Ceramic and Pottery Defects 8: Defects Generated During Enameling Operations

John T Jones, Ph.D.
 


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Color is what sales ceramics.

Okay, I lied. Shape is more important.

But color is very important and is used under-glaze, over-glaze and in-glaze. If the color sinks deep into the glaze on firing, the process is called in-glaze.

Many manufacturers use pad printing or silk screening to apply colors and enamels to bisk or glazed ware. Stamping, bulbing, and hand painting are also used. Go to http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP_Features_Item/0,2710,8117,00.html for more information on specific decorating processes.

Some stains are more reactive than others. Iron and manganese compounds tend to be so. Sometimes a frit is added to the stain to increase the reaction with the body or glaze. Stains may be encapsulated. This is true of cadmium colors, for example, that are tied up in zirconium silicates. This prevents leaching of the cadmium from food ceramics. Grinding can destroy the encapsulation.

If the stain is made into a paint, as is the common method, it is called enamel.

Stains can also be applied in powder form. With tile, a binder pattern is silk screened onto the ceramic. Then stain or colored frit in powder form is dusted onto the tile and shaken off. When the ceramic is fired, the binder pattern is reproduced in color.

Hand painting is another method for applying color. It can be automated. Remember that bond signers on Wall Street in New York sign fifty or more bonds at a time. The movement of a painters hand can be recorded and then transmitted repeatedly to servo motors that drive the brushes that do the actual painting. They can even wipe the brush.

Bulbing can also be easily automated. (Well, maybe not that easy. It took some engineering for us to do it in two of the companies I worked. )

The Inkjet process can be used to apply colors but there are restrictions as to the amount of color that can be deposit. At least, that is what I last heard. The British Ceramic Research Association (BCRA) has put a lot of effort into this area so there may be advances.

The many variations in color formulations, and also in the application processes, can lead to defect generation. That’s what manufacturers don’t want. However, once a problem is resolved in a process, it usually doesn’t come up again.

For example, say that a formulation error in the color causes crawling, faint or off color, running, etc. Correcting the formula eliminates the problem forever (usually).

Close control parameters must be adhered to by both the color supplier and the customer. Color meters are not always the complete answer. I preferred to use printed color standards (like you see at the paint store except the chips are much larger and there are a lot more of them. )

After instituting this, the design department could just give us the color number which we could match without argument. Most people have eyes that are more color discriminatory than any color meter. That’s the reason why some scientists believe in God.

Colors can be purchased in enamel form so that little or no preparation is needed at the factory. Often, the pigments are purchased and then frits and binders are added as needed at the factory.

The particle size of the prepared enamel must be controlled as must the specific gravity and the viscosity. A hand painter can adjust for variations in these properties but a machine can not.

Note that in continous operations the enamel must be checked often enough as to not let specific gravity and viscosity parameters vary.

Drying is the usual process before firing. However, some enamel can be cured much faster by using ultraviolet light. You must purchase the u. v. light-sensitive resins. Infrared lights can dry regular enamel. Burner heat perhaps from radient burners may be required in automated enameling and glazing lines.

Enameling on decaled ware requires skilled workers or precise machinery. Errors are easily corrected before firing. The enamel can be removed and you can start over. After firing, forget it!

Enamels are usually fired on decal or glazed ware in a 1-3 hour cycle.

I didn’t mention that stains can be applied to green ware by stamping, bulbing, or hand painting.

Well, I'm old and I forgot.

John T. Jones, Ph. D. (tjbooks@hotmail.com, a retired VP of R&D for Lenox China, is author of detective & western novels, nonfiction (business, scientific, engineering, humor), poetry, etc. Former editor of Ceramic Industry Magazine, Jones is Executive Representative of International Wealth Success. He calls himself “Taylor Jones, the hack writer. "

More info: http://www.tjbooks.com

Business web site: http://www.bookfindhelp.com (IWS wealth-success books and kits and business newsletters / TopFlight flagpoles)

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