DecalA method of printing designs using ceramic inks onto specially prepared paper (a glue, then a base coat of clear material is applied followed by the inks). The design on the clear material is then transferred to glass or ceramic. To apply the decal you put it in water for a minute or so to loosen the glue enough to slide the clear decal onto the ceramic surface (slightly porous paper is used so that water can soak through it).
Decal ware must be fired to precise temperatures to develop and mature the color properly. The are many mistakes to be made in application and firing that will compromise the quality of the final product.DecompositionMost materials used in ceramics do not just simply melt when fired in a kiln, they go through one or more state changes. Most often these changes result in loss of 'volatiles'. Clays, for example, typically lose about 5-8% (can be up to 12%) of their weight during firing as CO2, H2O, SO3. Calcium carbonate loses 45% as carbon dioxide gas. Materials often generate different gases at different stages of their decomposition. Assessment of the suitability of a material in a body or glaze must include this information. Many materials are impractical for use as body and glaze materials because of the way they decompose and the temperature at which they do so (decomposition is also accompanied by physical state changes, for example, they can expand). For example, hydrated lime is a good source of CaO, but 25% of its weight is converted to water at 500C (to say this would be an inconvenient event in a firing would be an understatement!).In Bound Links
Deflocculate, deflocculation, deflocculantThe process of making a clay slurry that would otherwise be very thick and gooey into a thin pourable slurry. Deflocculants (or electrolytes) are liquids or powders added in small amounts and they work their magic by imparting electrical charges to clay particles making them repel each other. It is the opposite of flocculation.
To deflocculate a slurry properly it is very important to be able to measure its specific gravity and viscosity accurately. Yet it is very common for slip casters to be tied to a recipes and have little understanding of how to control their slip. Many will work for years with substandard slip without knowing it, others will throw away all scrap rather than reprocessing it simply because they do not understand slip rheology.
It is common for potters to mix slips using clays intended for modeling or sculpture. Far better casting mixes can be made using mixes of materials that emphasize permeability instead of plasticity. Once you have used a slip properly formulated and deflocculated for casting you will never go back to using an inadequate slip.
Sometimes glazes are deflocculated to reduce their water content, this is most likely where glaze is being applied to once-fire ware.
Common deflocculants are sodium silicate, Darvan, Calgon.Out Bound Links
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Devitrification, CrystallizationUnwanted crystallization occurring in a glaze during cool-down in the firing. This spoils gloss surfaces. Very glossy or well-melted glazes can be subject to this because they likely either contain alot of SiO2 (which combines with other oxides to form silicate crystals) or have a very fluid melt (which enables crystals greater freedom to form). When devitrification is desired it is simply called crystallization.Out Bound Links
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Dimpled glaze'Dimpled' glaze surfaces are those that have tiny holes that do not go down to the ceramic underneath (a pinhole). These holes look as if they were produced by a pin-point being pressed into the surface of the glaze when its melt is very stiff. Dimples are considered a glaze imperfection and are usually caused by firing too quickly.Dimpling, Orange PeelA glaze defect where the glaze surface has not flattened properly. Other than the obvious cause of under firing, the problem can also occur when bubbles entrained in an overly thick glaze matrix push the surface up at each bubble site.
Pictures
Dimples occurring in double-thick section of a glaze
Dolomite MatteReduction fired cone 10 glazes that have a pleasant-to-the-touch silky feeling matte surface. This surface is a product of phase changes in the glaze melt associated with the sudden melting of the MgO from the dolomite (or talc). Many dolomite matte glazes are improperly formulated and are not melted enough.Out Bound Links
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- (Articles)
 G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes These starting recipes use no frits and work in oxidation/reduction and are inexpensive to make. The... - (Properties)
Glaze Variegation
In contrast to the typical homogeneous surfaces of... - (Properties)
Glaze Matteness
Matte glazes are the opposite of glossy ones. They... - (Glossary)
Silica:Alumina Ratio (SiO2:Al2O3)
The ratio of silicon dioxide to alumina oxide is o...
Pictures
Classic dolomite glaze at cone 10 reduction on a speckle producing clay body (10R). The magnesia flux in dolomite creates a silky matte surface.
Cone 10R dolomite matte glaze with 5% manganese dioxide
Double Charge Dust PressingA dust pressing manufacturing process where one layer of porcelain powder is overlaid by another powder and the two are pressed into the mold together. This method is advantageous to produce a higher quality or more expensive surface using a less expensive base. It is also possible to fabricate and glaze ware (e.g. tableware) in one manufacturing step using this process. Manufacturers typically face challenges in matching the thermal expansions of the two materials, when the match is not right the tiles fire convex or concave.In Bound Links
Drying PerformanceRefers to the ability of a clay to dry without cracking. Lab results for drying performance and drying shrinkage give a much more complete picture than drying shrinkage alone. Clay with lower drying shrinkage normally dries well (without cracking) but can also dry poorly if it lacks strength. Likewise, clays with higher drying shrinkage normally dry poorly, but they can also be made to dry better by increasing dry strength or adding aggregate or fiber. Drying performance tests can be done in simple ways and they normally accelerate the drying of one section of a sample while slowing down water release in another section, this sets up a situation where the rigid section resists shrinkage of the undried section. Differences in the type of failure provides opportunities to rate one clay against another.Out Bound Links
DuntingCracking that occurs in ceramic ware that is cooled too quickly. Dunting can exhibit itself as simple hairline cracks or ware can fracture into pieces. Ware of uneven cross section, ware with glaze that fits poorly, or large pieces (i.e. large flat plates) are often subject to dunting. Ware with high amounts of cristobalite or quartz undergoes sudden volume changes when heated or cooled through the inversion temperatures of quartz.In Bound Links
Dust PressingA method of fabricating ceramic objects (typically flatware or tile) where powder of controlled water content (typically 6-8%) is pressed under high pressure (e.g. 500 kg/cm2) into metal molds. This method of production requires minimal drying facilities and lends itself well to mass production in a continuous process. In recent years companies have learned how to press and fire tiles of more than one square meter. The tile sector of ceramic industry is by far the largest and it has seen amazing automation and innovation in recent years. Large tile manufacturing plants measure their output in square kilometers. The largest tile producing countries include Spain, Turkey, Italy, Vietnam but dozens of countries around the world produce tile. Out Bound Links
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