In this study, the characterization of artistic ceramic clay that contains grog (chamotte), which is used in high-temperature firing, along with their shaping and firing behaviors that are important for their usage, were examined in detail. The plasticity limits, which are important for a ceramicist regarding the shaping of grogged clay, was specified by Pfefferkorn plasticity tester, giving exact plasticity limits. The products were fired at 1100-1200°, which stands for the firing range in order to avoid deformation, and the fired specimens were further examined for their shrinkage, strength, porosity and microstructural analysis levels. X-ray diffraction analyzes were implemented to raw and sintered bodies and their phase development were further analyzed based on temperature. In conclusion, the plasticity levels of grogged clays are applicable for hand molding. Concerning high-strength and non-deformed firing, a temperature range of 1150°C and 1200°C was determined. The implementation showed that the clay has a suitable firing tempereature and for firing non-deformed products that are resistant to higher temperatures, the temperature increase needs to be specified with the addition of various grog rates. However, it is detected that the open pores need to be reduced for products that are exhibited outdoors in order for them to resist atmospheric conditions and in order to develop acicular mullite crystals, the firing duration and vitreous phase ratio need to be improved suitably.
Keywords: Grog (Chamotte), Plasticity, Sintering, Artistic Ceramics, Characterization, Art Work.
Shining out as one of the most powerful factors in forming design, “material” concept moved forward in concurrence with the historical background of people in the frame of design culture and history, and people’s knowledge and dreams have increased the capacity of the material. As a material that inspires people towards forms, clay served as a significant test ground along with the use of building techniques and contingent structures, because they improved the constructiveness of people . In the art of ceramics, clay stands for a basic material in terms of bringing different properties to the end product, paving the way for artists to express themselves. Concepts such as preparing the clay, its formability properties [2-3], firing color  and firing temperature  are of capital importance.
Grogged clay is preffered in productions related to indoor and outdoor organizations, for its ease of formability in detailed artistic works. It has a limited amount of deformation after the firing process and owing to this material, three-dimensional forms, panels, urban furnitures or products known as garden tiles can be manufactured without any problems [6-7]. Deformation after firing is a serious problem that ceramicists don’t want to encounter with. Therefore the shaping and firing properties of the clay needs to be well-defined. While clay’s softness, plasticity and range of water use matter in shaping process, it is important for the artist to know at which temperature and how long one should fire the material to obtain desired properties [10,11-12]. In this process, collaborative works between artists and engineering sciences stand for a pivotal point. Thanks to the experience of both sides, there are long lasting and high quality products that have high artistic values, as a result of interdisciplinary studies. The grogged clay is prepared by crushing kaolins sintered as grogs, fired bricks and fire clays and mixing them with clay in certain sizes . The use of sintered material prevents deformation of clay in the course of the firing stage, creates large-scale products and products that are resistant to thermal shocks and atmospheric effects [14-15]. Therefore, preventing the problems that may arise from clay, ensures works to be preserved with their artistic values and exhibited visually for many years. In order to ensure that the artist is able to work with large sizes, it is required to adjust the plasticity levels of clay, ensure the workability in wide ranges of water content, and provide sufficient intervals between sintering temperatures.
Ceramics having low strength and high water absorption rates, which are used outdoors, start to become deformed over a specific period of time due to atmospheric effects [10-16]. The freeze-thaw phenomenon can cause ceramic forms or products to decompose and event fragmentize under these conditions. This is an undesired situation for a work that has been created through a labor-intensive, exhausting process, following the design and manufacturing stages. For this purpose, the grogged clay used in artistic ceramic works was characterized and its formability and firing behaviors were further specified, followed by researching its non-deformed manufacturing criteria.
