Purification Pre-Hard Mode (Achievment sake)

[Juloxia]

So my buddies and I are trying to get the "And Good Riddance" achievment and we thought the quickest way would be to purify a pre-hard mode world. We brought terraformers from our hard-mode world and even summoned a mechanical boss to cheese getting the steampunker for the solution. Issue is, we're stuck at 1 percent and can't find the last of it. We're wondering if the demon altars and demonite count as corruption or if we're just blind. Looking for general advice

Before anyone says anything about just going straught to hard mode cause the corruption would come back, we do not care that corruption would come back. We just want the achievment without having to fight the spread cause we couldn't do it elsewhere

Update: Top answer has been chosen based on info, but Hedning mentioned the following for reference too:
It is genreally not possible to find a missing block. You have to methodically respray everything. However before doing that consider these options:
a) Quit and reload to update the dryad. Then talk to her.
b) Find the block you are missing with TEdit.

Thank you both for the help

[Hedning]

It is genreally not possible to find a missing block. You have to methodically respray everything. However before doing that consider these options:
a) Quit and reload to update the dryad. Then talk to her.
b) Find the block you are missing with TEdit.

[Juloxia]

Originally posted by Hedning:

It is genreally not possible to find a missing block. You have to methodically respray everything. However before doing that consider these options:
a) Quit and reload to update the dryad. Then talk to her.
b) Find the block you are missing with TEdit.

I'll give Tedit a try first as I've done the other countless times. Thank you for the advice!

[prpl_mage]

Demon altars and demonite etc don't count. I specifically left all that in the world where I got the acheivement.
However, as mentioned above. I had to leave the world and reenter it for the "state" to update. The Dryad said 2% before i left the world.

I also got the achievement the first time in a prehardmode world so that shouldn't be a problem.