A B C D E F G J K L M N O P R S T U V W All
Words beginning with C
Candling
The practice of slow firing ware through the critical temperature surrounding the boiling point of water. This is done to prevent cracking and explosions associated with steam trying to vent out of ware which is either not completely dry or has a thick cross section. Kilns are often candled overnight on very low heat and then the firing is continued in the morning.Carbon trap glazes
Glazes with variegated patterns of grey and black from carbon trapped below the surface.
The effect is created by fuel firing without adequate oxygen in early stages to build up soot (carbon) on the surface of ware. As the firing continues, the carbon trap glaze begins to melt before the carbon sitting on the surface burns away. Carbon is a refractory material and will stay in a glaze as long as there is no oxygen to combine with it. Typically this type of glaze includes soda ash or other soluble alkaline fluxes which will migrate to the surface of the raw glaze as it dries, forming a crust of alkalis which will melt earlier than the rest of the glaze, thus facilitating the carbon trapping.Casting, slip casting
Forming pottery by pouring deflocculated (water reduced) clay slurry into plaster molds. The absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks slightly and can be removed from the mold. There are many high-tech variations to this process and a wide variety of materials can be cast to make tiny and very large shapes. The Magic of Fire book has a chapter on understanding casting slips.Celadon
An green or blue-green firing reduction high temperature glaze that has been stained using iron.Ceramic
A man-made solid produced by the fusion of mineral substances in a kiln.
The term 'ceramic industry' or 'pottery industry' are subjective terms that can mean different things in different circles. In recent years the field of non-oxide ceramics has become popular, thus the term 'ceramics' now generally refers to thermally treated, non-metal, non-gaseous products like glass, sanitary ware, spark plugs, porcelain, abrasives, etc.Coatings, ceramic coatings, rfc
Refractory ceramic coatings are sprayed onto the insides of kilns and on elements to achieve several goals. Coated elements are claimed to last up to 10 times longer. Coated kilns are more efficient and last longer. Coating technology has been developed into successful commercial products by Feriz Delkic ( pronounced Ferris) of International Technical Ceramics, Inc. Although ceramic coatings are expensive, they do save money in the long run.Coe, co-efficient of thermal expansion
A measure of the reversible volume or length change of a ceramic material with temperature. The more it expands during heating the more it contracts while cooling down. Glazes that do not have a similar thermal expansion to the body cause problems like crazing, shivering, and weakened ware.Colloid
Colloidal particles are so small and light that they do not settle in water. The movement of water molecules is enough to keep them in suspension. It is important to remember that colloidal particles occur in a suspension, not a solution. A simple way to tell the difference is to shine a beam of light through the liquid. If you can see the beam it is a suspension.Cone
A pyramid-shaped ceramic device used to quantify the amount of heat delivered by a kiln. These devices are formulated from different mineral mixtures and numbered accordingly. They are placed in a kiln so they can be viewed during firing and when a cone begins to bend it is closely monitored and the firing is terminated when it reaches a specific position.Cone plaque
A stand or rest for hold cones during firing. Plaques are important to assume that cones are placed at a consistent depth and angle firing after firing.Controller
An electronic device attached to a kiln. Controllers are usually capable of firing a kiln to a specific schedule and can shut it off at the right time, soak it for a specified period, and cool it down at a controlled rate.Crackle glaze, craquele
A type of glaze that is intentionally crazed. Stains and other colorants are often rubbed into the crack lines to heighten the effect. Crackled glazes typically severely weaken ceramic ware, especially if it is thin, low fired or porous.Crank, plate setter
A special kiln shelf that has legs (usually three) and stacks by interlocking with others. Cranks are used to fire plates and tiles (one per crank). They are employed to overcome poor use of space in kilns when trying to fire flat objects.Crawling
A condition where fired glaze separates into clumps or islands leaving bare clay patches showing in-between. More prevalent in once fired ware. There are many causes for crawling (typically glazes shrink too much during drying and don't have a good bond with the bisque) in the Magic of Fire book.
Some times glazes are made to crawl intentionally. One technique to make this happen is to add 15-20% magnesium carbonate (testing required to determine amount) to a low fire transparent glaze.Crazing
Small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. A glaze of higher expansion shrinks more than the clay to which it is attached and therefore crazes.
There are many treat-the-symptoms approaches to crazing but the bottom line is: If there is a thermal mismatch it will reveal itself sooner or later no matter how you adjust firing or glaze thickness to hide the problem. If crazing is visible, it is an indication of a significant problem. This is because long before crazing becomes visible, serious strength problems result where glaze and clay are not expansion-compatible.
See the Magic of Fire book for many chapters on dealing with crazing. Ceramic calculations are very useful in dealing with crazing and the INSIGHT manual has an example of dealing with a specific problem.Crystalline glazes
Crystals form in glazes during cooling depending on the oxides present. All glazes have the potential to crystallize if cooled slow enough. Crystalline surfaces form well in fluid glazes low in alumina because it is easier for the component oxides to migrate to the site of formation. Glazes prone to crystallization have a distinct "zone of crystallization". If the kiln is cooled slowly during this critical period of freezing they will grow.
Most crystals have a halo of a different color than the glaze. This surrounding area is reduced in crystal forming oxides and is thus a 'depletion zone'. In theory larger crystals should grow at the expense of smaller ones in a 'survival of the largest' situation.
Crystals demonstrate the phenomenon of phase separation, where a glass melt separates into two or more liquids. Coloring materials tend to preferentially and selectively gather at one of these, (it is not uncommon to see a crystalline glaze where one coloring oxide colors the crystals, another the glassy areas).
Crystal formation can be considered a mechanical imperfection in the glass since it is disrupting the homogeneity of the matrix and imposing discontinuities between glass and crystal phases.
For the best crystals, slow the firing at the peak to make sure all materials are fully dissolved in the melt and then cool to the point where the crystal forming material precipitates out into crystals floating on the surface. Once experience reveals at what temperature they grow best you can soak the kiln there for maximum benefit. Some people even reheat the ware to just below this temperature and they will grow further.
Some materials, especially those with high melting temperatures, seed crystals (give them a place to start) in melts with a chemistry that can supply the seed material. A good example is the formation of a calcium silicate matte with the addition of wollastonite (calcium silicate). Zircon materials and tin are other examples. Other materials will crystallize well if oversupplied.
An accurate electronic kiln controller is must to make results repeatable.
For more information see also:
The Studio Potter, Dec '96, Vol #25, #1, pg. 39-49
Ceramics Technical, Nov '96, #3"
Glazes for Special Effects, book by Herbert Sanders (1974)
Ceramic Review #137, Sept-Oct 1992, pg. 27-31
An article was advertised in the back of C.M. that sells for around $12.00 U.S. The potters name is Dan Turnidge, River Rock Clay Co., P.O. Box 3183, Salem, Oregon 97302 Phone (503) 581-3606
There are also web pages on the Internet dedicated to the production of crystal glazes. The can be found through the search engines.
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