Bending strength of porcelains O. Isik Ece*, Zenbe-e Nakagawa Research Institute of Materials and Resources, Faculty of Engineering and Resources Science, Akita University, 1-1 Tegatagakuen-Cho, Akita 010-8502, Japan Received 21 May 2001; received in revised form 6 June 2001; accepted 6 July 2001 Abstract Porcelains in the quartz–metahalloysite+kaolin–K-feldspar system with different quartz grain sizes were investigated to study the effect of firing temperature on the bending strength. The degree of vitrification of the fired bodies was determined measuring the bulk density, relative percentages of constituent minerals by XRD and their microstructures using SEM techniques. The maximum bending strength was obtained in the 10–20 mm particle size quartz containing porcelains at 1300 and 1350  C interval. The distribution of closed pores, their geometric shapes and possible link with each other control the bending strength of the porcelain body. In addition, the presence of unmelted fine quartz grains in the porcelain body also increases the bending strength. It was found that the bending strength increases with both increasing bulk density of the porcelain bodies and the firing temperature, but upon further heating, after reaching the maximum level, the bending strength decreases due to bloating of isolated pores and the disappearance of quartz, which are also associated with a decrease in bulk density. # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: Bending strength; Porcelain body; Particle size; Quartz content; Compression-prestress; Bloating 1. Introduction The main phase of the porcelain body is made up from heterogeneous glassy matrix containing closed irregular shaped porosities due to gas bubbles and the melted resi- dues of quartz grains, and numerous needle shaped mul- lite crystals < 3 mm length. The bending strength of porcelain bodies have been experimentally studied by many researchers [1–6], because of its economic impor- tance in ceramic industry. It is known fact that quartz grains in different sizes have significant effects on mechanical strength of porcelain bodies [1]. Especially, it is proposed that bending strength of the porcelains increases with an increase in inter planar spacing of quartz crystals, so the quartz is under the tensile stress, and con- sequently, the glassy matrix surrounding the quartz grains is a compressive stress which acts as pre-stress, improving mechanical strengthening [1]. The pre-stressing effect due to the residual compressive stress at the glassy phase around the grain is large [1,2], which is related to the quartz grain size and the firing temperature. A similar conclusion had been also reached that the mechanical strength of porcelain is influenced mainly by stresses set up in the glassy matrix rather than by the amount or size of mullite crystals formed [3]. In addition, it is also reported that with respect to the amount of quartz content of bodies, the pre-stress effect increases with an increase in the amount of residual quartz in bodies [3]. There are some other scientific arguments about the effects of quartz grains on mechanical strength of porcelain. It was experimentally documented that the presence of a higher amount of fine-grained quartz con- tent increases to a higher strength [4]. In contrast, other studies claimed that the low quartz content provides high strength [5,6]. The purpose of this study is to investigate the differential changes of bending strength- ening of porcelain bodies using various quartz grain sizes at different firing temperatures. 2. Experimental procedure Size separation of quartz and feldspar grains were done according to Stoke’s law after pulverizing the samples. Quartz and feldspar grains were dried in a 0272-8842/02/$22.00 # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(01)00068-2 Ceramics International 28 (2002) 131–140 www.elsevier.com/locate/ceramint * Corresponding author at present address: Istanbul Technical University, Faculty of Mines, Mineralogy-Petrography Division, Maslak 80626, Istanbul, Turkey. Fax: +90-212-285-62-79. E-mail address: ece@itu.edu.tr (O.I. Ece).