There are a few different methods you could use to rehydrate your clay if it is not totally dry, but is too stiff to work with:

Small Pieces
Wrap clay in a soaking wet towel and cover with plastic to keep airtight and allow to sit for a couple of days before wedging – this may work best with smaller pieces so might be worthwhile to cut a larger block into multiple pieces for this method.

Clay Block
For a full block, poke a series of holes in the clay using a screwdriver, wooden spoon, pencil or similar. Make sure the holes do not go right through, leave a few centimetres at the bottom and fill the holes with water, wrap in plastic to keep airtight and allow to sit for a couple of days until the water has been absorbed. Wedge the clay and you should end up with nice moist clay to work with.

If your clay has become dry and you want to rehydrate it, here are some specific steps you can follow:

STEP 1 Gather the dry clay 
Collect all the dry clay pieces or scraps that you want to rehydrate. It's important to note that this method works best for natural clay or clay that doesn't contain additives like wax or plasticizers.

STEP 2 Break the clay into small pieces
Use your hands or a tool to break the dry clay into smaller pieces. This will help it absorb water more effectively.

STEP 3 Place the clay in a container
Put the broken clay pieces into a plastic or glass container. Avoid using metal containers, as they can react with the clay or cause rusting.

STEP 4 Add water
Pour water into the container, covering the clay pieces completely. Start with a small amount of water and add more if needed. It's better to add water gradually to avoid making the clay too wet.

STEP 5 Soak Clay 
Let the clay soak in the water for several hours or overnight. The water will gradually penetrate the clay and rehydrate it.

STEP 6 Test Consistency 
After the soaking period, check the clay's consistency. If it still feels dry or has lumps, add a little more water and let it soak for some more time. Repeat this process until the clay reaches the desired consistency.

STEP 7 Wedge
Once the clay has absorbed enough water and feels pliable, take it out of the container and wedge it thoroughly. Wedging helps distribute the moisture evenly and ensures that the clay is fully rehydrated.

STEP 8 Adjust Moisture 
Depending on the specific clay and your desired consistency, you may need to adjust the moisture content. If the clay is too wet, let it air dry for a while or add dry clay to absorb the excess moisture. If the clay is still too dry, sprinkle some water and continue kneading.

 

NOTE It is especially important to wedge your clay when it has been rehydrated to even out the moist/dryer spots. If the clay is particularly dry, you may need to repeat the process. It's important to note that not all clay can be successfully rehydrated, rehydrated clay may have slightly different properties compared to fresh clay, so it's a good practice to test it before using it for important projects.