Glaze defects are any flaws in the surface quality of a

ceramic glaze Ceramic glaze, or simply glaze, is a glassy coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimize the adherence of pollutants. Glazing renders earthenware impermeable to water, sealing th ...

, its physical structure or its interaction with the body.

Glaze and body mismatch

Certain glaze defects are a result of differences in the thermal expansion coefficient of the glaze and the clay body.

Crazing

Crazing is a

spider web A spider web, spiderweb, spider's web, or cobweb (from the archaic word ''Wikt:coppe, coppe'', meaning 'spider') is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey ...

pattern of cracks penetrating the glaze. It is caused by

tensile stress In continuum mechanics, stress is a physical quantity that describes forces present during deformation. For example, an object being pulled apart, such as a stretched elastic band, is subject to ''tensile'' stress and may undergo elongati ...

es greater than the glaze is able to withstand."Ceramics Glaze Technology". J. R. Taylor, A. C. Bull. ''Institute Of Ceramics / Pergamon Press.'' 1986. Common reasons for such stresses are: a mismatch between the

thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

s of glaze and body; from moisture expansion of the body; and in the case of glazed

tile Tiles are usually thin, square or rectangular coverings manufactured from hard-wearing material such as ceramic, Rock (geology), stone, metal, baked clay, or even glass. They are generally fixed in place in an array to cover roofs, floors, wal ...

s fixed to a wall, movement of the wall or of the bonding material used to fix the tile to the wall.Dictionary Of Ceramics. Arthur Dodd & David Murfin. 3rd edition. ''The Institute Of Minerals.'' 1994. The cracks can allow the ingress of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

into the cracks. Once fired, ware tends to be more resistant to crazing due to better development of the glaze/body interfacial layer, which reduces stress gradients between the glaze and body. In pottery a distinction is often made between crazing, as an accidental defect, and "crackle", which is when the same phenomenon, often strongly accentuated, is produced deliberately. The Chinese in particular enjoyed the random effects of crackle, though it spans a spectrum: in

Ru ware Ru ware, Ju ware, or "Ru official ware" () is a famous and extremely rare type of Chinese pottery from the Song dynasty, produced for the imperial court for a brief period around 1100. Fewer than 100 complete pieces survive, though there are ...

it is a tolerated feature of most pieces, but not sought, while in Guan ware a strong crackle is a desired effect. The causes of crazing include: *Thermal expansion mis-match. Poor fit between the glaze and the body's thermal expansion is the main cause of crazing and can be due to: :*Under-firing resulting in failure to develop sufficient body thermal expansion. :*Firing too quickly, resulting in failure to achieve sufficient heatwork. :*Low thermal expansion body. :*High thermal expansion glaze. :*Over-firing of vitreous ware. *Moisture expansion of the body. Porous bodies swell slightly due to absorption of moisture. Where glazes are in only slight

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

this can be sufficient to bring them into tension. The problem results in delayed or secondary crazing, which occurs over a period of time after the ware has been produced. *Glazing too thickly. This is a common cause of crazing. Glazes, which should be craze resistant, can craze if applied too thickly. This is because the further the glaze surface is away from the body, the lower the compression acting on it. *Thermal shock. Opening the kiln too soon above 100 °C can cause crazing and dunting. Above 200 °C catastrophic failure can occur due to the volume changes at the cristobalite inversion (around 225 °C) ''Steger's Crazing Test'' is a method for the assessment of the glaze fit. It is undertaken by measuring any deformation on cooling of a thin bar that was glazed only on one side. A common method of testing glazed ceramic ware for crazing resistance is to expose pieces of ware to the steam in an autoclave at a minimum of 50 psi. ''Seger's Rules'' are a series of empirical rules put forward by Hermann Seger for the prevention of crazing and peeling. To prevent crazing, the body should be adjusted as follows: decrease the clay, increase the free

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

; replace some of the

ball clay Ball clays are kaolinitic sedimentary clays that commonly consist of 20–80% kaolinite, 10–25% mica and 6–65% quartz, along with small amounts of organic matter (such as lignite) and trace amounts of other minerals such as pyrite and siderit ...

kaolin Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina (). ...

