The British iron age ran from around 800 BC until the invasion by the Romans in 44 AD who brought with them the technology to make steel, though this new tech took years to spread around the country.
Before the Iron age was the bronze age, a time when the hardest metal that people could work with was a combination of tin and copper. Both of these metals have reasonably low melting points as metals go, with copper melting at 1,085 °C and tin melting much lower at 231.9 °C.
Both of these metals can be melted on a bonfire, especially if there’s some coal thrown in to make it hotter. To make metals in this way they would simply need to place some ore on a flat stone in the middle of the fire, and then pick it up with sticks to pour it out into a mould. Over time they would be able to make hammers and anvils to shape and work the metal better, but the one thing they couldn’t do was melt Iron.
Iron has a melting temperature of 1,538 °C, a level of heat that’s way to high to achieve in any bonfire, no matter the size. The other problem was that iron was normally within rock as it rusts and becomes useless if exposed, and due to how soft the metals they had were it made it near impossible to hack it out of the rock with bronze tools.
So how did they work out how to melt Iron?
As for how the technology came to be, no one knows for sure. It could be due to a mysterious worldly traveller coming to the shores of Britain and showing the locals what he knew, or it could be that someone simply worked it out.
The way to melt iron seemed to be as easy as continuously blowing air into the fire, and so the billows were invented. This most likely came to be from people realising that blowing into a fire helps it become hotter momentarily, and a giant version of a lung blowing into a much bigger fire would create the necessary heat to melt iron.
Crude stone smelters were built by stacking stone sealed with clay, and a hole at the bottom allowed the air from the billows to come through. Stacking the inside of the smelter with alternating layers of ore and coal would help to increase the temperature, and after it became hot enough the liquid iron would melt out the bottom and run out onto the ground.
This set up would take nothing more than stone, clay, coal and a couple of animal skins held together with wood, all of which the Britons had for thousands of years before someone realised putting them together can produce a new contraption to melt iron.
As for where they got Iron ore from in the first place the answer would most likely be from bogs. If you’ve ever been out walking in the countryside and seen a stream with a rust red coloured mud or sand, then its most likely been caused by a nearby iron deposit rusting into the water.
If you see this strange colour around a swamp or bog then you’ve got yourself a source of iron. When iron is within bog mud, the center of the ore becomes starved of water and oxygen and so only the outside would rust, leaving the center ripe for smelting.
After this new technology moved across the country it took many years until it actually became useful, as the first casting from iron ore is known as pig iron. The name comes from the way it was made, which was done by drawing a line in the soil starting at the point the metal would touch the ground and going in a straight line away from it, with ingot shaped run-offs either side. The pattern is supposed to look like piglets suckling from either side of their mother.
Pig iron contains a very high carbon content and is very brittle, to the point where it shatters almost like glass if struck. It needs to be melted a second time in order for the carbon content to be removed so it can become hard enough to be useful. After that it needs to be worked and further purified, but as for when people actually worked this out is any-ones guess.