terraria
I've seen videos of people making monster farmers with wires, switches, statues and a thin layer of lava to kill the newly created monster. They always seem to have a perfectly created layer of lava that doesn't destroy the item but still destroys the monster.
I would like to know if there is a special brick formation or number of bricks that have to be placed for this magical layer of lava to happen.
With 1.1 the previous answers are no longer correct. Since you are digging down through the lava, redirect some water into your mineshaft. As you dig into the lava, the water will hit it and form obsidian walls to hold the lava back. You will need a nightmare pickaxe or better to dig out the obsidian that's in your way.
Another alternative is to use an obsidian skin potion (and maybe a gills potion) to safely swim to the bottom of the lava and dig a drain for it.
The liquid physics in Terraria are not completely precise. You can abuse it to duplicate them. Good thing, too, since transporting a decent amount of lava/water anywhere is a huge pain.
Applying #2 and #3, this is a very simple liquid duping setup:
The lava is placed on the edge, where it flows into both the container and the 1-block "basin" on the right side. Both "portions" are enough to be picked up by a bucket again (though in this case, the first scoop will be spread too thinly in the container).
Best Answer
In order to create a lava trap that damages monsters but does not destroy items, you want to keep a ratio of 1 bucket of lava to six blocks of trench.
This will allow the lava to burn monsters (and you if you don't make an obsidian skin potion!), but will not be deep enough to destroy the drops -- even Coins are safe.
I didn't have a bucket of lava handy, but this is what your moat should look like - though I had to use water, which is significantly less lethal.
The last thing to note, is that your liquids may get, for lack of a better word, "stuck" on the edge of the trench. In that case, all you need to do is exit / reload the world, which will settle the liquids, and appropriately depth your trench.