No, actually: Japan had exceptionally poor iron deposits and could not make quality steel, as there were too many impurities.

By repeatedly folding the steel, impurities are slowly released. The fact they had to do it so many times shows how awful their native sources were.

In addition, this actually backwards in sword-making: folding the steel doesn't really make it stronger, and is a horribly time-consuming and inefficient process; worse, if there are any problems that remain with the blade, they'll be multiplied by the fact the steel isn't a solid, casted whole.

You always hear about ancient Japan and it's proficiency with steel. Why is this?

You hear about it because the Japanese sword is famous, and many people say it's the best sword of all time, and owes that "bestness" to Japanese super-steel and/or super metalworking skill. Myth, of course. There is also a common counter-myth that Japanese swords are inferior, super-brittle, need to be wielded by an expert or they will break, and that traditional Japanese steel was inferior.

I won't discuss whether the Japanese sword is the best sword of all time, or the worst, but I will say something about typical construction and metallurgy.

First: raw materials. To make steel, starting with iron ore, you need to convert your ore to metallic iron, and to have steel (a carbon-iron alloy), you need to add carbon. To smelt the ore ("smelt" = convert the ore to metal), which is typically iron oxide, you need something to bind to that oxygen. Carbon is used, either coke (made by baking volatiles out of coal), coal (if your coal is sufficiently pure), or charcoal (make by baking volatiles out of wood). The modern carbon source is coke, and the usual traditional source was charcoal. The charcoal also provides the energy source to heat the ore (through being burned). Since steel is iron + carbon, you can add the carbon during smelting, or you can add it later. (The modern method adds too much carbon during smelting, producing cast iron, and the excess carbon is removed during the steelmaking step. The Chinese started widely using a similar method during the Han Dynasty.)

The simple way to make iron is to make a furnace, fill it with layers of charcoal and ore, and light it. If the temperature is kept high enough for long enough, enough carbon can diffuse into the iron to obtain a useful amount of steel. There are two important points:

1. The ore is converted to metallic iron at a temperature well below the melting point of iron, and

2. If you do get the smelting furnace hot enough to melt the iron, carbon will dissolve in it very easily, and you will get a puddle of cast iron (with about 3.3-4% carbon) at the bottom of your furnace. Cast iron is typically very brittle, and if you haven't developed methods for removing the excess carbon, is a waste product.

So, it is usual to convert the ore to metal, possibly producing steel (otherwise, low-carbon iron), without melting the metal. The metal is a spongy mass of iron filled with non-metallic inclusions (slag) from the ore. This spongy mass is called a bloom in English, kera in Japanese. The type of furnace that produces a bloom is a bloomery, or in Japanese, a tatara. This is the usual method of iron-smelting before the introduction of the blast furnace (Han Dynasty in China, late Medieval in Europe). Essentially, the Japanese method for iron- and steel-making was the same as the European method before the European adoption of the blast furnace and steel-making through decarburisation of cast iron.

Q: If the iron and steel technology was the same, why was the Japanese product different?

A: It wasn't.

What about the myth that Japanese ore was terrible? The Japanese ore that was traditionally used was iron sand. The deposits are sand with grains of magnetite (an iron oxide). The sand as a whole only has a low iron content, but since the iron ore grains are denser than the silica grains in the sand, the iron ore can be concentrated by washing. This produces a good quality ore (the washing leaves some silica sand in there, but that's useful for forming slag that unwanted impurities will end up in). Iron sand is still used for iron and steel making, and is typically considered an excellent source of iron ore (modern separation of the magnetite grains is by electromagnet rather than washing). For the Japanese, the iron sand deposits offered an easier way to extract the ore (by washing) than hard rock mining (which would have been needed to get the ore from the sources from which the iron sand was produced by erosion). So far, nothing bad about the Japanese ores.

What can make iron ore bad? Impurities, especially phosphorus and sulphur. The Japanese ores are low in phosphorus and sulphur - Japanese iron ore is good quality iron ore.

