Microscopy and microanalysis of crystalline glazes

Abstract

SummaryCrystalline glazes on ceramic plates produced commercially in the U.K. and on ceramic pots produced commercially in Taiwan and Spain have been examined by X-ray diffraction, conventional and polarized light microscopy, and scanning electron microscopy in order to identify the crystalline phases present in the glazes and to ascertain through X-ray microanalysis the partitioning behaviour of the transition metal ions used to colour the glazes and the crystals within them. In each case examined, the macroscopic two-dimensional spherulites within the glazes clearly seen by the naked eye were found to consist of large numbers of radially orientated acicular crystals each 5 µm or less in width embedded within the silica-rich glaze. Energy dispersive X-ray microanalysis and X-ray diffraction of these crystals identified these crystals as willemite, α-Zn 2 SiO 4 . The strong [001] texture of these crystals within the glaze evident from the X-ray diffraction patterns was consistent with polarized light microscopy observations of the willemite crystals. In addition to willemite, small iron-doped gahnite (ZnAl 2 O 4 ) crystals were found in a honey-coloured crystalline glaze and acicular rutile (TiO 2 ) crystals were found in the Portmeirion Pottery plates examined. Transition metal ions with a preference for tetrahedral coordination were observed to substitute for Zn 2+ ions in willemite and to partition preferentially to the willemite crystals, whereas ions preferring octahedral coordination preferred to remain in the glaze.

The thermal and microstructural behavior of a R2O–RO–(ZnO)–Al2O3–(TiO2)–SiO2 based macro-crystalline raw glaze system

“…Zinc containing glazes consist of alkaline (R 2 O), rare earth alkaline oxides (RO), SiO 2 , and Al 2 O 3 in their chemical composition. Willemite (zinc silicate) is the main crystalline phase of these glazes [10][11][12][13][14][15][16][17]. The presence of TiO 2 has a favorable effect, and might act as a nucleating agent or an opacifier due to its amount in the glaze batch.…”

KRİSTAL SIR UYGULAMALARINDA ZnO’İN KRİSTAL SIR GELİŞİMİNE ETKİSİ

“…Böylece küresel bölgelerdeki villemit kristallerinin büyüme hızı belirlenebilir [3]. Literatüre göre, sanatsal anlamda üretilen çinko içerikli kristal sırların özellikle yavaş pişirilmesi ve özel bir pişirim rejimine tabi tutulması gerekmektedir [1].…”

“…Literatürde, makroskobik morfolojide tercih edilen küresel şekilli villemit kristallerinin oluşumu sınırlı difüzyon kümelenmesi şeklinde açıklanmaktadır. Fiber şekilli villemit kristallerinin meydana gelmesi ise trigonal yapılarının ve böylesi kristallerin küçük açılı dallanma yapabilmesinin bir sonucudur [3]. Sır içerisindeki elemetlerin camsı faz ve kristal olarak ayrışması yayınımla gerçekleşir.…”

“…Bu doğrultuda artistik bir çeşidi olan kristal sırlar pişirim sürecinde sır içinde dağılan mikro ve makro kristaller sayesinde sır tabakasında şeffaftan opağa, parlaktan satene pek çok farklı estetik etkinin eldesine yol açarak seramik ürüne görsel anlamda önemli bir katma değer sağlayabilecek alternatif sır çeşidi şeklinde değerlendirilebilirler. Ancak, literatürde bu tarz sır formülasyonları sınırlı sayıdadır [2][3][4][5][6][7]. Son yıllarda kristal sırlarla ilgili çeşitli çalışmalar yapılsa da [4][5][8][9][10][11][12][13] böylesi artistik sırların endüstriyel uygulanmasına ve pişirim rejimlerine uyumuna yönelik araştırmalara da literatürde rastlanılmamıştır.…”

The Effect of ZnO Addition on Microstructure, Phase and Color Developments of Copper Reduction Glaze

“…However, the introduction of Zn to GCZ14 and GCZ20 samples leads to formation of lath-like particles with morphologies almost similar to Willemite. EDS analysis of these particles reveals the formation of Willemite (Figures 2e and 2g, in points 2 and 3, respectively), while other observations indicate that, the same oriented Willemite is formed within the glaze matrix [22]. However, the authors have addressed the formation of acicular morphologies in the matrix and this change in the crystal morphology can be inferred to the uncontrolled reduction process.…”