Question time! Because Toby trivia is fun.

Iron and steel. I'm not an expert on metals, but I think most sorts of steel contain 90% or more of iron. What we call iron in everyday language isn't pure, and 100% pure iron is hard to come by and not a very good material for knives or dungeon door hinges. So what makes the difference between Fae Kryptonite Iron and Happy Toby Steel as in her Volkswagen?

This is something I've wondered about too, so much that I did some research. Your typical low grade, plain carbon steel has less than one percent carbon, and high grade, plain carbon steel is still more than ninety-eight percent iron. Now when you get into alloys the iron content drops down considerably, but plain carbon steels account for most of the steel we, and the fae, would encounter day to day.. My take on this would be that even the tiny amount of carbon in, say, a steel sword or dagger, is enough to alter it to the point where it is no longer poison to the fae. I mean look at the difference in appearance; my trusty cast iron skillet is black as can be, but my steel short sword and dagger are both shiny silver.

Yes, but "cast iron" is also an iron-carbon alloy -- specifically, one with less than ninety-eight percent iron (per Wikipedia).

Now, there are some notable differences in microscopic structure between wrought iron (which is very low carbon) and steel of the same overall composition, and the differences result from whether it's been melted and re-cast or not. Which is rather an interesting observation....

Oh, and the black appearance of well-seasoned cast-iron skillets is a coating that's a combination of polymerized oils, carbon, and magnetite (i.e., "black rust", iron oxide that doesn't have as much oxygen in it as "red rust", and forms a protective coating).

As a metallurgist it continuously amuses me that most steels contain more iron and less carbon than cast iron. Most cast irons are around 95% or less iron while most steel are 97%+ iron. Highly alloyed steels such as tool steel and stainless steels are the obvious exceptions, but they are uncommon in an industrial setting.

Most carbon steels are actually alloyed with around 1% manganese, while the most common alloy steels have manganese, nickle, chromium and molybdenum additions to make them easier to heat treat.