The bronze anvil requirement is not documented ingame

[Gazz]

Survival handbook > Guides > Progression

does instruct to work the iron bloom "on the anvil"... but then it doesn't work.

It does in fact require a bronze anvil which is not mentioned anywhere.
In the Wiki I think it's mentioned in exactly one spot. You'd really have to know it to begin with to find that information.

[Tyron]

thanks, I now added a sentence right after that one.

[Gazz]

Thanks.
(and not-sorry for all the nitpicking =))

One of the differences between a complex and a frustrating game is whether you can solve the ingame challenges with ingame means.