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Technical Analysis and Solution for Surface Cracks in Clay Fired Bricks I. Problem OverviewThe photo shows sintered clay bricks with visible surface cracking after firing. These cracks typically indicate internal stress imbalance or improper control during raw material preparation and kiln firing. Although the bricks may appear structurally complete, such cracks seriously affect the product’s mechanical strength, water absorption stability, and frost resistance — thus they are considered unqualified products in engineering applications. II. Causes from Raw Material Perspective1. Clay Plasticity and Shrinkage ImbalanceIf the clay has excessively high plasticity or contains a large proportion of fine particles (20%), drying stress increases sharply, making the surface prone to cracking before firing.Solution:(1) Control extrusion water content within 16–18%.(2) Use vacuum extrusion to remove air bubbles and achieve uniform density.3. Inadequate Aging or MixingInsufficient blending or aging leads to uneven moisture and plasticity in the clay body, resulting in internal stress concentration during drying and firing.Solution:(1) Increase mixing and aging time (at least 48 hours for new clay).(2) Ensure homogeneous blending of all additives and recycled materials. III. Causes from Firing and Kiln Control Perspective1. Rapid Drying or HeatingIf the initial drying or preheating temperature rises too fast, the surface of the green brick dries and hardens quickly, forming a “shell,” while the interior still contains moisture. The steam pressure generated inside causes the surface to crack.Solution:(1) Slow down the drying curve; control the initial heating rate within 20–30°C/h.(2) Extend the holding period in the drying zone to ensure even moisture removal.2. Overly Fast Temperature Rise in the Sintering ZoneWhen the temperature in the firing zone increases sharply, especially between 600–900°C (dehydroxylation and quartz phase transformation stages), the brick body expands unevenly and cracks.Solution:(1) Optimize the firing curve and smooth the temperature rise.(2) Keep the sintering zone temperature rise under 40°C/h through the quartz inversion phase.3. Improper Cooling RateIf cooling after sintering is too rapid, thermal shock causes cracks, especially for thick or dense products.Solution:(1) Control the cooling rate below 40°C/h from 900°C to 600°C.(2) Ensure the cooling air flow is even to avoid local thermal stress. IV. Process Optimization and Quality Control Recommendations1. Raw Material Testing: Regularly test plasticity index, drying shrinkage, and mineral composition of the clay.2. Forming Process: Ensure uniform extrusion pressure and avoid lamination defects.3. Drying Control: Use staged drying with automatic temperature and humidity adjustment.4. Kiln Operation: Monitor temperature curves and air distribution in real time; use infrared or thermocouple sensors.5. Post-Firing Inspection: Observe crack patterns — mesh-like cracks usually indicate shrinkage imbalance, while single long cracks often point to thermal stress. V. Brictec Conclusion1. Surface cracking in fired clay bricks is a result of combined effects of raw material composition, forming moisture, and firing regime.2. By optimizing clay blending, strictly controlling drying and firing curves, and improving temperature uniformity in the tunnel kiln, such defects can be effectively prevented.3. Through systematic process control, Brictec ensures that clay sintered bricks achieve dense texture, uniform color, and excellent mechanical properties, meeting high-end architectural and structural standards. Editor: JF & Lou

Water Absorption Test Report for Fired Clay Bricks (Compiled by Xi'an Brictec engineering Co., Ltd. ) I. Test Purpose The water absorption test is an essential step in evaluating the physical properties of sintered clay bricks. It mainly examines the compactness, durability, and weather resistance of the finished products. For BRICTEC’s fully automated production lines, the test serves as an important verification procedure to ensure that all fired bricks meet both national and international quality standards before leaving the factory. Water absorption directly affects the brick’s frost resistance, long-term strength stability, and service life. If the water absorption rate is too high, the bricks tend to develop cracks, scaling, or surface peeling after repeated wet–dry and freeze–thaw cycles. Therefore, maintaining water absorption within the standard range is crucial for ensuring the reliability and durability of masonry structures. II. Testing Method and Procedure The experiment follows the national standard GB/T 32982–2016, Performance Requirements for Load-bearing and Non-load-bearing Sintered Bricks. Samples were collected from BRICTEC’s automated tunnel kiln after the firing process was completed. Testing steps were as follows: The dry mass (M₀) of each sample was measured. Samples were then immersed in water for 15 hours under constant temperature conditions. After removal, surface water was wiped off, and the saturated mass (M₁) was recorded. The water absorption rate (W) was calculated using the following formula: W=M1−M0M0×100%Where: M0: Dry weight of the brick (g)；M1: Weight after 15 hours of water absorption (g) III. Test Results No. Dry Weight (g) Weight After 15h Soaking (g) Water Absorption (%) 1 2785.7 3117.1 11.90 2 2845.4 3193.0 12.22 3 2835.7 3171.7 11.85 4 2819.9 3137.2 11.25 Average Water Absorption: 11.81% According to GB/T 32982–2016, the 5-hour boiling water absorption rate for load-bearing sintered bricks should have an average value ≤18% and a single value ≤17%. The BRICTEC samples show a significantly lower absorption rate, demonstrating excellent density, low porosity, and outstanding overall performance. IV. Analysis and Discussion The low water absorption rate reflects the technological precision and optimized control of BRICTEC’s manufacturing process. The uniform temperature distribution within the