The firing process of a purple sand ceramic glaze flower pot is an art of dance between clay, glaze, and flame, with temperature control being the key. To achieve a uniform glaze color, precise control is required across multiple dimensions, including raw material characteristics, firing stages, and kiln environment, allowing every inch of the glaze to flow with the beauty of a fusion of nature and craftsmanship.
The color mechanism of the purple sand glaze originates from the oxidation-reduction reaction of metal oxides in the glaze. Under high temperatures, elements such as iron and copper undergo valence changes with temperature, resulting in a rich range of hues from light purple to dark brown. If temperature fluctuations are too large, the oxidation-reduction reaction becomes unbalanced, leading to defects such as color spots, glaze runs, or a matte finish. Therefore, before firing, the optimal temperature range must be determined through experimentation based on the glaze formula, providing a benchmark for subsequent control.
The firing process consists of five stages: preheating, oxidation, reduction, holding, and cooling. Each stage has distinct temperature requirements. The preheating stage requires a slow increase to approximately 300℃ to ensure even evaporation of moisture from the clay body, preventing cracking due to localized overheating. At this stage, a strong oxidizing atmosphere is maintained inside the kiln to ensure the complete combustion of organic matter in the glaze, laying the foundation for subsequent reactions. If the temperature rises too quickly, the violent vaporization of moisture may break through the glaze layer, forming pinholes or bubbles.
The oxidation stage is the critical period for glaze color formation. When the temperature reaches 600℃-1000℃, the coloring minerals in the glaze begin to develop color, and the metal oxides gradually dissolve and rearrange. At this time, the heating rate must be strictly controlled, usually not exceeding 5℃ per minute, to ensure uniform melting of the glaze layer. If the temperature is too high, the glaze fluidity increases, easily leading to glaze runs or an excessively thin glaze surface; if the temperature is too low, the glaze layer will not completely melt, resulting in a matte or rough texture. The kiln atmosphere must be maintained at a weak oxidizing level to avoid premature introduction of reducing gases that could interfere with color development.
The reduction stage is the "finishing touch" of purple sand glaze. When the temperature approaches 1200℃, ventilation must be reduced or a reducing agent added to create a weak reducing atmosphere inside the kiln. At this time, iron is reduced from trivalent to divalent, and the glaze color changes from brownish-red to deep purple, forming a unique sandy glaze texture. Temperature control during this stage is particularly delicate, requiring precise adjustments between 1180℃ and 1220℃. This ensures the reduction reaction proceeds fully while preventing excessive vitrification of the glaze, which would reduce its permeability. Craftsmen often judge the degree of reduction by observing changes in the color of the flames and the color of the clay body within the kiln, accumulating experience to develop a keen eye.
The holding stage is the "setting period" for glaze color stabilization. Once the target temperature is reached, it needs to be maintained for 1-2 hours to allow the glaze layer to fully melt and distribute evenly. During this time, the kiln temperature must be stabilized within ±5℃ to prevent uneven glaze shrinkage due to temperature differences, which could lead to cracks or color variations. The holding time depends on the glaze thickness and the size of the clay body; thicker glazed flowerpots require longer holding times to ensure complete glaze maturation.
The cooling stage also tests the craftsman's skill. The glaze is liquid at high temperatures; rapid cooling will cause the glaze layer to shrink too quickly, separating from the clay body and peeling off. Therefore, a segmented cooling method is required: first, the temperature is slowly reduced to 800℃ at a rate of 50℃ per hour to allow the glaze to gradually solidify; then, it is allowed to cool naturally to room temperature. During the cooling process, the kiln door must be closed to prevent direct cold air from blowing and to avoid cracking of the glaze due to localized cooling.
The uniform glaze color of the purple sand ceramic glaze flower pot is the result of the combined effects of temperature, time, and atmosphere. From raw material proportions to firing curves, from kiln design to operational details, every step requires the artisans to treat it with reverence. Through generations of inherited experience and continuous innovation, they have thoroughly understood the "temperament" of the flame, ultimately allowing the purple sand glaze to bloom with a unique beauty in the kiln transformation—sometimes like low-hanging evening clouds, sometimes like wisps of purple smoke; each piece is a perfect dialogue between nature and humanity.
