To make glazed terracotta, you will need to place any kind of stained terracotta inside a furnace. However, before you can do this, you must first make normal terracotta by finding clay blocks and chucking them inside a furnace.
If you don't have any clay on hand, you can find it at the bottom of lakes, rivers, and even the ocean, and mining it with a shovel will do the trick.
If you want more clay to drop, try enchanting the shovel with fortune or if you rather get the whole block, silk touch will be your thing. Don't have any of those? Well, just gather up the clay balls and take them to a crafting table and place them in stacks of four.
Doing so will allow you to make clay blocks which you can then turn into terracotta by melting the blocks down inside a furnace. Once you got terracotta, you can make stained terracotta by placing it again inside a crafting table with a dye.
The type of stained terracotta you make will depend on the color dye used, so place whatever dye you want inside the table. When you got a stained terracotta block, you can then place it inside a furnace to make a glazed terracotta block.
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