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Body Bloating | Body Cracking and Dunting During Firing | Clay body does not hold water | Clay Cracking During Drying | Clouding in Transparent Glazes | Dealing With Chrome Flashing | Foaming of glaze slurries | Glaze Blisters | Glaze Crawling | Glaze Crazing | Glaze is excessively runny on firing | Glaze is Off-Color | | Glaze Pinholes, Pitting | Glaze Shivering | Glaze Slurry is Difficult to Use or Settling | Glaze Staining | Over firing | Powdering, Cracking and Settling Glazes | Specking | Splitting at the Plastic Stage | Uneven Glaze Coverage

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.

Are marks difficult to remove?

Is the glaze mature?

Related Information


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
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.
Media Predicting Glaze Durability by Chemistry in Insight-Live
How to spot out-of-balance indicators in the chemistry of ceramic glazes that suggest susceptibility to scratching or cutlery marking.

By Tony Hansen

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