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Decal
A 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.
Decomposition
Most 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!).
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Deflocculate, deflocculation, deflocculant
The 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.
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Deoxylidration
Deoxylidration is elimination of oxydriles (OH-) from chemical composition by water evaporation.
Devitrification, Crystallization
Crystals can grow in cooling glaze melts if one or more of the following conditions are present: the glaze melt is fluid, cooling rate is slow, oxides that like to form crystals are present (e.g. ZnO, TiO), oxides that can form crystals are present in high proportions (e.g. CaO), oxides that stiffen the melt are not present or present in low percentages (e.g. Al2O3, Zr, MgO). Crystals are normally silicate or borate compounds, thus SiO2 and B2O3 need to be present in significant amounts. Crystals can be seeded by incorporating them in a glaze batch. Glazes not normally prone to crystallization can sometimes be partially crystallized by slow cooling and glazes prone to crystallization can often be quickly cooled to prevent it.
Crystallization can be highly decorative but is difficult to maintain consistency and is only used in one-of-a-kind artware. Unwanted crystallization occurring in a glaze during cool-down in the firing is called devitrification, it spoils gloss surfaces and can be a real plague to industry. It can be dealt with by faster cooling, higher Al2O3, switching some CaO for MgO, reducing B2O3.
Very glossy or well-melted glazes can be subject to this because they likely either contain a lot 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. The chemistry of the host glaze is the key factor since it determines the amount of melting and the presence of oxides that can impede crystallization (e.g. Al2O3, MgO). Many mattes are simply glossy glazes in which the entire surface has been invaded by micro-crystals. Purely decorative highly crystalline glazes are high in Na2O and thus almost always craze badly.
A dramatic example of crystallization can be demonstrated by melting (and cooling) a powdered mix of 50:50 Ferro frit 3134 and cobalt oxide in a crucible at cone 6. The frit is a very active melter (it contains no alumina) and the cobalt is also an active melter, together they can work real magic!
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Pictures
The variegating effect of a thin layer of titanium dioxide (outside of bowl) on GA6-D Alberta Slip glaze at cone 6
Example of variegation and phase separation with about 5% rutile added to a dolomite matte cone 10R glaze.
Crystallization in a high MgO matte at cone 10R
Crystallization of rutile is completely subdued using Ferro frit 3249 (20% with Alberta Slip) on the right (the left is frit 3134 20%)
crystalline and vitreous silica matrix
Example of a crystalline glaze
This high boron cone 04 glaze is generating calcium-borate crystals during cool down
An example of the same cone 6 high iron (9%) glossy borate glaze slow cooled (right) and free-fall cooled (left). Notice how the iron silicate crystals have invaded the surface.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
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. They are often associated with pinholes. Dimples are not always immediately evident if the glaze, closer examination is needed. Dimples are considered a glaze imperfection and are usually caused by firing too quickly or by particulates in the body and/or glaze (which generate gases during firing). Industrial producers pay great attention to the particle sizes of their body and glaze materials and process them to whatever size is necessary to assure defect free glazes (even wet ball milling, filter pressing and spray drying if needed).
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Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light.
Dimpling, Orange Peel
A 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.
Dolomite Matte
Reduction fired cone 10 glazes that have a pleasant-to-the-touch silky feeling matte surface. This surface is a product of localized phase changes in the glaze melt associated with the sudden melting of MgO particles (discontinuities in the melt produce discontinuities in the glaze surface, this produces the pleasant silky feel). Since dolomite normally sources some or all of the MgO, these are often called dolomite mattes. Since this unique surface effect is a product of MgO sourcing minerals melting at a specific temperature, it is not possible to reduce the firing temperature of a specific dolomite matte. Rather, it is more practical to use mechanisms that produce the same surface effects at the intended temperature (e.g. refractory powders in fluid base, minerals that resist dissolution in the glass and melt suddenly at the intended temperature). Actually, MgO is a refractory at lower temperatures, so it can play a part in the development of these surfaces, but as a different mechanism.
Many dolomite matte glazes are improperly formulated and are not melted enough or have high percentages of feldspar and craze.
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Pictures
Dolomite matte glazed cone 10R mug courtesy of Susan Clarke.
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 Pressing
A 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.
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Drying Crack
When stresses are present within a drying ceramic item cracks can appear to relieve them. These stresses appear when a ceramic item does not dry evenly. If one part dries ahead of another it also shrinks ahead of the other. Thus when the latter part needs to shrink and dry a crack can appear to relieve the stress. Small drying cracks will normally grow during firing, especially if significant firing shrinkage occurs. Different types of clays (e.g. kaolin, ball clay, bentonite) have different characteristics (e.g. shrinkage rates, shrinkage curves, drying speed, ability to withstand the stress, dry strength, ability to terminate micro-cracks at pores and particles, etc). A common type of crack is called the 'S-crack', it is a signal that ware is being dried unevenly, contours are too angular or thickness too uneven, stress areas have surface imperfections that can foster cracking that might not otherwise occur, or the type of clay being used is too plastic. Fast even drying will get better results than slow uneven drying.
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An example of an S-crack in the bottom of a porcelain mug.
DFAC dried disk showing soluble salts and drying performance crack typical of a plastic pottery clay body.
Example of how simple placement of mugs during drying can prevent cracking of handles
Drying Performance
Refers 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.
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DFAC dried disk showing soluble salts and drying performance crack typical of a plastic pottery clay body.
Drying disks used for the DFAC test to measure the drying performance of a clay
Dunting
Cracking 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.
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Example of a dunting crack in a cone 6 porcelain
Dust Pressing
A 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.
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