Reactive glaze is a type of glaze that undergoes a chemical reaction during firing, creating a unique pattern or color. The glaze is made from different raw materials, including organic, mineral, or metallic, and then applied to the pottery piece after biscuit firing. The glaze is then smooth-fired, which results in a unique finish that can't be predicted. This means that no two pieces of reactive glaze pottery are identical, and each piece has a unique appearance with features like blemishes, dimples, aesthetic cracking, and unevenness to the touch.