Kaolin or Kaolin, is a clay group mineral and is widely used in ceramic glaze and body production. It is generally used as a floater, strength enhancer and alumina source.
The characteristics that determine the quality of kaolin are primarily the impurities found in kaolin, the low iron, titanium and sulfur ratio, the floater feature, the casting feature and the high alumina ratio.
In the hobby ceramic sector, it is used between 10% and 40% in clay production according to the properties of Kaolin and the clay ratio in the recipe. Kaolin used in the clay recipe is preferred due to cost, local kaolins (AL2O3 ratio between 14-26%). These are mainly Düvertepe, Şapçı, Mazardere, Bilecik, Uşak, Çanakkale region kaolins. Since these kaolins are in rock form, they are passed through the crusher and added to the mill under 10 mm.
Since Düvertepe and Şapçı region kaolins have sulfur in their structures, it is necessary to pay attention to this when buying kaolin, otherwise it may cause glaze errors during production.
Kaolin used in ceramic glazes must be filtered (pasta) kaolin. The reason for this is the process of filtering the organic materials in the Kaolin structure and removing them from the structure, thus turning it into a purer and cleaner product.
The filtering process of kaolin is; It is the removal of sand and organic materials by mixing in sedimentation tanks and passing through sieves of different diameters. This process also increases the Al2O3 ratio in kaolin. It is preferred that the Al2O3 ratio of kaolin used in glaze production is between 34-37%.
The Al2O3 ratio in kaolin, the smoothness of the glaze surface, the increase in the melting point of the glaze and the decrease in the expansion coefficient, thus preventing the glaze from cracking.
However, the main purpose of kaolin used in the glaze recipe is to prevent the glaze from collapsing during waiting and application and to provide the shaking feature during grinding in the mill in fritted glazes. Otherwise, since frit is a heavy material, grinding problems occur.
Kaolin ratio in fritted glazes varies between 6-10% according to the user's request. There are advantages and disadvantages to using more or less here.
If the Kaolin ratio is used low;
-The surface will be brighter (because the matting feature of kaolin will be low, so the glaze will be bright)
-The fluctuations called corrugations on the glaze surface will be more.
-The melting degree of the glaze will be lower
-The glaze cost will be higher
-The glaze will settle more during waiting.
If the kaolin ratio is higher;
-The glaze surface will be more matte
-The surface will be smoother
-The glaze will melt later
-The glaze will settle less
-The cracking and abrasion resistance will be high
Evaluation and conclusion;
The kaolin to be used in clay production should be used by making stock analyses and the kaolin to be used should definitely be white imported filtered pasta kaolin with an L value above 92% (Alumina ratio min. 35%).