Crackle Glaze: The Vital Fractures on Porcelain

Crackle glaze is a natural cracking phenomenon on the surface of porcelain glaze, which can occur during the ceramic firing process or due to historical reasons. Most kilns and a significant portion of ceramic varieties from ancient ceramics exhibit varying degrees of crackle glazing, which was initially considered a flaw in the firing process. However, some varieties have developed a specific artistic effect that has been favored by literati and appreciated by creative craftsmen who intentionally utilize the crackle effect to produce unpredictable artistic outcomes. The interwoven lines and patterned pieces evoke a sense of imagination and a mesmerizing, dynamic allure. As the saying goes, "It is the cracks in life that let the light in," crackle glazing adds vitality to static porcelain, endowing it with enduring artistic life.

During the porcelain firing process, there are two main reasons for the glaze to crack: first, the clay extends in a certain direction during shaping, affecting the arrangement of molecules; second, the different expansion coefficients of the body and glaze cause inconsistent contraction ratios when the piece cools after firing. Originally a flaw in porcelain making, the clever artisans mastered the regular pattern of cracking, and the resulting crackle glaze (cracked glaze) gradually became a special decoration on porcelain. Porcelains with crackle glaze characteristics have been passed down from the Song Dynasty's Ru, Guan, and Ge kilns.

There is also a type of crackle glaze that forms naturally over time, leaving traces of the years. As time passes, the glaze on the surface of the porcelain gradually develops internal cracks, either resembling fish roe patterns or ox hair patterns. Regardless of the pattern, the crackle is unrelated to the porcelain blank and never penetrates deeply, only cracking on the glaze surface. Therefore, any porcelain that naturally develops crackle over time will have a glaze that is subtly visible, while artificially cracked porcelain will have a stark revelation on the ceramic body.

The types of crackle glaze mainly include ice crack, wenwu, fish scale, hundred garbage broken, and crab claw patterns. Ice crack glaze describes the clear cracks on the glaze surface, resembling ice and snow. These cracks are very dense, with no gaps, preventing impurities from penetrating, and remain crystal clear after hundreds of years. Wenwu crackle refers to the overall cracking of the porcelain from top to bottom and left to right. The large, irregular cracks are metaphorically called "wen" (civil), and the smaller cracks nested within are called "wu" (martial). This type of crackle is most characteristic of the porcelain from the Song Dynasty's official kilns and Ge kilns.

Northern Song Dynasty Ru Kiln Tianqing Glaze Ice Crack Round Wash (Collection of the British Museum)

 

Fish scale crackle is a very unique phenomenon, mainly found on the products of the Northern Song Dynasty Ru Kiln, Song official kilns, and a few Southern Song Longquan kilns. The form is the crackle phenomenon that occurs between the layers of the thick glaze. These types of porcelain have very thick glaze layers, some even exceeding the thickness of the body. Such thick glaze layers are formed by multiple applications of glaze, each layer requiring drying before the next application, resulting in uneven thickness between layers. After the porcelain is fired, stress differences between layers cause fractures.

Southern Song Dynasty Official Kiln Pink Glaze Three-Legged Zun(Collection of the National Palace Museum, Taipei)

 

Regarding the golden wire and iron line crackle, the "Concise Dictionary of Ceramics" explains it as "a type of crackle, a special decoration of the Ge kiln porcelain handed down through the ages. Due to the large and small differences in cracks, the large crackle appears dark gray like iron, and the small crackle appears sauce-brown, resembling golden wire, hence the name." The glaze of Ge kiln porcelain is thick and the glaze color is steady. Due to the high content of iron oxide in the body, the glaze at the mouth edge melts and flows down during firing, thinning the glaze layer and revealing the sauce-purple body. Ancient craftsmen used a method of sudden cooling before the kiln was opened for Ge kiln, causing the internal stress of the glaze to release abruptly, producing large crackles. Then, the porcelain was immersed in an iron-containing solution, and the cracks absorbed iron substances into the cracks, creating the "iron wire" effect. After that, this once-cracked porcelain was placed in the kiln for baking, and before the glaze melted, it was suddenly cooled again, causing the glaze to produce small crackles within the large crackles. Then, the fired porcelain was placed in aged tea water, allowing tannin and other pigments to be absorbed into the crack gaps, or through long-term use and handling, forming the artistic effect of large pieces with small pieces, iron wires wrapped in golden wires. This unique process of Ge kiln is a typical example of the Song Dynasty craftsmen's control over crackle and has been inherited by the Yuan, Ming, and Qing dynasties, continuing to this day.

Qing Dynasty Yongzheng Ge Glaze Golden Wire and Iron Line Plate (Shanghai Auction 2012 Spring Auction)

 

Hundred garbage broken crackle is also a type of Ge kiln crackle, but the crackle is more finely broken. Due to the rapid cooling before the kiln is opened, the glaze is shattered, and the cracks are as dense as beads.

"Hundred garbage broken" crackle

 

Crab claw pattern crackle is a phenomenon caused by the different expansion coefficients of the body and glaze when they are heated. This physical phenomenon is manifested on the glaze of Ru kiln, presenting a trace similar to the marks left by a crab walking on mud and sand. During the Song Dynasty, some people called it chicken claw pattern, as recorded in the Southern Song Dynasty's "Ba Bao Zong Zhen Ji Volume Nine: Blue Ware" which says, "Ru kiln soil is moist, similar to Goryeo utensils, with chicken claw patterns as genuine, and those without patterns are even better, this item comes from the north. New kiln, built by the repair department. Since then, counterfeiters are all made by Longquan." Later, people might have felt that the description of "chicken claw" was not elegant, so the term crab claw pattern spread more widely. Ming Dynasty Cao Zhao, who wrote China's earliest work on cultural relic appraisal, said, "Ru kiln utensils, from the north. Those fired during the Song Dynasty, light blue, with crab claw patterns are genuine, those without patterns are even better..."

Song Dynasty Ru kiln crab claw pattern coiled dragon washbasin (collected by the National Palace Museum, Taipei)

 

In addition, fly wing crackle is a unique phenomenon of Tang Sancai, with discontinuous, irregular glass fine cracks between the large crack patterns. Sesame seed pattern crackle has a form similar to the glaze crackle of Tang Sancai, with Tang Changsha kiln products being the most representative. Its fine pattern is like sesame seeds, but because it is attached to the porcelainized body, the adhesion is relatively strong, and there is no lifting of the glaze layer. Cow hair pattern crackle is intermittent, sparsely and orderly on the utensil, similar to cow hair, and like the faint glow in the sky. There is also eel blood pattern crackle, named for its red-brown color, similar to eel blood.

 

"Yin Liu Zhai Shuo Ci" says, "A small utensil with large patterns, a large utensil with small patterns, both are valuable. In recent times, people do not value the crackle of Ge kiln, but the crackle of Lang kiln. Generally, the porcelain with crackle is considered valuable if there is no trace when touched." This has also been the standard for appreciating crackle since ancient times. At the same time, crackle is one of the porcelain firing processes, and the distinction between old and new crackle can sometimes be a key point in identifying the authenticity of ancient porcelain. For example, a large part of the crackle porcelain that has been used for many years will have the original sharp edges of the openings become dull and round, and the organic matter deposited at the openings for many years will be decomposed by microorganisms to produce pigments. These pigments spread through the aging glaze to the glaze, causing the bubbles around the openings to turn various colors; while the edges of the openings of the modern imitation crackle porcelain are sharp, and there is no dulling phenomenon, and the color deposited at the mouth is single, generally made old with acidic substances after cracking.