also works if  you screw a wooden dowel rod into a   hole in a different wood block, like  making a hammer or a wood lollipop.

In either case, once the lignin cools down,   it solidifies just like glue and  bonds the two pieces together. Some studies show that wood-welded joints can   even end up stronger than those  joined with regular wood glue. And wood welding hasn’t taken  over large-scale production yet.

But some researchers are looking into bringing it  into applications such as furniture manufacturing. So, with some careful physics, we get  wood that melts just when we need it to,   and we still get to enjoy our campfires. [♪ OUTRO]