The Grey Cast Iron Bar is a bloc of metal you get by melting Iron Ore in a Melting Furnace.
The Stainless Steel Bar is a bloc of metal you get by melting Iron Ore, Chrome Ore, Nickel Ore in a Melting Furnace.
The Bronze Bar is a bloc of metal you get by melting Copper Ore and Tin Ore in a Melting Furnace.
The Titanium Bar is a bloc of metal you get by melting Titanium Ore in a Melting Furnace.
The Aluminium Bar is a bloc of metal you get by melting Aluminium Ore in a Melting Furnace.
The Defective Alloy is what you get from Melting Furnace when you miss to achieve a correct alloy composition. This voxel has no other use than decorative purpose. It cannot be recycled in any way, so what you used to make it is definitely lost.