Glaze faults

US term for a glaze fault revealed by a reduction in gloss, and caused by surface devitrification. 

Dulling. A glaze fault characterized by the ware having poor gloss when drawn from the kiln the cause is surface devitrification, which may result from factors such as sulphuring (q.v.) or too-slow cooling. 

Bone China has also for long been available of eggshell thinness. Eggshelling. (1) A glaze fault resulting in potteryware coming from the glost-kiln with an egg-shell appearance. The fault is caused by gas bubbles that have burst... 

Feather End. See feather brick. Feather Side. See feather brick. Feathering. A glaze fault caused by devitrification. It is particularly liable to occur in glazes rich in lime. To prevent the fault the initial rate of cooling in the kiln, after the glaze has matured, should be rapid. (See also feather.)... 

Dry Edging. A fault sometimes occurring in pottery manufacture as a result of insufficient glaze application it is shown by rough edges and corners. Dry Gauge. See drag-ladle. 

Green Spot. A fault that occasionally becomes serious in the manufacture of sanitary fireclay and glazed bricks. The green spots are comparatively large and frequently of an intense colour. The usual causes are the presence of chalcopyrite (CuFeS2) in the raw clay or accidental contamination by a particle of copper or copper alloy, e.g. a chip off a bronze bearing. 

Over-glazed. Pottery-ware having too thick a glaze layer, particularly on the bottom this thick glaze is likely to be crazed. Causes of this fault are incorrect dipping, the use of slop glaze of too high a density, or biscuit ware that is too porous, i.e. underfired. 

Scalding. A term that has been used to describe the fault in the glost firing of pottery when glaze falls off the ware before it has fused a cause is too great a difference between dimensional changes of body and applied glaze. 

Silver-marking of Glazes. Silver 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. 

White Dirt A fault sometimes occurring during the manufacture of tableware as a result of the protrusion of white particles through the glaze usually, if the glaze layer is adequately thick, any such dirt is hidden. Sources include the bedding medium (alumina or sand), fragments of the body itself, and scale derived from hard water. 

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