How to craft it
Furnace
Work BenchShimmer
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Throwing Hellstone Brick into Shimmer transmutes it into Ancient Hellstone Brick. All transmutationsUsed to craft
it still burns you
Baking the ore into brick does not make it safe to stand on. Touch it without an Obsidian Skull or one of the accessories built from it and you catch fire the same as with the raw ore. Mining it and carrying it are both harmless, and enemies ignore the burn completely, so it is a hazard for you and nobody else. Anyone building an underworld base with it should be wearing the skull first.
getting hold of it
The ruined houses scattered across the underworld are made of it, so pulling one apart is usually faster than crafting a supply from scratch. One warning while you are down there: unlike the ore, this brick has no resistance to explosions at all, so a bomb run aimed at clearing a path through a ruined house will take the house with it.