; decrease the

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

; grind the silica more finely;

biscuit A biscuit is a flour-based baked food item. Biscuits are typically hard, flat, and unleavened. They are usually sweet and may be made with sugar, chocolate, icing, jam, ginger, or cinnamon. They can also be savoury, similar to crackers. ...

fire at higher temperature. Alternatively, the glaze can be adjusted: increase silica and/or decrease fluxes; replace some SiO₂ by B₂O₃; replace fluxes of high equivalent weight by fluxes of lower equivalent weight. To prevent peeling, the body or glaze should be adjusted in the reverse direction.

Shivering

Shivering describes the breaking away of glaze from ceramic ware as a result of greater compression in the glaze layer than the body caused by the glaze having an expansion coefficient below the clay body's. It is the opposite of crazing, as are the preventative steps: see ''Seger's Rule'' above. Shivering is also known as ''peeling''.

Metal release

Regulations have existed since the late 1960s to protect consumers from the potential risk of toxic materials, mainly metals, being released from glazes into drink and foodstuffs.

Lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

and

cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

are the metals of greatest concern, although testing can be extended to include others. The propensity for any glaze to release metal may depend on complex interactions between the formulation used, any applied decoration and the kiln atmosphere. Monitoring the level of metal release from glazed ware forms part of the

quality control Quality control (QC) is a process by which entities review the quality of all factors involved in production. ISO 9000 defines quality control as "a part of quality management focused on fulfilling quality requirements". This approach plac ...

procedures of all reputable producers. Test methods are specified according to national and international standards, although testing usually involves: the ware being immersed or filled with a 4%

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

solution; covered and left for 24 hours at room temperature, although if cooking ware is being tested higher temperatures are needed; the acetic acid solution decanted from the ware and the concentration of leached metal measured by

Atomic absorption spectroscopy Atomic absorption spectroscopy (AAS) is a spectro-analytical procedure for the quantitative measurement of chemical elements. AAS is based on the absorption of light by free metallic ions that have been atomized from a sample. An alternative tec ...

. Acceptance limits are enforced by legislation, and whilst varying between countries all are within the ppm range. Some of the most well recognised legislation are: across Europe 'EC Directive 84/500/EEC 1984'; for the UK 'GB Ceramic Ware (Safety) Regulations SI 1647, 1988'; and for the USA 'FDA Compliance Policy Guide 7117.06 and 7117.07 for cadmium and lead.'

Glaze surface defects

Blisters

A large bubble sometimes present as a fault in ceramic ware. Blisters appear as large bubbles either just below or penetrating the surface, leaving sharp, rough edges that collect dirt. The surface of the glaze is very unpleasant and looks like a boiled mass of bubbles, craters and pinholes.

Crawling

A defect that appears as irregular, bare patches of fired body showing through the glaze where it has failed to adhere to or wet the body on firing. The cause is a weak bond between glaze and body; this may result from greasy patches or dust on the surface of the biscuit ware or from shrinkage of the applied glaze slip during drying. The fault is more likely to occur with once-fired ware such as sanitaryware.

Metal marking

Metal marks are dark lines, often accompanied by damage in the glaze, caused by the deposition of metal during the use of metal utensils. The cutlery, or other relatively soft metal, will leave a very thin smear of metal on pottery ware if the glaze is minutely pitted. A glaze may have this defective surface as it leaves the glost kiln, or it may subsequently develop such a surface as a result of inadequate chemical durability. The fault is also known as cutlery marking.

Pin-hole

A fault that is commonly the result of a bubble in the glaze when it was molten that burst but was only partially healed. The bubbles are most often from

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

that originates from air trapped between the particles of powdered glaze as the glaze begins to mature, or from gases evolved from

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

compounds. A specific example of pin-holes is ''Spit-out''. These are pin-holes or craters sometimes occurring in glazed non-vitreous ceramics while they are in the decorating

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or Chemical Changes, chemical changes. Kilns have been used for millennia to turn objects m ...

. The cause of this defect is the evolution of

water vapour Water vapor, water vapour, or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor ...

, adsorbed by the porous body, during the period between the glost firing and the decorating firing, via minute cracks in the glaze."The A. T. Green Book". pp. 255–268 "Spit-out". W. T. Wilkinson, A. Dinsdale. ''The British Ceramic Research Association.'' 1959.

References

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External links

A study of Glaze expansion
Pottery Ceramic art Ceramic engineering Ceramic glazes