For millennia ceramic artists have covered their work with glazes that concealed the fired clay beneath. Seems easy, right? Because a glaze begins as molten glass, it is more likely to be transparent, so formulating an opaque glaze requires a surprising amount of attention to detail.

Considering the Nature of Light

To be adequately opaque for ceramic use, a glaze must reflect and scatter, rather than transmit, the light that falls upon it. To understand how light scatters, consider the nature of light. Light cannot pass through a totally opaque material. A sheet of steel and a wall of concrete are examples of opaque materials. An opacifier, however, is not necessarily opaque. If it is not opaque, then how does an opacifier render a ceramic glaze opaque? In a vacuum all light, say sunlight, for example, travels in a straight line from its source. When it encounters some material, the light is scattered; that is, it is either reflected back at an angle of less than 180 degrees or deflected forward at an angle greater than 180 degrees. Few materials are totally opaque (reflect all light). If we slice them thinly enough, some light will pass through virtually all glazes. However, that light may be heavily scattered. That means the formerly straight beam of light changes direction when it hits some surface in the glaze. The surfaces on and within glazes that scatter light are most often those of small crystals. These crystals, which opacify glazes, may be zirconium silicate (zircon) ground to a fine powder and added to the recipe. An alternative is to design the recipe and firing conditions so crystals will form in the glaze, either when it melts or as it cools. Wood-fired and wood-ash glazes are almost always opacified in this way. In some cases, glazes are opacified by the formation of separate glass phases or by the addition of specific chemicals that do not fully dissolve in the glaze. Tin is the most common example of the latter method of opacification. The first attempts by European artists to duplicate the fine white porcelains imported from China relied on low-fire clay bodies and low-fire, lead-fluxed, tin-opacified white glazes. If the cooling portion of a firing cycle is slow enough, crystals will always precipitate from highly flux-rich glazes. At high-fire temperatures, calcium, magnesium, and barium, if present in a sufficient proportion, all produce such crystals. At mid-range, sodium, potassium, and lithium do likewise. While there is considerable overlap in the temperatures at which these fluxes dissolve and later precipitate, zircon opacifiers work at all firing temperatures. A separate glass phase must have chemistry sufficiently different than that of the glass surrounding it in order to have a different physical structure. Opalescence in phosphorous-rich glazes is an example of light scattering caused by phase separation. Titanium, either pure or in the form of rutile, while it’s usually added as a colorant, can also be a glaze opacifier. Depending on its concentration and particle size in the glaze, its effect will vary. Finally, an abundance of fine bubbles in a cooled glaze will render it translucent to opaque.

Metal Marking and Zircon

Zircon is currently the most common glaze opacifier. It has been sold under many names over the years—Ultrox, Zircopax, and Superpax are such names. Zircon is mined from naturally occurring mineral sands, which are then cleaned and ground to different levels of fineness. Particle size and chemical purity vary depending on source and processing. The recipe amount of zircon used for glaze opacification and whitening is usually less than 15% of the batch weight, sometimes much less. Where other light-scattering crystals also form in the glaze, 3–5% zircon may be sufficient. While zircon is exceptionally effective as an opacifier, it can be problematic when added to glazes used on dinnerware. That’s because zircon contributes to metal marking. Metal marking on glazes opacified with zircon occurs because the opacifier is much harder than eating utensils. Unsightly gray lines on the glaze surface are evidence that knives, forks, and spoons have been drawn across the pottery. When the metal marks cannot be removed by washing it’s because the metal has been left in tiny pits in the glaze. The pits are created when the hard zircon is dragged out by the use of the eating utensils.1 Glossy and matte glazes opacified by means other than zircon typically do not suffer from permanent metal marking. That’s because eating utensils, while softer than zircon, are harder than most glazes.

Testing for Resistance to Knife Marking2

The best test for knife marking is simply to use your pottery yourself. It is always easier to see on white or light colored glazes and you will know after a few months whether or not this is a problem for a particular glaze. This test takes time and an accelerated version would be useful. Perhaps the easiest test is to take any coin, knife, or metal tool and firmly drag that metal across the glazed surface to be tested. After a few tries with both a glossy and a matte glaze you will learn how much pressure to use to leave metal marks. Repeat several times and examine the surface carefully. The marks you see (if any) are tiny particles of the metal which have been abraded by the ceramic surface. They will lodge in microscopic crevices of the glaze and can be easy or difficult to remove (1). If you can remove them by rubbing with your thumb, they are probably at a satisfactory level. The best situation is, of course, not to have made any visible marks. Sometimes the marks will not come off even after vigorous scrubbing with a scouring powder. If they are this difficult to remove, the glaze should not be used on surfaces subject to metal contact such as plates, the insides of bowls and cups, etc. It is also important to point out that a glaze that is attacked by acids or bases will metal mark worse over time. The etching from the acids/bases will leave a rougher surface that will abrade metal more easily and be more difficult to clean.

1 Metal Marking of Dinnerware Glaze: Correlation with Friction and Surface Roughness, Hyojin Lee, Dr. William M. Carty, Robert J. Castilone, in: “Whitewares and Materials: Ceramic Engineering and Science Proceedings,” Volume 25, Issue 2, 2004.

2 Exerpted from Mastering Cone 6 Glazes by John Hesselberth and Ron Roy. The book is available on Apple’s iBooks app or, a black-and-white version, at To learn more check out

This article was written by Dave Finkelnburg and excerpted from the October 2014 issue of Ceramics Monthly.