So as the title says, what was the Eiffel Tower made of. Iron or Steel. The reason I ask is that we touched on it in History class lately and all sources I come across say Iron but the teacher says Steel as it was made in the Steel Era.
A short answer: Iron; wrought iron to be exact. But there's a couple of things that have to be addressed here, specifically:
Let's start with basic chemistry and metallurgy - chemically iron is a rather reactive metal, and in nature as iron ore, it is almost never found elementally pure, like you could find less reactive metals - gold, silver, platinum. Iron ore is full of impurities, other elements like silicon in small traces, and what we know today as cast iron is very brittle and unsuited to take any kind of tensile load - however it is cheap as it requires minimal metallurgic processing. Today, items in architecture you'll find made of cast iron limits itself to street drainage hatches, for example. Since the dawn of the iron age civilization, we've been aware of this and metallurgy in terms of iron has developed processes that allow for these impurities to be extracted from the ore. The basic idea is two fold :
Before the industrial revolution, this was mostly done manually, on smaller amounts - a blacksmith would take an iron ingot, heat it up, and keep folding it while striking it with a hammer - the folding allowed for the impurities from the center of the ingot to come up to the surface, and run off as dross. Afterwards, we've developed huge furnaces where molten iron was mixed with rods, and even later, a steady flow of air was supplied to the surface of the molten metal - this allowed for the impurities to chemically react with the oxygen from the air, whereas the more inert iron wouldn't. Impurities' oxides would then be much easier to remove, and you'd be left with a batch of much purer iron - opposed to the blacksmithing process where you had to do this individually with each ingot.
Now you've got wrought iron - but how to make it into steel? The problem is, since the process of removing impurities doesn't discriminate, you're also removing some elements that are actually helpful in terms of constructional characteristics - specifically carbon. The difference between wrought iron and steel is in steel, after removing those impurities, you're adding carbon back into the mix, depending on what specific alloy you need. In the past, this was mostly done with adding charcoal to the mix, but these days we have more refined processes. Construction wise, construction or structural steel has better properties than wrought iron, mostly tensile strength and ductility, and that's why given the technological advancements these days we're no longer producing wrought iron on an industrial mass-production scale.
Now let's talk 1889., or specifically the Paris Exposition for which the Eiffel Tower was designed and constructed - even though the technology of making commercial steel via Bessemer furnaces had existed at that time, the problem was most of the metallurgy infrastructure at the time was still the old wrought iron furnaces. As such, steel was much cheaper than 50 years before, but given it was still a 'new' technology, it was still much more expensive than wrought iron. Given that the Eiffel Tower weighs some 8000 tons, this was clearly a matter a budget - riveted wrought iron had enough structural ability to allow the construction to be built, and within a sensible budget - as much as we can argue that 'sensible budget' is a euphemism when talking about 300 meter high collosi.