1. IntroductionProduction of unglazed porcelain tiles is definitely one of the hardest works in ceramics. So many different sizes, thicknesses, body recipes, colors and different body combinations (double charge, micronized powder etc.) may create so many problems.
The microstructure of porcelain tile consists of mainly mullite and quartz minerals in a matrix of glassy phase, and the expansion differences between quartz and the glassy phase may cause essential issues for the bodies having high amounts of free or coarse quartz minerals.
Usually a porcelain body expands 0.7% up to 1000°C, shrinks with firing 7.5% from 1100°C to 1200°C and contracts with cooling 0.5% up to 500°C in the kiln. This article involves primarily experiential reviews of two main defects associated with the linear contraction of unglazed porcelain tiles.
2. Cooling CracksWith their typical curved shape and sharp, clear-cut edges like broken glass (Fig. 1), cooling cracks arise due to the immediate linear contraction caused by the polymorphic transformation of quartz minerals from beta to alpha at 573°C (Ref. 1). These cracks are seen if tiles pass through the temperature of quartz inversion outside of the indirect cooling zone which starts around 590°C at the end of the rapid cooling zone and ends around 550°C at the beginning of the final cooling zone.
2.1 Possible Reasons
- Dirtiness of the channel from the rapid cooling zone to the kiln exit (broken pieces of tiles and rollers on the floor of the channel) creates an uncontrollable energy and definitely causes cooling cracks.
- If the indirect cooling zone is not working well, then the issue is inevitable (the pipes of indirect cooling must be sucking air from the environment).
- Direct air may be entering into the indirect cooling zone: If the vacuum pressure of the cool air suction fan is too high or too low, then cooling and indirect cooling zones drift and thus do not start and end at where they are supposed to be. Also if the pressure of the final cooling fan is too high then again the direct air enters into the indirect cooling zone.
- Damages at the sides or corners of the tile (happened before firing) may also conclude with cooling crack, due to the very high thermal expansion coefficient of the body.
- If tiles come out of the kiln like torn apart or cracked (mainly the colder ones closer to the walls of the kiln), this may show that it happened in the rapid cooling zone and somehow the real temperature on tiles at the end of rapid cooling is lower than it should be. Then increasing the temperature of the beginning of indirect cooling can be a solution.
- If tiles come out of the kiln in one piece but show the same crack when they are checked (mainly the hotter ones in the middle of the channel), this also may indicate that the damage is occurring in the final cooling zone and somehow the real temperature on tiles at the end of indirect cooling is higher than it should be. Then decreasing the temperature of the end of indirect cooling can fix the problem.
- Elongating the indirect cooling zone: Closing off the last blowers of rapid cooling (just before the indirect cooling zone) and the first blowers of final cooling (just after the indirect cooling zone), or directly slowing down the kiln (firing schedule) may resolve the issue by elongating the indirect cooling zone.
- If the cooling cracks only appear with the first tiles coming after a gap, then the maximum air pressure of the rapid cooling can be adjusted manually so that it can not go too much over the normal working pressure when the zone suddenly gets hot after the gap.
3. Planarity DefectsPlanarity defects may arise from several reasons. Uneven compaction, mould shape and type for the bottom of the tile (especially for the corners), wrong roller speeds in the kiln, deformed or dirty rollers, and over-firing or long final firing can cause irregular defects like upturned or downturned corners, and curly (roller effect) deformations which are not mentioned here.
Non-synchronized firing shrinkages of the top and bottom of the tiles during pre-heating, cause tiles to bend and become convex or concave. However, as the tiles soften and move into semi-liquid state in the firing zone, they bend again to the opposite direction (Ref. 2) as a result of the collapse of their deformed parts through gravity. Then the tiles eventually become straight if the maximum temperature and duration of the firing zone platform is sufficient (Fig. 2).
Most of the planarity problems occur in the rapid cooling zone where porcelain moves back into solid state and starts to get its final shape. Tiles solidify and get stronger in the cooling zone, and contraction differences between the top and bottom of the tiles may cause permanent planarity defects. In this case, corrections in the rapid cooling zone can be made by cutting off the cool air above the rollers to fix convexity, and by doing the opposite to cure concavity. Because the contraction of the faster cooling part is primary and therefore limited more by the still part, slower cooling part of the tile contracts more than the faster cooling one (Fig. 3).
Rapid cooling zone begins approximately at 1080-1050°C where cool air is directy blown from the holes of the cooling pipes inside the kiln. Since rollers already prevent the effectiveness of cool air blown below the rollers for the bottom, planarity can be simply adjusted by playing just with the amount of the cool air blown above the rollers for the top, and thus by changing only the contraction of the top of the tiles.In some cases (especially for the glazed porcelains with big sizes), tiles come out of the kiln straight but bend afterwards (sometimes even after 24 hours). This problem is probably caused by the very high thermal expansion coefficient of the body. In this situation, increasing the maximum firing temperature may help; because this way, quartz dissolves more into the glassy phase and so the thermal expansion coefficient of the final product decreases due to the reduction in the quantity of free quartz (Ref. 3).
4. DiscussionLinear contraction defects of porcelain tiles may become severe with high free quartz content in the body. Quartz increases the thermal expansion coefficient of the body and high thermal expansion brings more linear contraction during cooling.
Porcelain tiles have a high linear shrinkage rate (7-8%) due to their high flux content, and they shrink as a result of the closure of open porosity with glassy phase during firing. Therefore, linear shrinkage of porcelain is also inversely proportional with particle size and compaction of porcelain body. Because the decrease in particle size (finer milling) and compaction (pressing with low pressure), increase the total volume of porosity in tiles.
However, firing shrinkage differences between the top and bottom of the tiles create fewer defects compared to linear contraction differences, since contraction occurs during cooling where tiles solidify and get their final shape.
Continuous bending of tile after kiln a while may refer to the residual stresses trapped inside, which may be due to high thermal expansion, coarse quartz particle size, high content of free quartz, extreme corrections in rapid cooling and inadequate final cooling (very hot tiles at the kiln exit); especially if the problem is accompanied by cooling cracks.
1. P.J. Heaney, and D.R. Veblen, Observations of the alpha-beta phase transitions in quartz: A review of imaging and diffraction studies and some new results, American Mineralogist, 1991, 76, p 1018-1032.
2. SACMI IMOLA S.C., Applied Ceramics, Tipografia Moderna di Ravenna, Italy, 2002, p 386-394.
3. P.W. Olupot, S. Jonsson, and J.K. Byaruhanga, Study of Glazes and Their Effects on Triaxial Electrical Porcelains from Ugandan Minerals, Journal of Materials Engineering and Performance, 2010, 19(8), p 1133-1142.