What about quantity? As Japan industrialised in the late 19th century, the supply of Japanese iron ore was not sufficient, and the Japanese depended on imported iron/steel. However, this was not a problem before industrialisation, when the scale of iron and steel making was much smaller.

If the Japanese iron ore was good, why did they need to fold their iron/steel to get rid of impurities? Because the iron/steel was produced from the ore without being melted, the furnace produced a bloom (or kera), which is a spongy mass of iron with lots of slag. The usual procedure it to hammer it, fold it, hammer it, fold, etc., until the excess slag is removed. This was the usual procedure, not just in Japan, but in ancient Europe, Medieval Europe, Africa, SE Asia, and many other places. You can sometimes read on www that folding the steel was something special and Japanese, but it was the standard procedure for making bloomery steel over much of the world.

Japanese steel was made in the same way as steel was made in other countries through direct steelmaking in bloomery furnaces. The bloom is not a finished product, and is folded to produce wrought iron (if low carbon) or steel (if medium/high carbon). Japanese steel was not inferior, but was also not a super-steel - just good high carbon low-phosphorus low-sulphur steel as was produced in other parts of the world.

Compared to Chinese and modern steelmaking methods (e.g., the Bessemer process), Japanese steel was a lot more energy and labour intensive to make, but the final product used in Japanese swords was very similar to modern low-alloy high-carbon steel, typically approximately 1070, albeit with more slag inclusions (but not enough to affect the strength of the final product). While more expensive (more energy and labour needed), the final product was not of inferior quality (nor was it super-steel).

If the steel was OK, why did the Japanese use laminated construction for swords, making different parts of the blade out of high carbon steel, medium carbon steel, and low carbon iron?

1. High carbon steel was more expensive that iron, and if high carbon steel was used only where it was needed, a sword could be made more cheaply.

2. While modern heat-treatment to produced hardened steel is well-known and reliable, consisting of quenching (a red-hot blade is rapidly cooled in a liquid like water or oil) to harden the blade, and tempering (re-heating to an intermediate temperature to reduce the brittleness (at the cost of reducing the hardness somewhat), typically in a temperature-controlled oven for a specified duration), pre-modern heat treatment was much more uncertain. In particular, the smiths didn't know the exact composition or carbon content of their steel, and didn't have modern thermometers, thermostats, or temperature-controlled ovens. Tempering was commonly done by auto-tempering, using residual heat in the blade from an incomplete quench (slack-quenching) or a slow quench. This involved some risk of leaving an all-high-carbon steel blade too brittle, and composite/laminated construction would ensure that the blade would have softer but much tougher portions to support a brittle edge.

While it can be read online that laminated construction as used by the Japanese was something specially Japanese, it was widely used. The common Japanese lamination types of three-layer (Japanese sanmai, Chinese sanmei, hard steel core with soft iron sides), inserted-edge (hard steel edge inserted into a soft iron body), and kobuse (hard steel skin wrapped around a soft iron core) were also used elsewhere. Sanmei and inserted-edge were very common in China, and elsewhere in Asia, inserted-edge was common in Europe, and there are European examples of kobuse laminated (e.g., recovered from the Mary Rose).

The third claim of special Japanese techniques is for differential hardening, where even if the steel is uniform, a blade can be produced with a hard edge and softer body. Differential hardening was widely used in pre-modern Europe, and much of Asia. A variety of techniques were used, including edge-quenching (where only the edge to be hardened is quenched), differential quenching by slack-quenching, differential tempering through reheating the spine/body of the blade and not the edge, and differential auto-tempering. The Japanese method of differential quenching via a clay coating to reduce the cooling rate of the body was used outside Japan (in SE Asia, at least) so even that isn't uniquely Japanese.

So, in summary, myths about Japanese steel and steel-making are just myths. The main things often claimed as uniquely Japanese (folding, laminated, differential hardened) were done in much of the world. Japanese iron ore was of good quality, and the resulting steel was good quality bloomery steel, more expensive but otherwise competitive functionally with modern low-alloy high-carbon steel. Japanese iron was sufficiently plentiful to sustain the traditional iron/steel industry.