The only way to reach ore smelting temperatures without a pickaxe is by making charcoal. Here are the steps:
- Dig out a small ditch on the ground and fill up the hole with firewood.
- Cover it up with non-combustible blocks, except for the one in the middle.
- Place a firepit there and quickly cover it up. You need at least 32 firewood on one block to be able to place the firepit.
After 18 in-game hours, the pit will stop smoking and the firewood will have converted to charcoal. Your ditch can be of any shape, just be sure that all sides are covered up, i.e. you can also make a pyramid-shaped pile on the surface and cover it with dirt if you like.
Careful: After the firepit is placed you have 30 seconds to completely cover up the pit, otherwise your firewood will burn down to ash and not yield any charcoal, thus it is recommended to have all sides covered before placing the fire pit.