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Glaze defects are any perceived flaws in the surface quality of a ceramic glaze, its physical structure or its interaction with the body. What counts as a "defect" can be subjective depending on the ceramic tradition. For example, while crawling is often seen as undesirable, it is a characteristic feature of white Shino ware, known as yuzu-hada or "yuzu skin" due to its resemblance to the peel of the said fruit. In Japanese pottery, crackles in the ceramic glaze may also deepen in colour with use over time, e.g. from tea stains, much like the patina on copper objects.
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 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:
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.W. Steger. Ber. Deut. Keram. 9 (4). 1928."Evaluation Of The Steger Method In The Determination Of Ceramic-Glaze Joining". M. Peterson, A. M. Bernardin, N. C. Kuhnen, H. G. Riella. Materials Science and Engineering A 466 (2007). pp. 183–186"Temperature Gradient Method for Determining Firing Range of Ceramic Bodies". R. Stone. Journal of the American Ceramic Society. Volume 36 Issue 4, pp. 140–142. 2008. 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.British Standard 3402 and British Standard 4034"Autoclave Test Results On Crazing Of Sanitary Ware And Wall Tiles". Journal of the American Ceramic Society. Volume 29 Issue 7, pp. 203–204. 2010
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; replace some of the ball clay by kaolin; decrease the feldspar; grind the silica more finely; biscuit 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. The Collected Writings Of Hermann August Seger. H. A. Seger, E. Cramer. Nabu Press. 2010 POD edition.
It is the opposite of crazing, as are the preventative steps: see Seger's Rule above. Shivering is also known as peeling."Glazing Faults In Raw Glazes". Ceram. Inf. 18, (207), 348, 1983."Ways Of Eliminating Glaze Defects In Practice. Pt. 1. Crazing And Peeling". H. Simonis. Keram. Z. 31, (12), 717, 1979.
Monitoring the level of metal release from glazed ware forms part of the quality control procedures of all reputable producers."Countermeasures For Lead Release From On-Glaze Decorating Of Porcelain Products". A. Fukunaga. Ceram. Jap. 37, no. 8. pp. 628. 2002. 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 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."Whitewares: Production, Testing And Quality Control". W. Ryan, C. Radford. Pergamon Press / Institute Of Ceramics. 1987. 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.'"Decoration Trends In The Chinese Tableware Market". H.Zhou. Ceramic Forum International. DKG 85, no.5, 2008, pp. E30–E31."Understanding Heavy Metal Limits". Ceramic Industry. 158, no. 4, 2008, pp. 36–38."Toxic Metal Release From Ceramic Tableware: A Guide To Worldwide Regulations". J. Dawson. Ceram Research. Special Publication no. 145, p. 18. 2007.
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. The cause of this defect is the evolution of water vapour, adsorbed by the porous body, during the period between the glost firing and the decorating firing, via minute cracks in the glaze.The Causes of Pinholes in Vitreous China Sanitaryware. M. E. Twentyman, P. Hancock. The British Ceramic Research Association. RP. 706. 1979."The A. T. Green Book". pp. 255–268 "Spit-out". W. T. Wilkinson, A. Dinsdale. The British Ceramic Research Association. 1959.
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.
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 and cadmium 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.
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."Formation Of Blisters And Pinholes In Raw Glazes Of Ceramic Sanitaryware. Pt. 1. Literature Review On Their Formation And Determination". G. Schopwinkel, H. W. Hennicke. Keram. Z. 41, no. 11, pp. 830–834. 1989.
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."Crawling—A Serious Defect In Glazes After Firing". S. K. Mukherji, B. B. Machhoya, R. M. Savsani, T. K. Dan. Ceramurgia 23, no. 6, pp. 269–275. 1993"Problem Of The Glazing Defect Crawling". H. Mortel, I. Berger. Keram. Z. 45, no. 5, pp. 259–262. 1993.
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."The Marking of Glazes by Cutlery". W. P. Howells, W. Roberts. British Ceramic Research Association. Technical Note 98. 1966."Metal Marking Of Dinnerware". C. Prieur, G. Bisson, C. Casset. "Industrial Ceramics" no. 927, pp. 458–461. 1997."Testing For Metal Marking Resistance". Z. C. Seedorff, R. C. Patterson, H. J. Pangels, R. A. Eppler. Ceram. Eng. Sci. Proc. 13, no. 1/2, pp. 196–209. 1992.
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 that originates from air trapped between the particles of powdered glaze as the glaze begins to mature, or from gases evolved from carbonate compounds."Practical Suggestions For Control Of Glaze Defects. Pt. 2". H. Simonis. Keram. Z. 32, (10), 593, 1980.Pinholing in Sanitary Whiteware. E. G. Walker. The British Ceramic Research Association. TN20. 1962.
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