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Sintered ceramics, represented by ceramic tiles or clinker bricks, are widely used in the building materials industry due to their technological properties and visual qualities. They are characterized by a low level of water absorption and high levels of frost resistance, mechanical strength, and color stability. 1 The values of the mentioned characteristics are largely dependent on the firing temperature, that is, the degree of sintering of the ceramic body. Earlier studies 1 show that the water absorption values can range from 8.5% at a firing temperature of 1150℃ to even 0.1% at a firing temperature of 1230℃, depending on the raw material mixture used.Methods for coloring ceramic masses are widely known and cause no technological problems. However, the issue of color changing of ceramic materials has not been sufficiently explored to date. Color and coloring process of sintered ceramic products are very important issues, mainly due to the aesthetics of the product, but also other important properties, for example, mechanical properties or abrasion. There are some well-known techniques, that are used for coloring ceramic materials, for example, glazes or mass coloring. Mass coloring is about the direct introduction of the dye to the ceramic mass. Unglazed tiles (mass colored) have a very important advantage over glazed tiles-excellent mechanical properties and low abrasion. This characteristic makes mass colored products very desirable for use in public buildings, sports facilities, industrial halls, etc. 2 The aim of this paper was to examine the effects of the addition of different grain sizes of dolomite, as colorant to cream-firing clays on the technological and color properties of sintered ceramics.The impact of calcium-magnesium carbonates on the technological properties of ceramic materials, consisted of cream-firing clays is known. Iglesias, Garcia-Romero Acosta 3 proved that the amount and the granulation of dolomite has a significant impact on the control of the clay mixture plasticity. The high value of plasticity indexes for the test mixtures (32.60% and 47.53% depending on the amount of the raw material containing dolomite on its composition) was a consequence of the presence of micrometer-sized calcium-magnesium carbonate particles, which were contained in the clay

This study is focused on the behavior of the cream-firing clays from Opoczno region (Poland). The ceramic masses on which tests were carried out consisted of cream-firingBorkowice clay and dolomite in two different grain sizes as an additive that changes the color of ceramic materials. Test samples were prepared by plastic method and fired at range of 1100–1240 °C. Phase composition of theinvestigated materials was characterized by XRD method with quantitive analysis of the amorphous phase determined by the Retvield technique.Color properties of the surface of the obtained ceramic materials were determined in CIE-Lab color space. The phase composition of the obtained ceramics depends on the firing temperature. The color of the surface of the ceramic materials also depends on the firing temperature. There was a tendency to decrease the brightness, decrease the blue shade, and increase the yellow shade of the surface of materials with increasing the temperature. The conducted tests allowed to conclude that the color of ceramic materials depends on their phase composition. The most important role in the formation of color correspond to the amorphous phase, formed during the process. The lower content of the amorphous phase in the material allows to obtain brighter products with a lower proportion of yellow, and therefore closer to white. Moreover, following tests were carried out: total water absorption, total open porosity, linear shrinkage, and flexural strength. With increasing the temperature, total water absorption and total open porosity decrease, and total linear shrinkage increases due to the progressive sintering process. Flexural strength increases with the increase of the firing temperature for materials consisting of Borkowice clay. The addition of dolomite introduced new pores into the material, which resulted in an increase in flexural strength at lower firing temperatures and a decrease in flexural strength at higher firing temperatures.

The study focused on determining color changes in materials made of cream-firing clays from the Opoczno region (Poland) due to the addition of calcium carbonate in the form of limestone. Moreover, the influence of the grain size distribution of this additive on the color properties of the materials and their phase composition was determined. Test samples were prepared using theplastic method and fired at four different temperatures: 1120, 1140, 1160 and 1180 °C. The color properties of the surface of ceramic materials were determined in CIE-Lab color space using a colorimeter. Quantitative phase analysis was performed using the Rietveld method. The research showed that the addition of calcium carbonate causes an increase in the yellow color factor and a decrease in the red color factor and the brightness of the material. Moreover, it was proven that the grain size distribution of the additive used significantly influences the phase composition of the materials, thus determining the values of physical properties and the color of the materials.

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