In glazes with this fault, rubbing a metal knife or spoon on the surface will leave black marks that cannot be completely rubbed off. This is a common fault in glazes, especially mattes. Even commercial tableware often exhibits this problem. It usually happens because the micro-surface of the glaze is not smooth, it has angular protrusions that are actually abrading the metal, taking off tiny bits. Micro-crystalline surfaces will do this (this is a common mechanism to produce a matte). Also, glazes having a high zircon content can mark because the unmelted zircon particles have sharp corners and edges and can protrude from the surface. Of course, glazes that are not completely melted will also have a rough micro surface that will mark easily (because the metal is actually removing fragments of glaze and exposing sharp unmelted particles that can abrade). Some glaze formulations are inherently softer and less durable, for these metal is hard enough to actually scratch the surface, however for these the cutlery mark is not nearly as pronounced.
Severely 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.
Cutlery marking 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).
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!
Adding an opacifier can produce cutlery marking!
G2934 cone 6 matte (left) with 10% zircopax (center), 4% tin oxide (right). Although the cutlery marks clean off all of them, clearly the zircopax version has the worst problem and is the most difficult to clean. To make the best possible quality white it is wise 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.
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.
Tuning the degree of gloss on a matte black glaze
These 10 gram balls were fired and melted down onto a tile. The one the left is the original G2934 Plainsman Cone 6 MgO matte with 6% stain. On the right the adjustment has a 20% glossy glaze addition to make it a little less matte. Notice the increased flow (the ball has flattened more) with the addition of the glossy. In addition, while the percentage of stain is actually less (on the right), the color appears darker! Tuning the degree of matteness when making color additions to a base is almost always necessary to achieve a glaze that does not cutlery mark.
Why does the left glaze not cutlery mark when it is barely melting?
Left: VC71 cone 6 silky matte glaze. Right: An adjustment that adds boron melter and SiO2/Al2O3 (which preserving their ratio). The dramatic improvement in melting was unexpected. Even though B has the same Si:Al ratio, it is completely glossy. Why? A (left) is simply not melting completely, that is why it is silky matte (not a true matte). Yet A feels like a good silky matte and is resistant to cutlery marking. Why? Feel alone can be misleading. Cutlery marking usually happens with matte glazes or heavily opacified whites, this is neither, it is an under-fired glossy glaze, fired just high enough not to mark.
The Best Cone 10R Dolomite Matte base recipe
This is G2571A cone 10R dolomite matte glaze with added 1% cobalt oxide, 0.2% chrome oxide. The porcelain is Plainsman P700, the inside glaze is a Ravenscrag Slip clear. This recipe can be googled, it has been available for many years and was first formulated by Tony Hansen. This base is very resistant to crazing on most bodies and it does not cutlery mark or stain. It also has very good application properties.
A true matte is still matte when you over fire it
The top glaze is VC71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).
A good matte glaze. A bad matte glaze.
A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.
Tune your matte glaze to the degree of matteness you want
G2934 is a popular matte for cone 6 (far left). It is not matte because it is not melting enough or is covered with micro-crystals, it is an MgO matte (a mechanism produces a more pleasant surface that cutlery marks and stains less). But what if it is too matte for you? This recipe requires accurate firings, did your kiln really go to cone 6? Proven by a firing cone? If it did, then we need plan B: Add some glossy to shine it up a bit. I fired these ten-gram balls of glaze to cone 6 on porcelain tiles, they melted down into nice buttons that display the surface well. Top row proceeding right: 10%, 20%, 30%, 40% G2926B added (100% far right). Bottom: G2916F in the same proportions. The effects are similar but the top one produces a more pebbly surface.
Can a cone 6 functional glaze having only whiting and feldspar melt enough?
This flow test compares the base and base-plus-iron version of a popular CM recipe called "Tenmoku Cone 6" (20% whiting, 35% Custer feldspar, 15% Ball Clay and 30% silica, 10% iron oxide). Although iron is not a flux in oxidation, it appears to be doing exactly that here (that flow is just bubbling its way down the runway, the white one also fires to a glassy surface on ware). It looks melted in the tray on the right but notice how easily it is scratching on the tile (lower left). This demonstrates that looks can be deceiving. Cone 6 functional glazes always have some percentage of a power flux (like boron, lithia, zinc), otherwise they just do not melt into a hard glass. Maybe a glaze looks melted, but it has poor durability.
Out Bound Links
In Bound Links