Glaze Basics

March 1, 2005
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With a move toward leaner, more efficient manufacturing processes, the basics of glaze formulation and application should not be overlooked.



Over the past decade, and especially within the last several years, many manufacturers of tile, dinnerware and sanitaryware have begun embracing leaner, more efficient production processes in an effort to remain competitive. As part of these initiatives, some companies have installed new glazing equipment, modified existing kilns or installed new kilns to optimize energy efficiency, developed new glaze and body formulations, investigated and/or implemented new glaze application processes, developed specially treated glaze materials, or combined different application processes (such as dry/wet) to produce new effects.

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.

Material Analysis

A set standard for quality and purity should be established for each incoming material (raw materials or finished glazes), and all future materials should be compared to that reference. A number of different test procedures are used throughout the industry to develop these standards. For example, impurities are commonly identified by firing a sample of the material at cone 8-10; a change in color and/or the appearance of black specks indicates that the material is contaminated. Coarse particles in the material can be tested by passing a sample through a fine-mesh screen (325-mesh or finer) and evaluating the remaining residue. The melting behavior of a material can be determined by substituting a standard material with the new material in a known, standard glass batch. Likewise, a known colored glaze can be used to check the quality of an incoming raw material. For example, a green pigment consisting of equal amounts of Zr-V blue and Zr-Pr yellow might appear blue-green or yellow-green in a glaze, depending on the impurity and its concentration.

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.

Glaze Formulation

New glazes can be an easy way to bring a unique look to a new or existing product. However, before a new glaze is evaluated in pilot plant trials, it should be carefully developed for the intended application and rigorously tested in a lab to identify any potential problem areas. If the glaze development is outsourced, the glaze technician should be provided with information such as the type of product on which the glaze will be applied (tile, dinnerware or santaryware), the thermal expansion of the ceramic body, the firing process (e.g., single- or double-fire), the firing time and temperature, and the equipment that will be used for glaze application. In most cases, a range of standard frits is used to develop the glaze. All of the necessary parameters are typically finalized during the pilot test with the developed glaze.

Wet grinding using a conventional ball mill is one of the most popular glaze preparation methods.

Batching

The glaze preparation method can affect the quality and application of the final glaze, as well as the overall efficiency of the production process. Commonly used batching techniques include wet grinding, dry blending, liquid/slurry, granulation and composto/composti.

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.

Modern waterfall glazing machines used for mugs and cups...

Application

As with glaze preparation, a variety of different application methods are used in the industry to achieve the desired results. Waterfall, disc and spray application are the most commonly used wet methods, while dry pressing and dry glazing are the most widely used dry methods.

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.

...and flatware at Homer Laughlin China in Newell, W.Va. The ware is automatically transferred to the revolving table for glazing, and the glazing unit cycles forward until the glazed ware has dried sufficiently for removal.
Disc. Where a perfectly smooth surface is not required, the disc method can be used to apply glaze to the ceramic body. By varying the speed of the disc, any desired glaze weight can be obtained.

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.

High-Quality Glazes

New manufacturing equipment and processes must be embraced by today's tile, dinnerware and sanitaryware producers if they are to remain competitive on a global scale. However, basic techniques, such as glazing, are an equally important part of the manufacturing process. With the right glazes and application methods, combined with modern production practices, today's ceramic manufacturers can efficiently produce high-quality products that meet market demands.

For more information about glazes, contact Fusion Ceramics Inc., P.O. Box 127, Carrollton, OH 44615; (330) 627-2191; fax (330) 627-2082; e-mail fusion@raex.com ; or visit http://www.fusionceramics.com .

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