All Glossary

Cutlery Marking

Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.

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Details

In glazes with this fault, rubbing a metal key, knife or spoon on the surface will leave black marks. Often the mark can be rubbed off with your finger. But when the problem is worse, it cannot be completely removed or washed off.

This is a common fault in glazes. Even commercial tableware can exhibit the issue. In stable and durable glazes, cutlery marking usually happens when the surface is not smooth (e.g. mattes). Micro-topology can be wavy, as is the case with high-MgO silky mattes, the more irregular the surface the more the marking occurs. Fixing the problem is a matter of choosing a compromise between matteness and marking (by adjusting MgO content or increasing the cooling speed of the kiln). The micro-surfaces of durable glossy glazes can also have angular protrusions because on the presense of zircon particles or particle agglomerate (added for opacification), these can abrade metal and trap metal particles. Solutions are to find a balance between the amount of zircon needed to opacify and the need to minimize marking, making sure you are using a 5 micron grade of zircon, using high energy mixing to make sure particle agglomerates are being separated. Compromising the degree of matteness in a glaze will also help.

Of course, glazes that are not completely melted will mark easily (because the surface both abrades metal and the metal itself removes fragments of glaze exposing more sharp contours). The degree to which the chemistry of a glaze is unbalanced relates to how soft and susceptible to abrading it is (adequate Al₂O₃ and SiO₂ and a sufficiently melted matrix, for example, is very important to hardness and durability). If a glaze is not adequately melting, other issues besides cutlery marking are likely more important (e.g. staining, leaching; use a GLFL melt flow test to see how fluid it actually is).

Sometimes glazes that appear poorly melted are actually the opposite, they have completely crystallized surfaces that are a product of very high melt fluidity and consequence devitrification. Crystalline mattes (e.g. those high in CaO, SrO) can have surfaces covered by a fine mesh of angular micro-crystals, these commonly cutlery mark badly. These types of glazes, by design, have low Al₂O₃, that means they also inherently lack durability. Fixing this type of cutlery marking problem will depend on the degree to which you are willing to compromise the fired appearance (when you add Al₂O₃ and SiO₂) or how much faster you are willing or able to cool the firing (so crystals have less time to grow).Other oxides used to promote crystal growth (e.g. ZnO, Li₂O) can produce surfaces that are even more markable than CaO.

Related Information

Cutlery marking in commercial tableware

Even commercial dinnerware can suffer cutlery marking problems. This is a glossy glaze, yet has a severe case of this issue. Why? Likely the zircon opacifier grains are protruding from the surface and abrading metal that comes into contact with it.

Commercial ultra gloss glazes from IKEA are cutlery marking

Both of these have likely seen a year of service. Both are highly opaque. While that enables a thinner application and more even colouration, the higher zircon content needed can produce cutlery marking, even though the glaze is glossy. Notice the green one is also crazing.

Adding an opacifier can produce cutlery marking

This is G2934 cone 6 matte (left) with 10% zircon (center), 4% tin oxide (right). Although the base unopacified recipe does not cutlery mark the other two do. Although the marks clean off all of the two on the right, the zircon version (in this case Zircopax) version has the worst and is difficult to clean. Thus, a small change is all that is likely needed. One solution is to reduce the matteness of this glaze, moving to more toward a satin surface. A way to do this is to line-blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain. Another option is to switch to 400 mesh silica in the recipe, that will enable many more of the particles to go into solution in the melt, thus increasing the gloss a little (an improving the firing surface in other ways).

Cutlery marking on matte black glaze

This is G2934BL glaze fired using the C6DHSC slow-cool firing schedule, this has enabled too much crystallization. This mark cannot be rubbed off, matte black glazes show the marks more than others. If I had fired it using the PLC6DS free-fall schedule, it would have been glossier. Or, I could blend in a greater percentage of the glossy black. This being said, for this type of piece some cutlery marking can be tolerated.

Severe cutlery marking in a glaze lacking sufficient Al2O3

The glaze is cutlery marking (therefore lacking hardness). Why? Notice how severely it runs on a flow tester (even melting out holes in a firebrick). Yet it does not run on the cups when fired at the same temperature (cone 10)! Glazes run like this when they lack Al₂O₃ (and SiO₂). The SiO₂ is the glass builder and the Al₂O₃ gives the melt body and stability. More important, Al₂O₃ imparts hardness and durability to the fired glass. No wonder it is cutlery marking. Will it also leach? Very likely. That is why adequate silica is very important, it makes up more than 60% of most glazes. SiO₂ is the key glass builder and it forms networks with all the other oxides.

Yikes. Cutlery marking this bad on a popular glaze!

An example of how a spoon can cutlery mark a glaze. This is a popular middle temperature recipe used by potters. The mechanism of its matteness is a high percentage of zinc oxide that creates a well-melted glaze that fosters the growth of a mesh of surface micro-crystals. However these crystals create tiny angular protrusions that abrade metal, leaving a mark. Notice the other matte flow on the left (G2934), it not only has a better surface (more silky feel) but also melts much less (its mechanism is high MgO in a boron fluxed base). It is does not cutlery mark at all!

Matte glaze cutlery marks. Add 10% glossy glaze to it. No marking.

This is G2934Y (a version of the G2934 cone 6 matte base recipe that supplies much of the MgO from a frit instead of dolomite). Like the original, it has a beautiful fine silky matte surface and feels like it would not cutlery mark. But, as you can see on the left, it does! The marks can be cleaned off easily. But still, this is not ideal. The degree of matteness that a glaze has is a product of its chemistry. But can we fix this without doing any chemistry? Yes. By blending this with G2926B clear glossy (90:10 proportions) the marks are gone and the surface is only slightly changed.

Cutlery marking here is directly related to the chemistry of the glaze

This is an example of cutlery marking in a cone 10 silky matte glaze lacking Al₂O₃, SiO₂ and having too much MgO. Al₂O₃-deficient glazes often have high melt fluidity and run during firing, this freezes to a glass that lacks durability and hardness. But sufficient MgO levels can stabilize the melt and produce a glaze that appears stable but is not. Glazes need sufficient Al₂O₃ (and SiO₂) to develop hardness and durability. Only after viewing the chemistry of this glaze did the cause for the marking become evident. This is an excellent demonstration of how imbalance in chemistry has real consequences. It is certainly possible to make a dolomite matte high temperature glaze that will not do this (G2571A is an example, it has lower MgO and higher Al₂O₃ and produces the same pleasant matte surface).

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Why does VC71 not cutlery mark when it is barely melting?

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