Notes on iron and steel
=======================

Alloying iron with carbon is of huge importance, but in some processes
the alloying is an implicit side effect rather than the product of
explicit mixing, so it is a complex area.  In the real world, there is
a huge variety of kinds of iron and steel, differing in the proportion
of carbon included and in other elements added to the mix.

The Minetest default mod doesn't distinguish between types of iron and
steel at all.  This mod introduces multiple types in order to get a bit
of complexity and flavour.

Leaving aside explicit addition of other elements, the iron/carbon
spectrum is here represented by three substances: wrought iron,
carbon steel, and cast iron.  Wrought iron has low carbon content
(less than 0.25%), resists shattering, and is easily welded, but is
relatively soft and susceptible to rusting.  It was used for rails,
gates, chains, wire, pipes, fasteners, and other purposes.  Cast iron
has high carbon content (2.1% to 4%), is especially hard, and resists
corrosion, but is relatively brittle, and difficult to work.  It was used
to build large structures such as bridges, and for cannons, cookware,
and engine cylinders.  Carbon steel has medium carbon content (0.25%
to 2.1%), and intermediate properties: moderately hard and also tough,
somewhat resistant to corrosion.  It is now used for most of the purposes
previously satisfied by wrought iron and many of those of cast iron,
but has historically been especially important for its use in swords,
armour, skyscrapers, large bridges, and machines.

Historically, the first form of iron to be refined was wrought iron,
produced from ore by a low-temperature furnace process in which the
ore/iron remains solid and impurities (slag) are progressively removed.
Cast iron, by contrast, was produced somewhat later by a high-temperature
process in a blast furnace, in which the metal is melted, and carbon is
unavoidably incorporated from the furnace's fuel.  (In fact, it's done
in two stages, first producing pig iron from ore, and then remelting the
pig iron to cast as cast iron.)  Carbon steel requires a more advanced
process, in which molten pig iron is processed to remove the carbon,
and then a controlled amount of carbon is explicitly mixed back in.
Other processes are possible to refine iron ore and to adjust its
carbon content.

Unfortunately, Minetest doesn't let us readily distinguish between
low-temperature and high-temperature processes: in the default game, the
same furnace is used both to cook food (low temperature) and to cast metal
ingots (varying high temperatures).  So we can't sensibly have wrought
iron and cast iron produced by different types of furnace.  Nor can
furnace recipes discriminate by which kind of fuel is used (and thus
by the availability of carbon).  The alloy furnace allows for explicit
alloying, which appropriately represents how carbon steel is made, but
is not sensible for the other two, and is a relatively advanced process.
About the only option to make a second iron-processing furnace process
readily available is to cook multiple times; happily, this bears a slight
resemblance to the real process with pig iron as an intermediate product.

The default mod's refined iron, which it calls "steel", is identified
with this mod's wrought iron.  Cooking an iron lump (representing ore)
initially produces wrought iron; the cooking process here represents a
low-temperature bloomery process.  Cooking wrought iron then produces
cast iron; this time the cooking process represents a blast furnace.
Alloy cooking wrought iron with coal dust (carbon) produces carbon steel;
this represents the explicit mixing stage of carbon steel production.
Additionally, alloy cooking carbon steel with coal dust produces cast
iron, which is logical but not very useful.  Furthermore, to make it
possible to turn any of the forms of iron into any other, cooking carbon
steel or cast iron produces wrought iron, in an abbreviated form of the
bloomery process.  As usual for metals, the same cooking and alloying
processes can be performed in parallel forms on ingots or dust.