You must make 10,000 bars of metal to get this achievement. Metal bars are made from smelting ore you collect at a forge, and are iron, lead, tin, copper, silver, gold, tungsten, cobalt, palladium, mythril, orichalum, platinum, titanium, or adamantite. A simple forge can be made/crafted or purchased from the merchant. You will need these bars of metal you are smelting to craft various things (especially weapons and tools) to progress in the game. You obtain ore by mining it from the world in the underground. They are typically found in clusters/pockets. The deeper you go, the better ore you find. You dig using a pickaxe, and stronger pickaxes dig faster.
You should get this achievement through natural progression in the game (although it will take a while), but there are a few methods to make it go faster. Unfortunately, there is no progress tracker for this achievement, so you must keep track yourself.
QUICK SOLUTION*** Spoiler - click to reveal ***Go into a friend/booster's world that has a chest full of ore that requires only 3 to make a bar. Empty your inventory, and remove 6 stacks of 999 ore. Use that to make the bars and then trash the bars (this will make 1998 bars). Take 6 more stacks and repeat until you have the achievement. This should take 5 trips (1998 x 5 = 9990 bars) unless you have never smelted anything before, in which case a 6th trip with 1 stack of 999 ore should do it easily.
DUPLICATING METHOD
This method will get you extra items without having to find them in the world. This trick works for any item at all, including ore, bars, weapons, armor, money, etc. That means anyone who has this game finished will be able to duplicate any item for you from later in the game. This method has not been patched out of the game in any update. This will take a "Giver" and "Receiver".
1. TURN AUTO-SAVE OFF for the Giver!
2. Receiver enters Giver's World and picks up any item they want to duplicate. This items can be placed into a piggy bank, safe, or the Receiver's inventory.
3. Receiver saves game and can then drop all items before exiting (or put them back into chests).
4. Receiver re-enters Giver's World and will have all items. Receiver can then pick up dropped items and save again.
5. Drop all items again (now two copies of each) and repeat Steps 3 & 4.
6. Giver exits their world without saving when finished.
7. Receiver can then bring in all duplicated items to Giver's World and exit without saving so both players have copies of items.
Another boosting method if you are playing alone, is to collect as much ore as you can in a Chest or your inventory. Then turn the Auto Save feature off under the Help & Options in the Main Menu or by hitting . Then smelt all of the ore into bars. You can then choose to go the Main Menu. You will choose the second option of "Exit without saving" Then reload your world, and all your ore you had will still be there. Take it again and smelt all of it into bars. Repeat quitting without saving until the achievement pops. This is very similar to the duplicating method, only for one person.
This game saves things to the character (your profile) as you do them, such as boss kills, items crafted, etc. whether you save the game or not, using Auto Save or manually saving and regardless of what world you do it in.
NOTESAll smelting is done at a Furnace using various ores.
A (regular) furnace is made at a workbench using 20 stone, 4 wood, and 3 torches. This smelts all the basic metals.
A Hellforge is found in the Underworld, and can be removed using any Hammer of 60% power or higher. You can place this in your base or anywhere you want. A Hellforge can smelt Hellstone in addition to the basic metals.
A Hellforge is required to make a Titanium/Adamantium Forge. This is made from 30 Titanium/Adamatium ore at a Mythril/Orichalum Anvil.
The Titanium/Adamantium Forge can smelt ANY metal ore, effectively replacing the need for any previous Furnace.
ORES TO BARS CONVERSION(Regular) Furnace3 ore --> 1 Bar : Copper, Iron, Tin, Lead, Cobalt, Palladium
4 ore --> 1 Bar : Silver, Tungsten, Gold, Platinum, Demonite, Meteorite, Mythril, Orichalum
Hellforge4 Hellstone Ore AND 1 Obsidian --> 1 Hellstone Bar
Titanium/Adamantium Forge5 ore --> 1 Bar : Titanium, Adamantium, Chlorophyte