To make a mold for casting a mask, soften the clay by kneading it, and flatten it until it is 1/4 inch thick. Pull the model's hair away from her face, and cover it with a bald cap. Apply a thin layer of baby powder to the model's face and bald cap to keep the clay from sticking to her.
Preheat an oven to 250 degrees Fahrenheit. Have the model close her eyes and mouth, and lay the clay over her face. Create holes where her nostrils are, and delicately press the clay into her facial features. Start at the sides of the nose and eyes, and move outward, keeping the thickness of the clay uniform. If needed, add more clay as the mold is being pressed.
Once the mold is pressed completely into the model's features, have the model bend forward with her face pointed toward the ground, and slowly remove the mold. Check the features to ensure they look right before placing the mold onto a baking sheet lined with baker's parchment paper. Place the mold so the nose is pointing upward to protect the facial features.
Bake the mold for 20 minutes if the clay is still 1/4 inch thick. The clay is ready when the mold has hardened. Allow the mold to cool before using it to cast a mask. If the mask mold requires features more exaggerated than a typical human face, carefully create the exaggerations with tools after the clay is removed from the model's face.