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Glaze Marks or Scratches

'Cutlery Marking' occurs where metal instruments leave marks on glazed functional ware. This happens because the glaze is not smooth, it is abrasing microscopic particles of the metal. However if the marks left by these particles cannot be removed easily this is more than a cosmetic problem. It suggests that they are trapped in surface pores or irregularities (pores are a possible sign of under melting).

This is a very different situation than if a sharp hard metal object can scratch the surface. Such a glaze is definitely soft and lacks resistance to wear (and has the potential of harboring bacteria). Even glossy glazes that appear hard can often be scratched easily. In general, the higher a glaze is fired, the better the potential to produce a hard and smooth surface. This is because high fire glazes require less flux and therefore have more silica and alumina. While a capable technician can produce a relatively hard glaze at any temperature range, a less knowledgeable or attentive person can make soft glazes in any range also. The chemistry principles of making a hard glaze are well known.

Compare the glaze to a known hard glaze using a simple scratching test.

Use a concrete nail or the sharp corner of a file (these are about 6.5 hardness on the Mohs approximate scale of 1=talc, 2=gypsum, 3=calcite, 4=fluorite, 5=apatite, 6=orthoclase, 7=quartz, 8=topaz, 9=corundum (ruby or sapphire), 10=diamond). Another excellent hardness testing method is to direct a sandblast at the surface at a 45 degree angle. Microsurface optical or electron analysis can then be used to accurately rate abrasion resistance (equipment to do accurate surface plots is now quite common in many industries, search the internet or check with some labs or universities).

Is the surface smooth? 
Can you mark the surface with a fork or knife?

If a glaze surface has angular protrusions then it will be abrasive. This is often the case in glaze that feels silky to the touch. Microscopic sharp edges will cut away minute chunks of metal, possibly holding them in surface voids.

  • Does the glaze contain zirconium opacifier?
    Zirconium-silicate particles do not enter the melt and they are angular and can protrude from the glaze surface. If you can make a line even with a hard metal object this confirms that the surface obstructions are very hard.
    You may need to ball mill finer, use a different or less opacifier, use a transparent overglaze, or employ a different base glaze that better envelopes the zircon. Use a microscope to check this.
  • Does the glaze contain calcined alumina?
    As with zircon, you may need to use a finer size or mill the glaze more.
    Don't assume your ball mill is doing the job without testing particle size or surface area, a badly configured mill won't grind fine enough no matter how long it runs.
  • Surface crystallization can produce an angular irregular abrasive surface. Islands of micro crystallization may be occurring even though the surface looks and feels smooth. Use a microscope. Check the glaze's chemistry to see if it is susceptible to crystal growth during cooling. Typically glazes low in alumina will devitrify (crystallize) during cooling. Increase the alumina to stiffen the melt and reduce the problem. Try cooling the kiln faster if other factors allow. Sometimes a slightly faster cooling cycle will not only reduce the crystals, but change their character to be less problematic. Is something nucleating the crystals (i.e. illmenite, wollastonite, titanium)?
  • If the glaze is a crystalline matte you will need to rationalize it's appearance. Changes made to reduce or eliminate crystallization will affect the visual character. Sometimes smaller changes to glaze make-up to simply reduce devitrificaion are helpful. Or changes to the firing curve can be made to grow a finer crystal mesh. Consider switching to a high alumina matte since they have smooth (although not flat) surface. Or you might consider employing a different crystalline mechanism.

Are marks difficult to remove?

Is the glaze mature?

  • If the glaze is not fired high enough it will simply not melt adequately. The incompletely developed surface will be both abrasive (from undissolved abrasive particles) and lacking in hardness. Try firing the glaze higher to see if it improves. If it does, adjust your body to work at higher temperatures, or adjust glaze chemistry to melt lower. Sometimes only small additions of Li2O or ZnO, for example, can give much better melts. 
  • Some soft glazes are volatile. If fired exactly right they are OK, but variations in the process result in problems with cutlery marking from time to time. Test your glaze at higher and lower temperatures to span variation typical in your kiln. Volatile glazes are typically unbalanced in their chemistry (one oxide will be very high or silica/alumina very low).
  • Alumina is a key to glaze hardness, the more present the harder a glaze will be. Inadequate alumina will contribute to glaze solubility also. While it is true that matte glazes often have high alumina, glossy results are dominant and most glossy glazes can tolerate additional alumina without noticeable visual change. Higher temperature glazes or low to medium ones containing significant boron can often tolerate a higher than expected alumina increase, especially if you source it from a feldspar or frit. Thus you might even consider adding a little boron to lower firing glazes so they can accommodate more alumina. Although keep in mind that excessive alumina in a well-melted glaze can crystallize aluminates.
  • Glazes lacking glass former SiO2 are likely to lack hardness. Check typical limits for the temperature range and type of glaze. If your glaze will tolerate more silica then put it in. If not then firing higher or adding some B2O3 will enable the use of more SiO2. Better yet, use a finer grade of quartz (i.e. 15 micron, however make sure it is does not agglomerate during application). 
  • Zircon will improve hardness so use it as the opacifier (however remember that it can contribute to cutlery marking as outlined above). Although zirconium is considered an opacifier, many transparent glazes can tolerate 3-4% of a fine grade without loss of transparency (especially borate glazes). Put as much in as your glaze will tolerate. Source it from a zircon frit if necessary).
  • Magnesia can reduce hardness so reduce it if you can. Magnesia holds thermal expansion down (and therefore tendency to craze) so consider carefully what to replace it with (perhaps one or more of SrO, Li2O, CaO).
  • If you are firing ware at low temperatures, consider using a fritted base or a commercially mixed powder. While durable ware can be made at lower temperatures, it is much more technically challenging. High borate glazes are often unbalanced and not only lack resistance to marking, but are leachable.
  • Flux saturated reactive art ware or pottery glazes are often lacking in hardness. It is common to see high temperature glazes, for example, that contain 70% or more feldspar and little or no silica or kaolin. While they are visually pleasing, they lack the necessary silica and alumina to form a hard glass.

Out Bound Links

  • (Articles) Low Budget Testing of the Raw and Fired Properties of a Glaze

    There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.

  • (Glossary) Cutlery Marking

    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 ...

In Bound Links

By Tony Hansen

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