With excerpts from Linda Bloomfield and Dave Finkelnburg
Testing standards are important to both the studio artist and his or her customers. While we may give specific instructions on the care, handling, and durability of our work, the truth is those tips can easily be forgotten and buyers expect our pots to act as well as commercial products. We are responsible for properly testing our ware, accurately reading the results, and ultimately selling a safe, durable product.
One important glaze consideration in making food-safe surfaces is resistance to metal marking. This is when metal deposits are left on ceramic surfaces from utensils or other metal objects (mainly on matte surfaces). If marks won’t wash off, the glaze is not satisfactory.
Hardness and Scratch Resistance Science
by Linda Bloomfield
In general, it’s better to use a mixture of several different fluxes than to rely on only one or two. Potash feldspar will make harder glazes with better scratch resistance than soda feldspar. The alkaline earths—calcium, magnesium, barium, and strontium—will increase glaze stability, as will zinc oxide. Adding titanium or rutile will also help to improve resistance to acid attack. Adding zirconium silicate makes the glaze opaque and also improves durability and scratch resistance. Low-alumina glazes may become scratched more easily than those within the limits for stability.
In general, the higher the firing temperature, the harder and more durable the glaze will be, provided it contains enough alumina and silica. For low-fire and mid-range glazes, enough boron must be added to allow complete melting at the chosen firing temperature.
Matte and crystalline glazes may be marked by cutlery. This occurs when metal from the cutlery scrapes across and is deposited on the microscopically rough edges of crystals in the glaze. It’s better not to use matte glazes on surfaces of functional pots that will come into contact with cutlery, although most marks can usually be removed. Zirconium opacified glazes may also by marked by cutlery, however, these marks may not be removable.
Metal Marking and Zircon
by Dave Finkelnburg
Zircon is currently the most common glaze opacifier. It has been sold under many names over the years—Ultrox, Zircopax, and Superpax. Zircon is mined from naturally occurring mineral sands, which are then cleaned and ground to different levels of fineness.
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 harder than eating utensils. Unsightly gray lines on the glaze surface (see below) 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 don’t suffer from permanent metal marking. That’s because eating utensils, while softer than zircon, are harder than most fired glazes.
Testing for Resistance to
by Dave Finkelnburg
The best test for knife marking on your functional ware is simply to use your pottery yourself for a few months. It’s always easier to see marks on white or light colored glazes. Alternatively take any coin, knife, or metal tool and firmly drag that metal repeatedly 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. 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. If the marks will not come off even after vigorous scrubbing with a scouring powder, don’t use the glaze on surfaces intended for functional use.
It’s also important to note that a glaze that’s repeatedly in contact with acids (coffee, vinegar, lemon juice, etc.) or bases (some soaps, cleaning products with ammonia) will metal mark worse over time. The etching from the acids/bases leaves a rougher surface that will abrade metal more easily and be more difficult to clean.
Choosing the Right Surface
by Editorial Staff
Choosing the glaze and surface type that won’t metal mark, whether commercial or homemade, requires testing. Glossy glazes are most often, but not always, immune to metal marking while matte and semi-matte glazes are generally more prone to scratches. Metal markings on a hard matte or semi-matte glaze will wipe off if you take the time to clean it properly, but you can’t always expect your customers to do that and keep themselves safe. If you want the aesthetic look of a matte glaze on your functional ware, finding the perfect version will most likely require one without zircon as the opacifier. Two solutions are possible without a lot of chemistry or advanced glaze mixing knowledge:
1: The combination of a matte glaze with a satin version thinly sprayed over the top of it. This turns the matte into a slightly more satin glaze, which should stop the marking.
2: Blend your matte glaze with a clear glossy glaze (90:10). The glaze will harden enough in the firing causing the metal marks to cease, and the surface will only be slightly changed.
Excerpted from Science for the Potter,
Linda Bloomfield. The American Ceramic Society, 2017.
Excerpted from “Opacifiers” Ceramics Monthly, October 2014, Dave Finkelnburg.
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 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 www.thebookpatch.com. To learn more check out www.masteringglazes.com.