2. Material and Method
The grogged clay that was used in studies, was procured from Anadolu University, Faculty of Fine Arts, Department of Ceramic and Glass. The particle size analysis of clay was performed by Malvern laser particle size analyzer. For the particle size analysis of ceramic clay, the clay was dissolved in deionized water and the measurements were performed between 0,020-2000μm. A total of five specimens were used in experimental studies and obtained values were calculated by statistical methods. In order to measure plasticity limits, Pfefferkorn plasticity test was applied [17-18]. Contingent on water content, a crushing level graph was drawn to determine plasticity values according to different crushing levels. Rigaku ZSX Primus XRF device was used for the chemical analysis of the used formula. The ceramic clay was directly reshaped as 150x10x10 mm in plaster molds. These reshaped specimens were dried in 110°C drying-oven until they reached to a constant weight. In order to detect their sintering behaviors, they have been fired in an electrical shuttle kiln having 1100-1150-1200°C temperatures, with each one of them was subjected to 30 minutes of maturing, and natural cooldown in the air atmosphere.
The mineralogical analyzes of raw and fired specimens were carried out through powder method. These specimens were grounded under 63μm with an agate mortar. The levels of 30kV and 15 mA (Cu-Kα, 2θ 5-70º, 2º/min, 0,02 parameters) were measured using X-Ray Diffractometer (XRD, Rikagu, Rint 2000 with monochromatic filter,Japan). Archimedes’ method was used for measuring bulk density and porosity values of the fired products. The strength values were measured by Gabrielli S.R.L (Italy) three-point flexural device (TS EN ISO 10545-4) , and the L*a*b* coordinates of color were measured by Minolta 3600d (Japan) colorimeter device. In the frame of micro structure analysis, the fractured surfaces of samples were examined. In fractured surface studies, samples were cauterized 30 seconds in 10% HF solution at ambient temperature, and filmed with gold-palladium mixture. Their surface images were later analyzed by Zeiss Supra 50VP Scanning Electron Microscope (SEM).
3. Results and Discussion
3.1. Chemical and mineralogical properties
Table 1 exhibits the chemical analysis results that were performed by XRF in order to identify oxides and minerals in grogged clay, and Figure 1 shows the X-ray patterns. Upon the chemical and mineralogical content of clay, the containing grog addition designates sintered kaolin. Through sintering kaolin at 1200 °C or above, mullite, cristobalite and vitreous phases come into existence . High kaolin content in clay, increases Al2O3 levels and prevents the deformation of clay by increasing sintering temperature . When aimed at firing non-deformed products and light-colored bodies, raw materials and their proportions affect quality. The fact that fire shrinkage is low even though the Al2O3 level is high, is because the sintered kaolin and Al2O3 act as corundum. Cristobalite and mullite seen in grogged clay’s mineralogical analysis, are formed through sintering kaolinitic clays . Quartz, kaolin, cristobalite and mullite are detected in the mineralogical examinations of clay.
3.2. Particle size distribution and plasticity properties
Particle size distribution in clays; are among the main factors that affect packing density and sintering in formability. The fine and coarse grain distribution in certain proportions, affects clay’s formability and dry and fired strength properties. The grain size distribution of the formula was detected as D (0,90): 80,6μm, D (0,50): 7,2μm and D (0,10): 1,5μm. Figure 2 gives the size distribution graph, along with cumulative undersize graph.
Figure 2. Size distribution graph of the ceramic clay
The fact that the clay is whether hard or soft during usage, is significant for shaping. If the clay is hard, the lather adds water in the first stage and adjusts consistency, in order to give softness to the clay. The vacuumed clays are mostly preferred in products that are pressed with machines or shaped with automatic lathes such as pots and crocks. In order to hand mold the hard clay in lathe, additional quencing needs to be implemented along with applying much more force to soften the clay. This situation causes the laborer to get exhausted quickly and reduces the production efficiency. Therefore preparing the clay in suitable softness or hardness, has an important role for formability.
