Introduction to the Imperial “Golden Brick” Manufacturing Process in Ancient China Brictec – Clay Brick Technology Insi

Introduction to the Imperial “Golden Brick” Manufacturing Process in Ancient China

Brictec – Clay Brick Technology Insight Series

 I. Overview and Historical Background
The so-called “Golden Brick” (Jinzhuan) was not made of real gold. It was a high-grade square clay brick specially produced during the Ming and Qing Dynasties for imperial palaces such as the Forbidden City’s three main halls. Renowned for its smooth luster, dense texture, and metallic resonance, it was also called Jing Brick or Fine Clay Palace Brick.

Historical records indicate several standard sizes (e.g., 1.7 chi or 2.2 chi in length), and it was mainly used for floor paving in imperial halls and other royal venues.

The production of Golden Bricks was extremely complex and time-consuming, with a manufacturing cycle exceeding one year. In modern times, this process has been recognized as an Intangible Cultural Heritage of China.

II. Raw Material Sources and Selection — Why It Is Unique

1.Origin:
Traditionally sourced from Suzhou, Jiangsu Province, especially from areas such as Lumu Imperial Kiln Village and Taihu Lake mud. The fine-grained, iron-rich lakebed clay from the Jiangnan region was known for being “sticky but not loose, powdery but not sandy,” ideal for making dense, glossy brick bodies. Historical kiln records confirm this provenance.

2.Material Requirements:
The clay had to be fine-grained and low in impurities, with strict control of iron content, plasticity, cohesion, and organic matter. Since natural deposits varied, multiple clays were often blended to achieve the desired plasticity and firing color.

III. Overall Production Cycle and Key Stages

1.Historical and archaeological studies agree that Golden Brick production was a long, multi-stage process that included:

Soil selection → Clay refining (settling, filtering, drying, kneading, treading, etc.) → Molding → Natural drying → Kiln firing → Water curing (“Yinshui”) → Polishing and finishing.

2.The entire cycle typically exceeded one year, with some records citing 12–24 months from clay preparation to finished brick. The clay refining process alone often lasted for several months. Some documents describe 29 detailed sub-steps in total.

IV. Step-by-Step Technical Process (Grouped by Stage)

Note: Details varied by historical period and kiln site. The following represents common, technically refined practices documented by museums and scholarly research.

1. Raw Clay Pre-Treatment (Extraction → Mixing → Settling and Clarification)

• Clay extraction: Selected from lake mud or designated pits, avoiding sand and organic-rich layers.

• Coarse screening: Removed stones, roots, and large debris.

• Soaking and sedimentation (“Cheng”): Clay was soaked for long periods; gravity settling separated fine particles from impurities.

• Filtering and water replacement (“Lü”): Multiple filtrations and water changes improved particle uniformity and purity.

• Technical significance: Determines particle grading and purity, fundamental for the brick’s density and surface gloss.

2. Clay Refining (Long-Term Aging and Kneading)

• Drying and airing (“Xi”): Partially dried to suitable moisture for kneading.

• Kneading and treading (“Le” & “Ta”): Manual or foot kneading expelled air, improved cohesion, and homogenized texture.

• Repeated clay refining: Historical records emphasized repetition — months of repeated mixing, filtering, and aging.

• Technical significance: Long-term aging (analogous to modern “clay maturation”) improves plasticity, reduces internal stress, and ensures uniform shrinkage and dense firing—the key to the Golden Brick’s unique “metallic sound.”

3. Forming and Compaction

• Molds and pressing: Large square molds were used. Workers manually pressed or stepped on boards to compact clay evenly.

• Stamping and surface finishing: Some bricks bore imprints or royal stamps. Surfaces were carefully smoothed.

• Technical significance: Manual compaction and surface polishing created dense, smooth, low-porosity bricks.

4. Natural Drying and Controlled Air-Drying

• Long-term air-drying: Instead of fast drying, bricks were slowly air-dried for 5–8 months, minimizing cracks.

• Technical significance: Slow moisture release prevented shrinkage cracks and ensured even internal moisture before firing.

5. Kiln Loading and Long-Term Firing

• Kiln type and stacking: Imperial kilns like those at Lumu were large and meticulously managed. Stacking patterns optimized heat distribution.

• Slow temperature rise and long soaking: Firing took weeks or months, avoiding thermal shock and crystal stress.

• “Yinshui” water curing: Post-firing, bricks were soaked in water basins to stabilize structure and enhance the metallic resonance.

• Technical significance: Controlled, slow high-temperature firing plus water curing increased strength, density, and acoustic quality.

6. Post-Firing Finishing (Polishing, Sorting, Acceptance)

• Cooling and inspection: Bricks were cooled and manually inspected. Qualified ones were glossy, crack-free, and resonant when struck.

• Polishing and trimming: Edges were refined and polished before installation in palace halls.

V. Why Were Golden Bricks of Such Exceptional Quality?

• Extended clay refining and aging: Months of clarification and maturation yielded fine, pure, cohesive clay for high densification.

• Slow drying and firing: Prevented cracking and ensured homogeneous internal structure.

• Unique mineral composition: Iron content enhanced surface color and solid-phase reactions, improving hardness and hue.

• Post-treatment (water curing & polishing): Enhanced surface gloss, density, and acoustic resonance (“metallic sound”).

VI. Comparison Between Imperial Golden Bricks and Modern Clay Sintered Bricks

Comment:
Ancient Golden Brick production pursued ultimate craftsmanship and imperial aesthetics, trading enormous manual effort and time for rarity and perfection.
Modern brickmaking focuses on scalability, uniformity, and cost efficiency, achieved through mechanization, automation, and quality control systems.

VII. Material Science and Acoustic Interpretation — Why Does It “Ring Like Metal”?

The Golden Brick’s “metallic sound” arises from its high density, low porosity, and high elastic modulus.
When internal particles are tightly sintered with minimal pores, impact stress waves propagate with low energy loss, producing a clear, bright tone similar to ceramics or stone.
Long-term clay aging, water curing, and surface polishing further enhance this acoustic effect.

VIII. Institutional Legacy and Cultural Preservation

The Golden Brick technique has been listed as an Intangible Cultural Heritage of China.
Today, artisans in Suzhou and Lumu Imperial Kiln Museum continue to preserve and reproduce this craft for heritage restoration and cultural education.

IX. Technical Significance

• The superior performance of imperial Golden Bricks stems from the synergy of four factors:

  • Clay selection;

  • Extended refining and maturation;

  • Controlled slow drying and firing;

  • Post-firing water curing and polishing.
    Together, they yield extremely low porosity and exceptional density.

• Compared with modern industrial brickmaking, Golden Brick production sacrifices productivity and cost for ultimate quality, representing the pinnacle of manual craftsmanship and experiential control.
  Modern production prioritizes efficiency, consistency, and standardization — two technological paths reflecting different eras.

• In preservation and restoration, understanding and retaining key traditional steps — especially clay aging, slow drying, and water curing — is vital for replicating the authentic quality of historical palace bricks.

Brictec – Clay Brick Technology Insight Series
Written by: JF & Lou