It is not safe to eat directly off clay plates that have been baked in a kitchen oven for 30 minutes at 375 Fahrenheit (190 degrees Celsius) to "fire" them. Domestic kitchen ovens are not designed for firing pottery or clay at high temperatures, and the clay used to make the plates might not have been processed or glazed correctly for food use.
When pottery or clay objects are fired, they need to be fired at specific temperatures and in kilns that are designed for pottery firing. This ensures that the clay reaches the necessary temperature to vitrify or harden properly. Additionally, food-safe glazes need to be used to create a non-porous surface that won't leach harmful substances into your food.
Eating off improperly fired or unglazed clay plates can pose health risks because the clay might not be fully vitrified, making it porous and prone to bacterial growth. Also, clay that is not specifically formulated for food use might contain impurities or chemicals that could be harmful if they come into contact with food.
If you want to use clay plates for eating, it's essential to purchase them from reputable sources where the products are explicitly labeled as food-safe and have been fired and glazed properly. If you have made clay plates yourself and want to use them for food, consult with a pottery expert or kiln operator to ensure they are fired correctly and safely.
In general, it's best to use ceramics and pottery that are explicitly designed and labeled as food-safe to avoid any potential health risks.