The Secret of Glaze: From Accidental Discovery to Scientific Application

Glaze, the very soul of ceramic art, embodies human ingenuity and serendipitous discovery. This article will delve into the origins, evolution, scientific composition, and modern applications of glaze, along with crucial safety considerations.

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I. The Miracle of Fire and Clay: The Accidental Birth of Glaze

About 500,000 years ago, humans learned to use fire. In the process, they discovered that clay, when burned, became hard and retained its shape. Some "lumps of clay" with four upturned sides could even hold rainwater. Inspired by this, early humans began to intentionally create desired objects from clay and fire them. Thus, pottery was invented.

By the Shang and Zhou dynasties in China (approximately 3,000–2,500 years ago), our ancestors made another accidental discovery while firing pottery: "glaze." When firing pottery in kilns, the Shang and Zhou people noticed that some individual pieces emerged with sparkling, shiny spots on their surfaces. These spots were glossy like glass, did not absorb water, were bright and attractive, and could be wiped clean. This led people to wonder where these shimmering things came from. After much observation, they found that the "sparkle" was caused by ash from the wood and plant fuel in the kiln falling onto the surface of the wares during firing. Based on this discovery, people tried mixing burnt plant ash into a slurry and applying it to the exterior of unfired pottery before firing. As expected, a shimmering layer formed on the pottery's surface—glaze had been invented.

Glazed pottery not only possessed excellent sealing properties, making it waterproof and impermeable, but it was also extremely easy to clean, greatly increasing its practicality. This naturally formed glaze was not artificially applied but was a pure product of nature. During the Shang and Zhou periods, wood and plants were the primary fuels for firing pottery, so the falling ash was plant ash. Consequently, people in ancient times collected withered plants, burned them at high temperatures to remove fibers and retain the essence, ground them into fine carbonate particles, mixed them with porcelain stone, and after further grinding and washing, created the glaze for porcelain.

The invention of glaze was neither purely accidental nor purely inevitable, but the result of multiple interwoven factors. The pottery fired during the Shang and Zhou period was mostly hard earthenware, which required high temperatures of around 1200°C to be fired successfully. It was this high-temperature environment that caused the plant ash in the kiln to melt and fuse with the clay body, forming a glazed surface. If they had been firing soft earthenware, which only requires temperatures around 1000°C, the conditions for glaze formation would have been difficult to achieve.

Therefore, once the three key conditions—"glaze," "porcelain clay," and "temperatures above 1200°C"—were met, porcelain came into being.

Why did applying plant ash create a bright glaze on the pottery's surface? It turns out that plant ash contains **calcium oxide**. Calcium oxide acts as a **flux**, an agent that lowers the melting point of the quartz and feldspar in the clay body, causing them to melt and form a vitreous substance on the surface. This vitreous substance is the glaze.

Typically, making a cup involves steps like throwing, trimming, bisque firing, and glazing, with each stage determining the final glaze effect and the quality of the porcelain. However, these early "glazes" often had high concentrations of silicates and alkali metal oxides, but their composition was unstable and the craftsmanship was crude, resulting in thin, uneven glaze layers with varying colors and textures.

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II. The Evolution of Glaze: From Natural Ash to Intentional Application

Shang and Zhou to Sui and Tang Dynasties (Approx. 3,000–1,400 years ago)

• Shang and Zhou (Approx. 3,000–2,500 years ago): The earliest use of **Ash Glaze**. People discovered that ash from burnt wood and plants falling on pottery created a glassy luster.
• Eastern Han to Sui and Tang (Approx. 2,000–1,400 years ago): More stable **Lime Glaze** was developed, and white and black glazes began to appear.

Song and Yuan Dynasties (Approx. 1,000–700 years ago)

• Song Dynasty (Approx. 1,000–800 years ago): Ru ware, Longquan ware, and others used **Lime-Alkali Glaze**, which gave the glaze a softer, more lustrous texture, as seen in the famous "sky-blue" and "plum-green" colors.
• Yuan Dynasty (Approx. 700 years ago): Blue-and-white porcelain became popular. The glazes were still primarily made from natural minerals, but the formulas were more refined.

Ming and Qing Dynasties (Approx. 600–100 years ago)

• Ming Dynasty (Approx. 600–400 years ago): Glaze colors became richer, but still relied on natural minerals (like iron, cobalt, and copper oxides).
• Qing Dynasty (Approx. 300–100 years ago):
  • Kangxi Period (Approx. 350 years ago): With the introduction of European chemical technology, China began to use small amounts of **Chemical Glazes**, such as Cobalt Oxide for blue-and-white porcelain.
  • Yongzheng and Qianlong Periods (Approx. 250–200 years ago): More colored glazes appeared, but traditional ash glazes remained dominant.

Modern Era (Approx. 150 years ago to present)

• Late 19th Century (Approx. 150 years ago): Influenced by Western industrialization, **Chemical Glazes** (like Zinc Oxide, Titanium Dioxide) gradually replaced traditional ash glazes.
• 20th Century to Present (Within the last 100 years): Chemical glazes have become mainstream. Glaze research has become more systematic and scientific, with continuous optimization of formulas and improvements in application and firing techniques. Modern ceramic artists and engineers have developed a series of new glazes, such as crystalline glazes, flowing glazes, bubble glazes, and metallic glazes, as well as glazes with special functions (e.g., antibacterial, wear-resistant, easy-to-clean), further broadening the field of glaze application.

