For smelting iron, you need a fuel that's nearly pure carbon, which has typically been either charcoal or coal. The former is the more easily obtained; when the forests were too depleted to provide adequate supplies of charcoal, people started mining coal on a large scale.

Is it actually the case that charcoal is as good as the best coal, so that in a setting where adequate supplies of the former still exist, there would be no reason to start mining the latter, and a setting that was deficient in coal, would only be a hindrance when the forests started running out?

In particular, I noticed in an answer to How far could civilisation develop within one lifetime - starting from nothing?

anthracite coal. This allows you to fire kilns to a sufficiently high temperature to make steel.

I would be surprised if that implied anthracite is better than charcoal, which is as I understand it also pretty much pure carbon. But perhaps it just implies 'given that the forests are depleted and you have to switch to coal, you are much better off with anthracite, which is pure enough to fully substitute for charcoal, instead of having to burn something lower grade like lignite'?

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    $\begingroup$ Charcoal is pure carbon.... with about 20% impurities! Anthracite is pure carbon, with about 2% impurities. That's a huge difference, you can actually burn anthracite without an ash-management system, in sufficient volumes to smelt iron. Not the same for charcoal. $\endgroup$
    – PcMan
    Jun 28 at 15:45
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    $\begingroup$ @PcMan this could be fleshed out into an answer $\endgroup$
    – John
    Jun 29 at 5:11

As stated in the Wikipedia page for steel

Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the introduction of the blast furnace and production of crucible steel.

Quantity is for sure a problem:

Since the 17th century, the first step in European steel production has been the smelting of iron ore into pig iron in a blast furnace. Originally employing charcoal, modern methods use coke, which has proven more economical.

And then it comes also the related temperature problem

Crucible steel is steel that has been melted in a crucible rather than having been forged, with the result that it is more homogeneous. Most previous furnaces could not reach high enough temperatures to melt the steel. The early modern crucible steel industry resulted from the invention of Benjamin Huntsman in the 1740s. Blister steel (made as above) was melted in a crucible or in a furnace, and cast (usually) into ingots.

Same concept is expressed in the page for charcoal

Charcoal burns at temperatures exceeding 1,100 degrees Celsius (2,010 degrees Fahrenheit).[9] By comparison the melting point of iron is approximately 1,200 to 1,550 °C (2,190 to 2,820 °F). Due to its porosity, it is sensitive to the flow of air and the heat generated can be moderated by controlling the air flow to the fire. For this reason charcoal is still widely used by blacksmiths. Charcoal has been used for the production of iron since Roman times and steel in modern times where it also provided the necessary carbon. Charcoal briquettes can burn up to approximately 1,260 °C (2,300 °F) with a forced air blower forge.

In the 16th century, England had to pass laws to prevent the country from becoming completely denuded of trees due to production of iron. In the 19th century charcoal was largely replaced by coke in steel production due to cost.

I would also add that burning pure carbon produces a much more clean product, reducing less effort in purification, which again goes on the economic side. Also don't forget that to make charcoal you need to process wood, you can't just burn chopped tree. This again adds to the economy of the process.

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    $\begingroup$ I think the achievable temperature has more to do with whether you use ambient air, preheated air, or pure oxygen – not with the kind of coal burnt. $\endgroup$ Jun 28 at 16:47

Charcoal and coal are mostly equivalent though there are two crucial differences:

  • Coal often contains Sulphur, which is why charcoal was often preferred until the process for making coke from coal was invented (for brewing beer at first, the use in metallurgy came later)

  • Charcoal is far more brittle than coal, making long range transport difficult. What arrives after a bit of shaking along the road is a mix of fine powder and larger chunks.

If you want to imagine an industrialization fueled by charcoal, it should feature huge plantations of fast growing wood with coal kilns in between (check out Hoffman kilns for a rather efficient and almost high tech approach to charcoal making) and pellet presses to process the transported charcoal into solid bricks before burning.

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    $\begingroup$ Also I read that it's mechanical properties made building high furnaces impractical - compression at bottom of furnace created problems. $\endgroup$
    – Vashu
    Jun 29 at 7:57

I'm wondering what you are asking and what your ideas are about smelting iron.

Smelting "is a process of applying heat to ore in order to extract a base metal". Iron is a base metal.

Charcoal may have been used in the past to heat iron ore, but these days other sources of heat are used, depending on the facility used: coal, gas or electricity.

The thing about using charcoal or coal in smelting iron and making steel is they have had two uses: firstly to heat the ore and secondly to remove impurities within the ore to leave the metal required.

Iron ore is various forms of iron oxide: magnetite (Fe3O4, 72.4% Fe), hematite (Fe2O3, 69.9% Fe), goethite (FeO(OH), 62.9% Fe), limonite (FeO(OH)·n(H2O), 55% Fe) or siderite (FeCO3, 48.2% Fe).

In the smelting process, carbon is used to remove the oxygen and leave behind the iron metal. When the iron ore is being heated, by whatever means, carbon is added to the molten mix to reduce the iron ore, produce iron metal as the desired product.

Charcoal is a good source of carbon because it is made from partially burnt wood and it has had some of the impurities from the wood removed during the burning process in a oxygen reduced environment.

The current day substitute for charcoal is coke. In the same way that charcoal is partially burnt wood, coke is partially burnt oil or coal.

This is added to the iron ore melt. It reacts with the molten iron oxides. Oxygen from the oxides bond with the carbon in the coke (charcoal in older applications) to produce carbon dioxide (CO2), leaving behind molten iron metal.

When it comes to reducing the iron oxides during smelting, coke from coal is not inferior to charcoal from wood.

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    $\begingroup$ There's another reason big reason carbon is useful, it's needed to create steel and high-carbon iron. In pre-industrial times, charcoal was used for this. Today we use metallurgical coal. It's actually used to add impurities (carbon) to the iron. $\endgroup$
    – JimmyJames
    Jun 28 at 16:23

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