Glasuur

Glasuur is a vitreous coating fused onto ceramic bodies during a glaze firing, forming a glassy surface that seals, protects, and decorates the ware. It can alter the color, texture, and durability of pottery and porcelain.

It consists of a glass-forming silica network, fluxes to lower the melting temperature, and stabilizers, with colorants or opacifiers such as cobalt or copper oxides and tin oxide. The glaze must be chemically compatible with the clay body to prevent defects like crazing or peeling during cooling.

Types range from transparent glazes that reveal the underlying material or decorations, to opaque tin-based glazes that create a white surface for painted designs, and from matte to glossy finishes depending on the glaze composition and firing.

Application methods include brushing, dipping, or spraying glaze suspensions onto bisque-fired ware. After application, the piece dries and is fired in a kiln at specific glaze temperatures. The firing atmosphere and rate influence color development and glaze behavior, with common issues including pinholes, blistering, and crazing if misfired.

Historically, glasuur has been central to many ceramic traditions, such as Delftware in the Netherlands with tin glaze, Italian majolica, and various Chinese and Persian wares. Each tradition developed characteristic color palettes and surface textures through distinct glaze recipes and firing practices.

Safety and regulation: some traditional glazes contained lead or cadmium, leading to strict modern restrictions. Contemporary glazes are often lead-free and tested for food safety, but care is advised when using pottery for meals or drink, especially with older or improvised glazes.