Main Differences Between Chemical Glazes and Mineral Glazes:

• Composition and Stability:
  • Chemical Glaze: The composition is stable, refined industrially with precise formulas to ensure consistency in every batch, guaranteeing product stability.
  • Mineral Glaze: Uses raw, unrefined minerals. The mineral composition is unstable, and batches from different locations can vary significantly, leading to minor or even noticeable differences between products.
• Production Process and Cost Control:
  • Chemical Glaze: The process and costs are controllable, making it suitable for mass production and widely used in daily goods and crafts.
  • Mineral Glaze: Due to the uncertainty of raw materials, each batch requires test firings and adjustments, demanding high technical skill from the potter and increasing production time and cost. It is not suitable for mass production.
• Safety:
  • Chemical Glaze: Generally safe, unless harmful substances are intentionally added. The production and use of modern chemical glazes adhere to strict safety standards.
  • Mineral Glaze: While traditionally seen as "natural," its unstable composition may contain unknown or potentially harmful substances, posing certain safety risks.

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III. The Scientific Decoding of Glaze

Definition of Glaze:

A thin, glassy layer formed on the surface of a ceramic object. It is made by grinding certain natural minerals (like quartz) or plants into a powder, mixing it with clay to create a glaze slurry, applying it to the body of the ware, and then firing it. The thickness of the glaze on an unfired body is typically around 0.1 cm, but after firing, it adheres tightly to the porcelain body, making the vessel dense, giving it a soft luster, and rendering it impermeable to water and air, creating a bright, mirror-like appearance. It also enhances the strength and chemical stability of the object, preventing contamination, facilitating cleaning, and reducing corrosion.

Composition of Glaze:

I. The Four Basic Components of Glaze

Component Type	Function	Common Chemical Formulas	Raw Material Source
Glass Former	Forms the glass matrix of the glaze	$\text{SiO}_2$ (Silicon Dioxide), $\text{B}_2\text{O}_3$ (Boric Oxide)	Quartz sand, Feldspar, Borax
Flux	Lowers the melting point, promotes vitrification	$\text{PbO}$ (Lead Oxide), $\text{Na}_2\text{O}$ (Sodium Oxide), $\text{K}_2\text{O}$ (Potassium Oxide)	Red lead, Soda ash, Potassium feldspar
Stabilizer	Adjusts the physical properties of the glaze	$\text{Al}_2\text{O}_3$ (Aluminum Oxide), $\text{CaO}$ (Calcium Oxide)	Kaolin, Limestone
Colorant/Crystallizer	Imparts color and texture	$\text{CuO}$ (Copper Oxide) → Green, $\text{CoO}$ (Cobalt Oxide) → Blue, $\text{MnO}_2$ (Manganese Dioxide) → Purple	Metal oxide minerals

Types of Glaze:

• By Body Type: Porcelain glaze, Earthenware glaze, Stoneware glaze (for ceramics between earthenware and porcelain, like water vats, casserole dishes, etc.).
• By Temperature: High-temperature glaze, Medium-temperature glaze, Low-temperature glaze.
• By Appearance: Transparent glaze, Colored glaze, Opaque glaze, Crackle glaze, Crystalline glaze, Matte glaze, Gloss glaze.
• By Composition: Lime glaze, Feldspar glaze, Lead glaze.

Common types include transparent, opaque, and colored glazes. The firing temperature for colored glazes must reach above 1250°C. Cheralle's coffee cups use high-temperature, colored, and crystalline glazes.

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IV. A Guide to Selecting Safe Glaze

When choosing your favorite ceramic cup, safety and practicality are just as important as appearance. Today, let's talk about ceramic glaze safety and learn a few simple methods to easily select a cup that is both beautiful and safe.

I. Safety Assurances in Modern Ceramic Craftsmanship

Thanks to mature firing techniques and strict quality inspection standards, ceramic products from reputable brands on the market already meet safety requirements. The key is to look for the following guarantees:

• ✅ Authoritative Certifications: Look for certifications like FDA (USA), LFGB (EU), GB 4806.4 (China).
• ✅ High-Temperature Firing: Quality ceramics must be fired at temperatures above 1300°C to ensure the glaze is stable.
• ✅ Craftsmanship Choice: **Underglaze decoration**, which seals the pattern beneath the glaze layer, is safer and more durable.

II. 3 Easy Steps to Identify Quality Ceramics

1. Check for Certifications: Look for FDA/LFGB/GB logos on the packaging or instructions.
2. Feel the Surface: Choose products with a smooth glaze and a flat pattern, characteristic of underglaze decoration.
3. Choose Reputable Brands: Legitimate brands will provide complete inspection reports.

III. Cheralle Coffee Cups' Safety Promise

Every one of our cups:

• It is produced in Jingdezhen, the "Porcelain Capital of the World," using high-quality raw materials like kaolin, feldspar, and quartz.
• It is fired at a high temperature of 1320°C (2408°F), far exceeding safety standards.
• Has passed both GB 4806.4 and FDA dual certification.
• It is regularly sent to international testing agencies like SGS for inspection.

When you purchase a Cheralle coffee cup, you can request the latest quality inspection report from our customer service. We promise that all our products meet international safety standards, so you can use them with peace of mind and drink with confidence!