Underglaze can be painted on clay at various stages of its drying process, depending on the desired effect and the techniques you plan to use.
Painting underglaze on clay that has reached the leather-hard stage is a popular choice. The clay is firm enough to support the application of underglaze without distortion.
Applying underglaze to bone-dry clay allows the underglaze to be absorbed into the clay, creating a different appearance compared to painting on leather-hard clay.
You can also paint underglaze on greenware (unfired clay), but extra care is needed to avoid damaging the delicate surface.
The stage of clay will influence the techniques you use, such as brushwork, sgraffito, or stamping, to create your desired design.
Experimenting with different stages of clay can lead to diverse artistic outcomes when painting underglaze, allowing you to achieve unique textures and visual effects in your ceramic pieces.