While these are all important steps, the basics of glaze formulation and application should not be overlooked. Simple procedures such as analyzing incoming materials, carefully developing and testing new glazes, and specifying or using the right batching and application techniques can mean the difference between increasing profitability and struggling with high losses due to quality problems.
Whatever quality control checks are put in place, they should be used regularly and consistently to ensure that only high-quality materials and glazes are being used in production.
Wet Grinding. Wet grinding using a conventional ball mill is one of the most popular glaze preparation methods. The glaze usually contains frit(s), raw materials such as clay and feldspar, and coloring agents. The grinding media is typically alumna balls or flint stones, and different-sized balls or stones are used to optimize grinding efficiency. After the glaze is thoroughly mixed, it is then dried and sent through a second dispersion process to remove any agglomerates formed in the drying process.
It is important to note that differences in glaze properties have been observed when ball mills of various sizes are used to prepare the same glaze. This is likely due to differences in variables such as batch size, grinding time, particle size distribution and/or the amount of water and suspension materials used. For this reason, all parameters must be carefully defined and rigorously followed to ensure the highest glaze quality and consistency.
Dry Blending. In the dry blending process, all of the glaze ingredients are milled to a small particle size (typically <40 mm) and are thoroughly mixed into a homogeneous glaze powder. In most cases, the finished glaze powder is ready to use in a dry application process or can be easily converted into a slip using high-speed dispersion equipment. If the materials used in the dry blending process are coarse, an additional wet grinding step using a ball mill will likely be required.
Liquid Glazes. To prepare a liquid glaze (also called a glaze slurry), all of the glaze ingredients are wet milled in a ball mill. After the glaze slurry reaches the desired particle size, it is passed through a screen and a magnetic filter to remove any impurities. The density and viscosity of the glaze are then adjusted to fit the application, and the finished glaze is ready to be used in production.
Granulation. With the introduction of fast firing in the whiteware industry, new glaze systems were also developed to create unique effects. For example, some manufacturers discovered that applying dry glaze materials to a flat tile surface creates the appearance of natural stone and marble. Some of these effects are only possible with the use of granulated glazes, which contain glaze materials, frit(s) and pigment(s) formulated through a defined process. The ingredients are well blended, and additive materials are used to build up the glaze granules. The resulting material is then separated into different sizes using multiple screens, and is classified into sized granules that are ready to use in a dry application process.
Composto or Composti. In this technique, unground and unblended materials are supplied to the ceramic manufacturer to prepare the glazes in-house, typically through wet milling. While this can enable the manufacturer to save some money up front, it creates the risk of quality problems and high losses if the glazes are not developed and tested correctly. For this reason, manufacturers should carefully evaluate their capabilities before choosing this route.
Waterfall. In this technique, which is the most widely used glaze application method in the industry, the liquid glaze is allowed to fall freely through a defined opening onto the surface of the ceramic body. With older equipment, the low density of the liquid glaze made it difficult to achieve a smooth surface, particularly when glazing high-gloss white or transparent-surface tile. The coated surface often appeared somewhat "hammered" or dented. However, newer bell glazing machines and high-density glazes were eventually developed to improve the glaze quality, and users were able to produce defect-free high-gloss glazes even under fast-firing conditions.
In the late '90s, a pressurized waterfall unit was introduced. Instead of relying on gravity to "pull" the glaze down onto the product being coated, the new system used pressure to force the glaze through a split and onto the product below. As a result, the glazing process was faster and more uniform, and even low-density glazes could be used.
More recently, new waterfall glazing machines have been constructed specifically for the dinnerware industry. Such machines allow complicated shapes to be easily coated, and they employ automated systems that increase the efficiency of the glazing process.
Spray. Spray guns are widely used to overspray the first coat with a cover coat of either a colored or clear glaze.
Dry Pressing. Primarily used for tile applications, dry pressing is a combined operation in which glazing and shaping are performed simultaneously. In this technique, the dry glaze powder is pressed into a mold filled with the supporting body materials. The process eliminates the need for wet grinding and a conventional glazing line, thereby providing a better work environment for employees, and it also creates tile with a durable finish and unique surface effects.
Dry Glazing. Dry glazing is also commonly used in floor tile production and consists of a dry glaze powder (granulated glaze) being scattered over a wet base glaze. During firing, the base glaze and dry powder smelt into each other and produce beautiful effects on the surface. The final tile surface is typically harder and more durable than surfaces obtained using traditional wet glaze application methods.