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Why is earthenware often glazed after firing?

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traditional ceramics Stages in the slip casting of a thin-walled whiteware container. Clay powder is mixed in water together with a dispersing agent, which keeps the clay particles suspended evenly throughout the clay-water slurry, or slip. The slip is poured into a plaster mold, where water is drawn out by capillary action and a cast is formed by the deposition of clay particles on the inner surfaces of the mold. The remaining slip is drained, and the cast is allowed to dry partially before the drain hole is plugged and the mold separated. The unfinished ware is given a final drying in an oven before it is fired into a finished product.

Earthenware is often glazed after its initial firing for both practical and decorative purposes. Because earthenware is not fired to the point of vitrification, the resulting body remains slightly porous.12

Practically, glazing is essential to overcome this porosity, which otherwise makes the vessels unsuitable for storing liquids like milk or wine. To achieve this, the fired object is coated with a finely ground glass powder suspended in a solvent and refired at a lower temperature. During this second firing, the particles fuse into a glasslike layer that seals the pores of the clay body, rendering it impermeable.213

Decoratively, glazing allows for the application of colors and finishes. For instance, opaque white tin glazes were historically employed to conceal the natural color of the clay body, which often ranges from buff to dark red due to iron content. Additionally, specific transition-metal or rare-earth elements can be added to the glaze to create various colors.32