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Thursday, August 19, 2010




The breaking away of glaze from ceramic ware in consequence of too high a compression in the glaze layer; this is caused by the glaze being of such a composition that its expansion coefficient is too low to match that of the body. It is the opposite of crazing, as are the preventative steps: see Seger's Rule above. Peeling is also known as shivering.


Crazing is a spider web pattern of cracks penetrating the glaze. It is caused by tensile stresses greater than the glaze is able to withstand. Common reasons for such stresses are: a mismatch between the thermal expansions of glaze and body; from moisture expansion of the body; and in the case of glazed tiles fixed to a wall, movement of the wall or of the bonding material used to fix the tile to the wall. The cracks can allow the ingress of water, dirt and bacteria 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
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 which collect dirt. The surface of the glaze is very unpleasant and looks like a boiled mass of bubbles, craters and pinholes
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.
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.
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.

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