| Brown and black glaze were colored glazes of high temperature with iron as the primary coloring agent.The brown glaze is brown,persimmon red,or sesame brown,while some appear purple-gold.There are weak optical anisotropic and very small speckles under observation from different angles.The black glaze was bright black lacquer,reddish black,purplish black,bluish black,or stripes on black.The crystal morphology of iron oxide in browns and black glazed glazes has been deeply studied in recent years.It is colorful and extremely precious.The existence of rare ε-Fe2O3 crystals has been found,which has become the research hotspot of ancient ceramics and modern materials scholars.However,the root cause of the strange light and color in brown glaze surface,the role and occurrence form of iron elements in the glaze,and the growth mechanism are still unclear,which restricts the deep understanding of its color formation mechanism and cultural relic value,as well as the in-depth analysis and inheritance of its scientific and technological connotations.This study uses the multi-scale analysis method,taking the paste glaze and black glaze porcelain of Yaozhou kilns in the Song Dynasty as the primary research object and the similar glaze of other kilns mouths as the contrast to carry out the detailed analysis of the composition,micro-nano structure and melting out a crystal of different kinds of glaze,to obtain the information of the valence,crystal type,morphology,distribution and other information of iron element,and to explore the color mechanism and high gloss mechanism of the paste glaze strange color and the formation of black glaze brown spots.The main progress and achievements of the research work are as follows:(1)Representative types of brown and black wares from the Yaozhou kilns(Song Dynasty)were selected to study the morphology,structure and distribution of the precipitated crystals in the glazes.The results show that the rare metastable ε-Fe2O3 phase,already observed in typical brown glaze,was also detected in other types of brown glaze decorations,such as light brown stripes on a darker background or irregular brown spots on a black background.Cross-section analyses also showed a three well-separated layered crystalline distribution in the glazes of lighter brown colors.Raman analyses revealed that the crystals contained in third layer are of varying nature:ε-Fe2O3 in the brown glaze and magnetite in the light brown stripes.The peak shifts and line broadenings observed in the Raman spectra of ε-Fe2O3 crystals were also investigated.They are the result of Al,Ti and Mg substitutions,which were identified using TEM-EDS.Such ionic substitutions would stabilize the metastable ε-Fe2O3 crystals by increasing the cationic disorder.In addition,Mg,Ca and Fe rich spinel crystals were observed for the first time in a black glaze from the Yaozhou kilns.Cordierite crystals were observed for the first time in the brown glaze from Yaozhou kilns.The results show that the existence of impurityε-Fe2O3 crystal phases such as zircon and hematite and the cation substitution in the ε-Fe2O3 crystal promote the formation,growth and stable existence of crystal phases.(2)The crystal morphology,structure,and light reflectivity of brown glazes from Yaozhou kilns in the Song Dynasty were systematically studied.The absorption spectrum analysis shows that the absorption band of the glaze is brownish red at 575～580 nm at different angles.Alkali etching was implemented to remove the glassy matrix surrounding these crystals to further investigate their microstructure.SEM observations emphasize that they are composed of well-arranged parallel branches separated by about 230 nm,each of which consisting of offset stacks of many small flake-shaped crystals,with the offset ranging from 100 to 110 nm.The results indicate that the reddish-brown color of the crystal and glaze is directly related.The special structure of this crystal can be seen as a diffraction grating,which interacts with incident light to cause local rainbow effect due to light interference.The relatively small size of the sheet like structure relative to the random orientation of the glaze results in the overall high gloss characteristics of the glaze.(3)The glaze is made from natural mineral raw materials and fired in a reductive atmosphere by archeological experiment verification and triangle batching methods.The paste glaze samples were obtained by sampling at different firing stages.The chemical composition,phase composition,thermal history analysis,micro-morphology and phase composition of the glaze were analyzed and studied.It is found that the main crystals on the surface of the glaze are arranged neatly with submicron iron-rich cells,and the main crystalline phases are iron spinel,magnetite,a small amount of quartz,cordierite and various titanium oxides during the heating process.The crystal morphology formed on the glaze surface at the initial cooling stage can be divided into sub-micron cells,irregular crystal epitaxy dendritic crystals and well-developed dendritic crystals with two-dimensional structures.The main crystal phases areε-Fe2O3 and α-Fe2O3.After furnace cooling,large amounts of dendritic ε-Fe2O3 crystals and small amounts of α-Fe2O3 crystals are distributed on the glaze surface.STEM-EDS analysis showed that the crystal growth of ε-Fe2O3 consists of substituting Al,Ti and Mg.The analysis results indicate that,ε-Fe2O3 crystals are formed during the cooling stage,and a set of equations are derived to simulate the growth process of the ε-Fe2O3 crystals in the glaze.(4)The chemical composition of glaze porcelain and the morphology,type,structure and distribution of crystals in brown glaze of Dangyangyu kilns,Lushanduandian kilns,Jingxing kilns,Yingou Site(Song Dynasty)and Hunyuan kilns(Jin and Yuan Dynasty)were studied.The results show that large amounts of crystals precipitated on both the surface of the purple-gold glaze and the brown glaze.The difference is that the Fe-rich crystals on the black glaze are mainly triangular dendritic crystals,dendritic or flake α-Fe2O3 and a small amount of ε-Fe2O3 dendritic crystals rich in magnesium and iron.Alkali etch-SEM analysis shows that the crystal structure of the dendritic magnesia ferrite spinel is composed of secondary branches arranged at 250-300 nm apart,and its branches are composed of flake crystals arranged at 80-90 nm offset.The results show that the crystal on the glaze surface is brownish red,and the crystal branching structure and incident light produce structural color,which synergistic produce purple-gold effect.Secondly,because the crystal structure is aligned to form specular reflection,the chemical coloring intensity of the crystal is reduced,resulting in silvery glossy spots on the glaze.By comparing the distribution,size and morphology of ε-Fe2O3 crystals on the glaze surface of different kilns,it is found that the glaze color tends to be red when the crystal size increases.The poorer the crystallinity of the crystal and the more types of unmelted crystals on the surface,the glossiness of the glaze decreases. |
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