Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Review article Study of borosilicate glaze opacification by phosphates using Kubelka-Munk model Y. Abouliatim a, ⁎ , Y. El Hafiane b , A. Smith b, ⁎ , M. Mesnaoui c , T. Chartier b , A. Benhammou a , A. Abourriche a , L. Nibou a a Laboratoire Matériaux Procédés Environnement Qualité, LMPEQ, Université Cadi Ayyad - ENSA de Safi, Route Sidi Bouzid, BP 63 - 46000 Safi, Morocco b Science des Procédés Céramiques et de Traitements de Surface, SPCTS, UMR CNRS 7315, Centre Européen de la Céramique – ENSCI, 12 rue Atlantis, 87068 Limoges Cedex, France c Laboratoire de la Matière Condensée et de l’Environnement, LMACE, Université Cadi Ayyad - FSSM, Avenue My Abdellah, BP 2390 – 40000 Marrakech, Morocco ARTICLE INFO Keywords: Borosilicate glaze Devitrification Kubelka-Munk Opacification Phosphates ABSTRACT The aim of this work was to study the effects of additions of sodium metaphospahte (1–50 Wt. %) on optical properties of a conventional borosilicate glaze. The optical properties were evaluated by diffuse reflectance measurements. The absorption and the scattering properties of the obtained glazes, were explained through the Kubelka-Munk model. The structural and microstructural properties were determined by X-ray diffraction and Scanning Electron Microscopy. It was shown that white opaque borosilicate glazes can be obtained for an addition of 5 Wt% to 50 Wt% of sodium metaphosphate and 20 Wt% seemed the optimum amount to obtain a perfect white opacity. The opacification process is based on the light scattering phenomenon wherein the nature of the scattering centers vary depending on the amount of sodium metaphosphate addition. In the case of 20 Wt% scattering centers have been identified as Ca 10 (PO 4 ) 6 O and NaCa(PO 4 ) crystals formed by a devitrification process. 1. Introduction The white opaque glaze is widely used in the ceramic industry (wall tiles, sanitary, etc.) [1–5]. Opacity results from the difference between the refractive index of an opacifier and a glassy matrix. Thus, the greater the difference between the refractive indices is, the higher the light scattering occurs and therefore, the better the opacity becomes. In the case of ceramic glazes, opacity is obtained in two completely different ways. The first one is devitrification: it happens when opacifiers dissolved in a hot glassy phase recrystallize homogeneously in the glassy matrix during cooling [2,6,7]. The second way consists in introducing into the glaze formulation compounds which do not dissolve in the hot glass and remain in the glassy matrix, thus forming an opaque veil on the glaze surface after cooling [7]. This corresponds to the case of white opacifiers such as titanium oxide (TiO 2 ), zircon (ZrSiO 4 ) or zirconia (ZrO 2 ) [1–10]. TiO 2 is somehow limitated in terms of whitening effect because a yellowish appearance is frequently found [10]. Concerning the use of ZrSiO 4 or ZrO 2 , a major limitation is the high price [3]. A few studies present the use of the phosphates compounds as opacifers [2,11,12]. For example Bou et al. [2] showed that the addition of small amounts of P 2 O 5 (about 1 wt%) in the SiO 2 -Al 2 O 3 -B 2 O 3 -CaO- K 2 O-TiO 2 systems converted a matt glaze into a glossy glaze. This was attributed to the presence of P 2 O 5 which inhibits devitrification of the wollastonite in the glaze melt and gives rise to changes in the crystal morphology titanite. The aim of this work is to study the opacification effect of sodium phosphate [13] on a borosilicate glaze in order to develop a new white opaque glaze. The opacification effect is examined by UV diffuse reflectance measurements of glaze formulations as characterized by the scattering (S) and absorption (K) coefficients which were obtained by the Kubelka-Munk analysis. 2. The Kubelka-Munk model The Kubelka-Munk (K-M) model is a phenomenological approach witch proved its ability to interpret measurements in different applica- tions (paint, pigment, coating, composite, etc.) [14–17]. The propaga- tion of light through a thin section of thickness dz at a distance z from the illuminated surface of the layer D is approximated by two fluxes: http://dx.doi.org/10.1016/j.ceramint.2017.02.046 Received 21 October 2016; Received in revised form 27 January 2017; Accepted 11 February 2017 ⁎ Corresponding authors. E-mail addresses: abouliatim.younes@gmail.com (Y. Abouliatim), agnes.smith@unilim.fr (A. Smith). Ceramics International 43 (2017) 5862–5869 Available online 14 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved. MARK