In the first stage of detecting the plasticity property, determining clay’s crushing level during its direct use without water injection, gives information about the softness of clay. In this regard, the primary crushing level of clay is important for revealing whether it is eligible for use. In the Pfefferkorn device, clay’s crushing strength was measured as 24 mm and its water content was detected to be 22,7%. The plasticity limits of clay was given with water contents that correspond to 30-24-16 mm crushing strength, depending on the crushing strength-water content curve, carried out by Pfefferkorn tester. Using excessive water during shaping, may cause the product to shrink heavily and create deformation. Such products are dried either in humid environments or environments where no air conditioning was present for a really long period of time. When large-sized products are shaped, adding excessive amounts of water softens the clay and create deformation by making it difficult for the product to support itself. Therefore, the clay to be used in such productions, needs to have a wide range of water intake levels and plasticity limit. As water is being added to clay, which has a wide range of water intake levels, the continuation of clay’s strength towards the load, provides convenience for the artist. Figure 3 exhibits the plasticity curve that shows the water content according to the specified crushing strength, along with regression equation and correlation coefficient value.
Figure 3. Pfefferkorn plasticity curve of the ceramic clay
The scientific studies show that the plasticity limit of hand molded clays, used for artistic works in Turkey, is usually 29-33 in vacuumed clays and 22-25 in non-vacuumed clays, for a 16mm crushing strength level . Accordingly, the plasticity limit of grogged clay is in the range of suitable limits for hand molding. Table 2 shows the first crushing strength and plasticity limits depending on the crushing level.While creating large-scale products, the artist reduces the risk of crack formation by adjusting the wall thickness of the product (owing to experience). However, these thickness values vary for each person who performs shaping, according to their experiences. Those who have dedicated their years to shaping clay on lathe, show their dexterity by using their practicality and creating thinner walled products, grounding on shaping the product with lesser amounts of water. As for those who have just started to work with lathe, create thicker walled products by shaping with more water. It is detected that grogged clay is medium-hard. In the scope of hand molding, 16 mm is selected as the baseline for crushing strength. Accordingly, it is determined that the artist can maintain the shape of the product without any deformation by using 5% more water for shaping, and that the product is able to stand still without deformation, when the softness levels reach 16 mm. This situation shows that the water to be used by the artist for shaping large-scale products, has a clay property that gives them the opportunity to create their products without any size limitation and problem.
3.3 Physico-mechanical properties
Table 3 shows the physico-mechanical properties of specimens fired at different temperaturs. With the rising temperature, water absorption and porosity values decrease. However, the fact that water absorption level is 9,67% in specimens that were fired at 1200°C, means that the cavities inside the body were not sufficiently filled up vitreous phase, in other words, there is a raw material that create insufficient amounts of amorphous phase. The water absorption values comply with TS EN 14411 standards for AIIb-2 ceramic tile groups . The water absorption range for this standard, needs to be between 6% and 10%. The water absorption values of specimens fired at 1200°C in this study,ensure TS EN 14411 standard with a value of 9,67%. Additionally, the water absorption values of the specimens fired at 1100 and 1150°C, comply with the >%10 water absorption levels of Group AIII, with the order of 13,68% and 11,86% . In conjunction with the rising temperature, the bending resistance of specimens tends to increase. The fact that resistance values increase by nearly 15% at 1150°C after 1100°C, 8% at 1200°C and when the temperature rises from 1100°C to 1150°C, the water absorption values show a decrease by 13%, along with 8% at 1200°C; shows that the clay is best suited to be fired between 1150-1200°C. According to TS EN 10545-4 norms; the fired products’ values of 19.40 N/mm2 for 1100°C, 22.21 N/mm2 for 1150°C and 23.96 N/mm2 for 1200°C, comply with the required standards. According to TS EN 14411, in the frame of outdoor art works, the water absorption values of fired specimens are high and the density of amorphous phase is low . These informations show that in order to reduce water absorption, increase strength and the proportion of mullite crystals; it is required to either enhance content that will ensure fusion or extend the duration of sintering during the firing process. Therefore, this will be important for ensuring the production and permanence of art works, which can withstand in atmospheric conditions for a long period of time owing to the increasing strength of the fired body.
