Available online at www.sciencedirect.com Journal of the European Ceramic Society 31 (2011) 1371–1376 Fluxing action of illite and microcline in a triaxial porcelain body Darunee Wattanasiriwech ∗ , Suthee Wattanasiriwech School of Science, Mah Fah Luang University, Chiang Rai 57100, Thailand Received 10 September 2010; received in revised form 10 January 2011; accepted 26 January 2011 Available online 17 February 2011 Abstract Comparative studies were made on two types of triaxial porcelain bodies; one used conventional potash feldspar (ST0) and the other used illite (IT0) as the fluxing agents. Scanning electron micrographs showed that densification and mullite formation in the IT0 body had already occurred at 1000 ◦ C suggesting that there was enough liquid to assist densification and mullite crystal growth. In the ST0 body, mullite crystals started to emerge around 1100 ◦ C, while potash feldspar completely melted around 1200 ◦ C. Full vitrification of the ST0 body required the firing temperature of 1300 ◦ C, which was around 50 ◦ C higher than the IT0 body. The IT0 body showed superior strength to the ST0 body at all studied firing temperature ranges and thus was possibly due to (i) a better densification, (ii) a higher amount of interlocking mullite crystals and (iii) the smaller amounts of residual quartz. © 2011 Elsevier Ltd. All rights reserved. Keywords: Triaxial porcelain; Illite; Microcline; Mullite; Vitrification 1. Introduction The rise in energy demand has accelerated firing technology of ceramic products so that a fast firing scheme is often used. One way forward to achieve such goal, new body recipes have been proposed. Conventional triaxial porcelain body mix contains three main components; clay quartz and feldspar. Vitrification of the body normally starts only when feldspar has melted. It was reported that feldspar melted around 1140–1150 ◦ C with- out changing its shape until 1200 ◦ C due to its high viscosity. 1,2 In order to achieve complete densification through the viscous flow process of the feldspar, the common firing temperature of the body mix is usually ≥ 1200 ◦ C. During firing, phase transformations and chemical reactions between compositions were observed and reported to affect properties of the final products. 3,4 The final microstructure of the body mainly contains different forms of mullite crystals and undissolved quartz grains dispersed in the glassy matrix. It has been reported that the presence of these crystalline phases were significant to the mechanical behavior of the products. 5 Phase transformation of undissolved quartz grains upon cool- ∗ Corresponding author. Tel.: +66 53 916263; fax: +66 53 916776. E-mail address: darunee@mfu.ac.th (D. Wattanasiriwech). ing cycle could generate local stresses and microcracks which diminished the flexural strength. In contrast, the presence of mullites, especially the long interlocking-needles was shown to enhance the mechanical properties. 5,7 Dana and Das reported that primary mullite occurred by decomposition of kaolinite crystals, while secondary mullite needles occurred from the reac- tion between feldspar relict and clay mineral relict. 4 Iqbal and Lee, on the other hand, suggested that primary mullite possi- bly acted as a seed for the nucleation of secondary mullite. 3 These mullites were distinguished by different morphologies and compositions. 6 Small needle-like structure and interlock- ing of mullite crystals were proposed to be the key factor for achieving high strength porcelains. 7 Clarification on the for- mation of mullites will lead to a better body formulation and firing scheme, improved microstructure and hence mechanical properties of ceramic products. Our previous study on illitic ball clay using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) showed that vitrification of this clay started around 900 ◦ C and was complete at 1200 ◦ C. 8 Illite is a clay-sized, non-expanding, dioctahedral, aluminous potassium mica-like mineral. 2 Recently illite was defined illite as a “K-deficient mica with an approxi- mate formula of K 0.88 Al 2 (Si 3.2 Al 0.88 O 10 )OH 2 . 9 The existence of liquid phases due to the dissociation of illite crystals in the matrix was thought to enhance the reaction kinetics and densifi- 0955-2219/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2011.01.025