A glaze is a specially formulated glass applied to ceramics.

In the case of electrical insulators the glaze must maintain specific electrical properties even in inclement weather.

For chemical porcelain the glaze must have high chemical durability.

For fine china, porcelain, and semi-vitreous china, the glaze must have high gloss and be resistant to dish washing chemicals, cutting by knives, and abrasion.

For floor or wall tile a glaze must be resistant to abrasion and scuffing.

In all cases a glaze must be resistant to thermal shock. (Yes, even for tile. A thermal shock test can prove the glaze is in proper compression. More later.)

The properties of a glaze are determined by its chemical composition. Potassium feldspar is a single-component high-temperature glaze for chemical porcelain. Lead is a component of fine china glazes because it "fixes” many of the application and flow problems of glazes and adds high gloss. Lead is not used as it once was, in the white lead form, which was desired in dipping glazes. All most all lead is now contained in frits.

A frit is a special glass used in compounding glazes. It ties up toxic and soluble materials and sometimes coloring oxides.

To form the final glaze compositon, clays, alumina, alkaline oxides and or carbonates, etc., may be added to the frit.

To learn how to formulate a glaze, frit, or ceramic body, see Ceramics: Industrial Processing and Testing, John T. Jones and M. F. Berard, Iowa State University Press.

While not all ceramic bodies are ground, all glazes are ground to a specific particle size.

Over grinding can cause crawlingof a glaze. Firing does not always fix this particular application problem.

The glaze is sprayed on to the ware in most cases. Many shapes are dipped.

Have you ever wondered how the innards of a toilet bowl can be glazed? A Nerf Ball® is soaked in glaze and sucked through the trap.

In the porcelain process the body is porous after a low-temperature bisk fire. Therefore it is easy to dip. This process is automated for dinner plates and such in modern factories.

In the china process the ware is vitrified and it must be heated during the glazing process. This is usually done by burners in the first section of the glazing tunnel.

Glazes can also be applied in powder form by electrostatic spraying. This is a good way to put enamel on household appliances. I have little experience in this area.

I might mention that in the zillion tile factories I have toured in Italy (and one in Columbia, S.A.) a water fall works well for glazing.

The tile industry has many clever ways of glazing and decorating the tile as it moves down the line. I've learned that people are generally not required in modern tile factories. Everything including lunch break is automated.

Typically most glazes have for binder a small amount of montmorillonite. This can be a refined synthetic white material in glazes for fine china or Wyoming bentonite for less restrictive glazes.

Methocellulose, dextrin, and gum Arabic are also used as binders. Crawling is a problem that can be prevented by using a proper binder system and avoiding over-ground glazes and overly-thick glaze application. Crawling usually appears during the firing process but it can occur on drying of the glaze.

Some glazes tend to craze after firing. This can occur during decorating operations (or sooner) or much later with time.

One form is delayed crazing that occurs when a porous body (semi-vitreous ware) absorbs water which can expand the body, stretch the glaze, and crack the glaze.

Usually adding nepheline syenite to form more glass in the body, adding silica to raise the body expansion, or other body adjustments can prevent this.

Crazing during decorating operations indicates that the glaze needs reformulating. This may be done by simply adding a small portion of silica or low-expansion frit to the glaze batch. Changing the cooling cycle during the decorating

process can help.

I've seen large shapes crack on cooling during glost firing, sometimes crazing the glaze. The solution was always in controlling the cooling cycle especially though the silica conversions. (See the reference.)

It is interesting that silica can raise the thermal expansion of a body and lower the thermal expansion of the glaze. Can you figure out why? (Answer: the silica is in crystalline form in the body and in the vitreous form (or very-low thermal expansion form) in the glaze.)

For all practical purposes the thermal expansion coefficient of the glaze must me lower than that of the body. This keeps the glaze in compression after firing.

Glazes always fail in tension.

Keeping the compressive forces higher than the tensile forces is the key.

Silica and boron compounds can lower the expansion coefficient of the glaze. Replacing alkalis with alkaline earths can help.

There is a thing I call multiplicity in glaze formulation: The more different materials used in compounding a glaze, the better. It minimizes the negative effects of a particular material while retaining the good effects. (Anyway, that is my theory and I'm sticking to it.)

Kiln contamination can be a problem in some operations. Pits, pin holes, and pocks caused by impurities dropping on the glaze during firing can be removed by grinding. The ware is resprayed with a thin coat of glaze and then refired. The thickness of a glaze before firing is always a concern so you don't want to over do it.

Keep your kilns clean. This should be a routine for the kiln loaders.

Pinholes can be caused by body contaminants. If body impurities are still releasing gases on refiring your problem may not be resolved until you fix the body problem.

There is a discussion of glaze defects at http://www.ceramicstoday.com/articles/122000.htm. One of the defects discussed there is shivering caused by too low a thermal expansion coefficient for a glaze.

In all my days I've seldom seen this except in the laboratory. The problem is almost always the opposite (crazing).

Anyway, what can be easier than raising the thermal expansion coefficient of a glaze?

I forgot to mention something of importance when I first put this article up. The foot of glazed ware can be a problem area. For green ware the foot is usually waxed before glazing. The glaze runs off the foot during the glazing process. This leaves a less than desired foot but is satisfactory for the price range of such ware. After firing, the foot is usually rubbed with an abrasive stone or polished on an abrasive belt to remove any burrs.

Bisk fired ware can be glazed and then the foot cleaned before firing the glaze by running the ware across a sponge belt. After firing, the foot must be polished to prevent scratching of stacked ware and to protect furniture, etc.

Also, ware can be pin-fired that leaves a glazed foot which is desirable. The pin marks may have to be removed by grinding but some setters use very small refractory balls at the contact point that leave very little after-firing work.

If your process will allow a glazed foot, then that is the preferred finish.

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