In the early stages of artificial pottery, there was no glazing on the ceramics. Even today, indigenous peoples such as Native Americans have preserved the tradition of unglazed pottery in various parts of the world.

Apart from some coarse pottery items like bricks, flower pots, and Yixing clay teapots, most ceramic household items available in the market today undergo glazing.

The majority of practical ceramics used for storing food require glazing for decoration and improved waterproofing. Ceramic decorating materials can be divided into two main categories: ceramic colours and glaze.

1. Ceramic Colors

a. Cosmetic Clay

Cosmetic clay refers to semi-fluid clay with a significant amount of moisture. It comes in various colours and is mainly used for body decoration. Cosmetic clay comes in rich colours such as red, black, white, brown, etc., commonly seen in Neolithic painted pottery.

Cosmetic clay, when fired alone, lacks a glassy lustre and has low strength, making it prone to peeling off or being plastered. It can be directly used for body painting or decoration techniques like inlay or coiling. Cosmetic clay is often used for surface decoration and carving. It is applied to the damp body, and after a shallow carving, the body exhibits both its original clay colour and the colour from the cosmetic clay.

Cosmetic clay can also be used as a base for glazing. For instance, if certain clay has an excessive iron content affecting the glaze colour, applying white cosmetic clay on the body before colouring the glaze ensures vibrant colours.

Cosmetic clay should be used when the body is still damp. When the body is completely dry, a powdery substance may form on the surface, affecting the bonding between cosmetic clay and the body.

b. Metal Oxides

Metal oxides are ceramic raw materials extracted from minerals or obtained by decomposing compounds. They are the main source of ceramic colourants and can produce a variety of effects depending on factors like raw material composition, formula ratio, firing temperature, and atmosphere.

c. Half-Glazed Colored Clay

This is a ceramic material used for surface decoration, with a chemical composition between clay and glaze. Artists add colourants or metal oxides to clay and mix in solvents used for the glaze, creating half-glazed coloured clay. It can be directly used for body decoration on semi-dry or bisque-fired bodies.

2. Ceramic Glaze

Ceramic glaze is a glassy material on the surface of ceramic bodies. The relationship between the ceramic body and glaze is like that of muscles and skin in the human body. Glaze serves multiple functions: it enhances the appearance, protects the ceramic from liquid and acid intrusion, facilitates easy cleaning, and adds aesthetic and decorative value.

Glaze materials come from various minerals, organic substances, and water-insoluble inorganic compounds, and are processed into powder or liquid forms. When glazed ceramic items are heated in a kiln, the glaze melts and adheres tightly to the body surface, becoming one with the body. If two glazed items are placed together in the kiln, they may stick together.

The colour development of glaze depends on factors such as ceramic raw material formulation, body composition, firing temperature (low, medium, high), firing time, firing method, and kiln atmosphere (reducing flame, oxidizing flame). Glazes can be categorized based on firing temperature (high, medium, low), composition (acidic, alkaline, neutral), and appearance effects (glossy, matte, transparent, opaque, crackle, lustre, crystalline, etc.).

Notable glaze types include:

a. Organic Ash Glaze: Natural ash is the basis for this glaze, containing solvent components. Examples include wood ash glaze and fallen ash from burnt firewood, primarily composed of potassium and sodium.

b. Lead Glaze: Known for vibrant colours and strong fluidity, with the Tang Tri-colored Glazed Pottery being an example. Lead acts as a flux, with a melting point around 800°C. Due to lead being a harmful heavy metal, lead glaze should not be used for the surface decoration of tableware or tea sets.

c. Feldspar Glaze: A natural flux, feldspar is divided into sodium feldspar, potassium feldspar, and sodium-potassium feldspar. It is mainly used to prepare medium to high-temperature base glazes.

d. Multi-Layer Overlapping Glaze: Also known as flower glaze, it can have a uniform monochrome glaze or a multilayered mixed-colour glaze. The appearance of coloured glaze can vary significantly based on firing temperature and atmosphere, sometimes resulting in unpredictable "surprises," commonly known as "Furnace-transmutation."

Furnace transmutation naturally brings out unique charm with irreplaceable effects. The colour development of some oxide colourants is not significantly affected by changes in firing temperature and atmosphere, such as cobalt oxide. During firing, coloured glaze exhibits a flowing appearance, unlike regular ceramic pigments that do not flow.