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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.
A second typical defect is from poor design and construction. An example of this would be a ceramic piece with a handle too thin to support the weight of the cup. A third manufacturing defect is careless firing: a ceramic piece that has been fired too rapidly or allowed to dry unevenly will crack or break. [4]: p.20
A parting and contraction of the glaze on the surface of ceramic ware during drying or firing, resulting in unglazed areas bordered by coalesced glaze. May be caused by uneven glazing, excessive glaze thickness or a greasy substrate. [8] Crazing A glaze fault characterised by the cracking of fired glazes and due to high tensile stresses.
Glass disease is caused by an inherent fault in the chemical composition of the original glass formula. [3] Glass contains three types of components: network formers establish basic structure, network stabilizers make glass strong and water resistant, and flux lowers the melting point at which the glass can be formed. [5]
The cavitation involved in crazing allows the material to achieve plastic strain faster. The presence of cracks or defects in bulky samples will favor the initiation of crazing, as these defects are points of high concentration of stresses and can cause the formation of initial microvoids. Crazes grow on the plane of maximum principal stress.
These deliberate glazing effects are usually known as "crackle", with crackle[d] glaze or "crackle porcelain" being common terms. It is typically distinguished from crazing , which is accidental craquelure arising as a glaze defect , although in some cases, experts have difficulty in deciding whether milder effects are deliberate or not. [ 10 ]
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 minimise the adherence of pollutants. It is used for decoration, to ensure the item is impermeable to liquids and to minimise the adherence of pollutants.
[1] [2] The most commonly used fluxing oxides in a ceramic glaze contain lead, sodium, potassium, lithium, calcium, magnesium, barium, zinc, strontium, and manganese. These are introduced to the raw glaze as compounds, for example lead as lead oxide. Boron is considered by many to be a glass former rather than a flux.