3.4. Phase Development
Phases that are formed after the firing process have a huge role in the physical properties of the body. Vitreous phase that is less than the required amount, causes mineral grains to have less interaction and bonding, the lack of strength in the body and a negative effect on other physical properties such as water absorption. The excessive formation of vitreous phase in the body, increases the risk of deformation . Figure 4 gives the X-ray patterns, which demonstrate the phase development of specimens that are fired at different temperatures.
indicates the amorphous phase. This shows that the alkaline and fluxing content or the temperature, is not enough for this ceramic body. Feldspar found in non-fired clay, and the cristobalite, tridymite, quartz and mullite phases found in sintered kaolin, which is used as chamotte, determine the properties of the body by reacting with each other during the firing process. The kaolin found in clay, decays after 980°C, forming the spinel structure and primary mullite developments [26-27]. As for feldspars, they melt at certain temperatures and increase the strength of the body, further accelerating the reactions of other raw materials with each other . The growth of secondary mullite crystals is also affected if there is insufficient vitreous phase in the environment or if the viscosity of the composed vitreous phase is not suitable.
There has to be a sufficient amount of vitreous phase for the development of acicular mullites, which increase strength. Cristobalite and mullite, found in chamotte, stand for high temperature-resistant phases . Regarding their reactions at low temperatures, the vitreous phase surrounding them becomes effective. Given the fact that the reactions will be weak when there aren’t any vitreous phases around them, there will be no changes to be observed before high temperatures. The fired specimens show mullite and cristobalite peaks before 1200°C. In conjunction with the rising temperature, the intensity of mullite peak increases. An obvious increase in strength values is expected when there is a secondary acicular mullite development due to the temperature. After 1150°C, the strength continues to increase with vitreous phase and developing crystals. This data shows that when clay is fired at higher temperatures, it can be shaped without any deformation.
3.5. Color development of fired specimens
Table 4 exhibits L*,a*,b* data, which indicate color-coordinate values of the fired specimens. In conjunction with the rising temperature, L* and a* values decrease. Beyond 1150°C, the L * values of the specimens fired at 1200 ° C did not change much and the a* value significantly decreased. Visually, the whiteness of the specimens fired at 1200 ° C was higher. It is possible to create visually beautiful works thanks to the fact that grogged clay paves the way for creating highly complex art works that require subtle and detailed practices. Figure 5 shows the works that are chosen by courtesy of ceramicists Mine Aktaş Poyraz and Özlem Öğüt, who used grogged clay, of which its characterization is made in this study, through hand molding and die casting, firing at 1160°C and using glazed and unglazed piercing technique. The fact that Mine Aktaş Poyraz (Fig. 5- A, B, C, D, E) easily engraved patterns on the artifact, shaped by the piercing technique, and created a lace-like work of art without any deformation; was made possible by grogged clays. The fired specimens are white and light yellow without any deformation, thus making the products visually appealing to the eye.
3.6. Microstructure analyzes
In structures consisting of kaolinitic clay, feldspar and quartz, clays increase the formation of mullite. Mullites are the only interphases that are stable between alumina-silica systems at atmospheric pressure. Mullite’s temperature stability and corundum (Al2O3), its natural refractory structure, are superior to certain high-temperature implementations. For this reason, mullite formation in ceramic structures is important in terms of improving the properties. The feldspars used in conjunction with clays to form these structures in ceramic bodies, stand for low-temperature minerals and as fluxing agents, they form the liquid phase, which fills the cavities between pores during the firing process [30,31]. Weak clay at low temperatures form mullites in which kaolin particles are in elementary position. Along with the presence of liquid phase, which was increased by feldspars, low-energy crystalline surfaces are exposed to reactions in liquid phase. On low-energy surfaces, acicular mullites are formed under these conditions [31, 32].
