Throwing with excessively firm clay is uncomfortable and rarely produces the best results. Sometimes the clay arrives from the supplier in that state, and sometimes it’s just because a bag has been sitting in the studio for a while and has dried out. Thankfully, with a little time and water, you can rehydrate the clay quite effectively.
Plastic clay generally has a water-content percentage in the low 20s and a difference of 1–2% can take a clay from firm to soft. For a 25–27 lb (12.5 kg) bag, this would mean adding 100–200 mL of water.
You can just add the water to the bag (1) and leave it for a while, but it’s more effective to submerge the sealed bag of clay in a container of water (2–4). This applies pressure to the water inside the bag, forcing it into the clay from all directions. Doing this the day before you need it will soften up the clay considerably (5).
It’s important to use a watertight bag or a lot more water can end up inside! As it’s fairly likely that an excessively dry block of clay is in a bag that isn’t perfectly sealed, it’s better to double bag with an outer bag that you know is watertight.