Terraria – Are altars able to spread crimson/corruption

You can use Corrupt Seeds on dirt to artificially create corrupted areas yourself. If you want to control how much the corruption spreads, plant Sunflowers on the left and right boundaries of your corrupt areas, as they stop the spread of Corruption.

According to this user, only Corrupt Grass spreads the Corruption (to other dirt blocks) – Ebonstone won't. However, another user states Ebonstone will in fact spread it, so I'm not sure who is correct. Thorny Bushes will spread the Corruption. I don't know if there's any data on the maximum size of Thorny Bushes.

With the Clentaminator present in the game, this is actually quite easy, if a bit time consuming and expensive. Dig hellevators every 120 blocks across the area you want to purify. With the Green Solution equipped, float down the each hellevator twice, first spraying to the left, then to the right. I recently did this in one of my worlds, and it worked perfectly. I'd recommend checking every so often after you're done to make sure you didn't miss anything. This will also turn any underground mushroom biome to jungle, so you may want to go back when you're done and convert it back with Dark Blue Solution.

When I did this, I left large swathes of land from the surface to the underworld converted, and even brought in Crimson from another world so I had all the biomes present. Just make sure that if you do this you put wider shafts (I left six blocks) around the corrupted/crimsoned/hallowed areas so they don't spread.