Mullites can be classified based on their sizes. According to this classification, those having (< 0.5µm) sizes stand for primary mullite, and those with (> 1µm) stand for secondary mullite. While primary mullites are formed at low temperatures as products of clay minerals, secondary mullites are formed through the recrystallization and dissolution of alumina silicate structures with the help of fluxing, when feldpars are added into the body [20-33]. The main differences between these two mullite types are the size, shape and fusion ratio of mullite crystals .
In cubic primary mullites (Type-I); crystals show an aspect ratio of (1-3:1), and the acicular secondary mullites (Type-II) formed by the liquid phase, which are generated by feldspars, the aspect ratio escalates to (3-10:1). Along with the proper mixing of clay, quartz and feldspar, and feldspars transforming into a totally viscous state, a higher aspect ratio (30-40:1) is reached and this ratio allows the formation of tertiary mullites . Given the fact that feldspar ratios are not too high, the formation is completed before secondary mullites. Figure 6-A shows that primary mullites are formed at low temperatures and secondary mullites start to form. When Figure 6-B is examined, it is detected that liquid phase and Type-I and Type-II mullites within, are formed. In conjunction with increasing temperatures, secondary mullite content also gradually increases .mullite formations tend to decrease. Rising temperatures in mullite formations, increase the aspect ratio of crystals. Figure 6-D exhibits the increase in density of mullite phase and its crystals, in conjunction with the rising temperature. Given the fact that liquid phase is not sufficient in rising temperature, because of less amounts of feldspar, it is unable to develop mullite crystals concentratedly. When the liquid phase ratio in augmented, the aspect ratio of mullite crystal also escalates in proportion to rising temperature, resulting in the formation of tertiary mullites having an aspect ratio of (30-40:1) .
Figure 6-C shows that while Type-II mullite formations increase in proportion with the aspect ratio (3-10:1), which increase with the rising temperature, Type-I
Discussion and Conclusions
In the frame of manufacturing by potter’s lathe and preparing ceramic clay formula, it is important to examine the chemical and mineralogical properties in order to improve the properties of the fired product. The plasticity limit of the artistic ceramic clay containing chamotte (grog), which is used in this study, is suitable for hand molding. According to the Pfefferkorn plasticity test for hand molding, the remaining crushing deformation level is 16 mm, which points out a a standard value. In studies performed with this standard reference value of 16 mm, it is detected that adding approximately 5% more water is sufficient for the artistic productions to be formed without any deformation. The bending strength values of all of the fired specimens, comply with TS-EN 10545-4 standard norms . As for water absorption values, they comply with TS EN 14411 standard norms .
If the softness and plasticity values of the ceramic body are used for shaping, depending on the product size and the sintering behavior during the firing process, it allows to designate the temperature limits for deformation and therefore artists can smoothly mold and fire the body without any issues and turn it into a product. They can produce large scale works without encountering with any problems. Regarding ceramic bodies containing chamotte, the 1150-1200°C sintering temperature range is ideal in firing process. However, it is necessary to examine thermal expansion behaviors between cristobalite and glaze. Additionally, a glazing study needs to be performed in accordance with the body, in order to obtain a crack-free glazing. In order to improve the physical properties, it is determined that secondary mullites and the vitreous phase formed in this phase, as well as the firing duration in the viscosity of the vitreous phase and viscosity-effective raw materials need to be doped at different levels and it is necessary to examine their effects in crystal development. It is detected that the whiteness of specimens fired at 1200°C firing temperature, is visually high. With the use of grogged ceramics, it is possible to obtain aesthetically-pleasing artistic works and it will be possible through more complex, subtle and detailed studies.
We hereby would like to thank SAM (Ceramic Research Centre, Eskişehir) and Anadolu University, Department of Materials Science and Engineering for their assisting in taking SEM images for this study; the ceramicist Mine Aktaş Poyraz for procuring her works exhibited in various venues, which were created by using grogged clay, the subject of this characterization study.
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