tunnel kiln ensures complete sintering and dense internal structure formation. The precise control of moisture and combustion air minimizes internal pores and enhances compactness. The advanced mixing and extrusion systems increase green brick density, improving impermeability and frost resistance. These factors together indicate that BRICTEC’s production technology guarantees consistent, high-density, and high-performance fired bricks, suitable for load-bearing structures and harsh environmental conditions. V. Conclusion Based on the test results and analysis, the average water absorption rate of fired clay bricks produced by BRICTEC’s fully automated line is 11.81%, which is well below the limit specified in GB/T 32982–2016. This confirms that: The bricks achieve excellent vitrification and densification during firing. The finished products exhibit superior resistance to moisture, frost, and weathering. The overall production process is technologically advanced, stable, and reliable. BRICTEC will continue to implement systematic quality monitoring and standardized testing procedures, ensuring that every fired brick produced meets international standards for durability, structural integrity, and environmental performance. VI. Further Testing Recommendations (Extended Quality Verification Items) To comprehensively evaluate the overall performance of the product, it is recommended to conduct the following supplementary tests based on the water absorption test results and establish corresponding benchmark indices: Open Porosity / Apparent Density / Bulk Density – for direct correlation between water absorption and mechanical properties. Compressive Strength / Flexural Strength – to assess mechanical load-bearing performance. 5-Hour Boiling Water Absorption Test – verification method required by Table 4 of GB/T 32982-2016. Freeze–Thaw Cycle Test – recommended for projects in cold regions. Salt Crystallization Resistance Test – for bricks used in coastal areas or road pavements. Microporous Structure Analysis (BET surface area, pore-size distribution, microscopic observation) – to identify structural causes and guide process optimization. Permeability and Pore Connectivity Analysis – for simulating long-term durability in engineering applications. These extended tests help establish a complete quality profile and ensure that the sintered bricks meet performance requirements under different environmental and structural conditions. VII. Key Elements of the Water Absorption Test Report (for Project Documentation) When issuing the official water absorption test report, BRICTEC recommends including the following elements to ensure traceability and technical completeness: Project title, sample ID, sampling date, and test date; Testing standard and reference (e.g., GB/T 32982–2016, including specific clauses); Model and calibration record of all instruments used; Drying conditions, immersion procedure/time, and weighing method (including scale precision); Detailed raw measurement data (m_d, m_s, and full calculation process), along with statistical values (mean, max, min, and standard deviation); Compliance assessment (whether the sample meets the relevant standards and project specifications, and if further freeze–thaw testing is required); Technical recommendations and proposed follow-up tests; Signatures of testing personnel and authorized quality supervisors. This standardized format ensures that the test documentation is suitable for international project submissions, EPC acceptance reports, and long-term traceability audits. VIII. Conclusion (BRICTEC Technical Evaluation Summary) Based on the 15-hour water absorption test of the four provided samples, the average absorption rate is approximately 11.8%, which is significantly below the limit value (≤15%) specified in Table 4 of GB/T 32982–2016 for load-bearing decorative bricks. From this single performance indicator, it can be concluded that the finished bricks exhibit good compactness and material quality. The results confirm that the current raw material formulation, forming density, and firing regime have achieved excellent densification. Under these conditions, freeze–thaw pre-screening is not required based solely on water absorption data (provided the testing method and standard comparison are consistent). However, for projects operating under more demanding environmental conditions or where long-term durability is a key design concern, BRICTEC recommends performing additional evaluations including: The 5-hour boiling water absorption test, Freeze–thaw cycle testing, and Other durability assessments as specified in relevant national or international standards. Based on the results, targeted optimization of the raw materials and firing process can be implemented to further enhance the product’s durability and reliability.

Introduction to the Imperial “Golden Brick” Manufacturing Process in Ancient China Brictec – Clay Brick Technology Insight Series I. Overview and Historical BackgroundThe 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 Item Ancient Imperial “Golden Brick” Modern Tunnel Kiln Clay Brick Raw Material Processing Special clay from designated sites; months of clarification and kneading Mechanized crushing, blending, and mixing (hours to days) Forming Method Manual molding and board pressing Vacuum extrusion and continuous cutting (automated, high output) Drying Long-term natural drying (months) Mechanical tunnel drying (hours to days) Firing Traditional kilns with slow heating, long soaking, and water curing (weeks–months) Tunnel or roller kiln; continuous and precisely controlled (hours) Productivity & Yield Very low output, low yield but supreme quality High output, standardized, stable yield Quality Features Extremely dense, glossy surface, metallic resonance High strength, consistent dimensions, controllable absorption Labor Intensity Labor-intensive, craft-based, long cycle Mechanized/automated, efficient, short cycle 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 SeriesWritten by: JF & Lou