Hardened clay is used to make stained clay and you can also use it in buildings.
To make Hardened Clay (Terracotta), you will need Raw Clay, Clay Blocks and then have to use Furnace. Please follow the following steps to make Hardened Clay (Terracotta).
In order to make Hardened Clay, the basic item you need to get is Raw clay. You will have to find and get at least 4 raw clays. You can find them under the ocean.
After that you will have to craft clay blocks from Raw clays you have collected. Go towards the crafting table and add 4 clays in any four adjacent cells of 3*3 crafting grid. You will get a clay block in the right side box of the crafting table. Add these clay blocks into inventory.
Hardened Clay using Furnace
Now you have to use the furnace to make Hardened Clay (Terracotta). Move to Furnace beside the crafting table and add a clay block in the upper box and coal in the lower box to get a hardened clay block as output in the right-side box of the furnace. Add this into your “ready to use” stock.