Fast Fire GlazesFast fire glazes are used in most industries now and many can fire up and down in less than two hours. Traditional alkali and boron glazes melt too early and gases of decomposition from the body cause them to bubble. Fast fire glazes thus need to melt late and quickly. Search for the term "fast fire" in the materials area to find frits intended for this purpose. This will help you to learn about the chemistry of fast fire glazes. Generally, they have much lower boron and sodium and higher zinc, calcia and silica.Out Bound Links
Feldspar GlazesQuite simply, glazes high in feldspar. Feldspar by itself melts well at high temperatures but it needs additions of other fluxes and silica to produce a balanced glaze that does not leach. The process of comparing the chemistry of a feldspar to a target formula for a typical medium or high temperature glaze, and adding materials to bring it into line, is quite fascinating. Since feldspar melts so well, it is common to find reactive glazes (ones with interesting visual surfaces) that contain high percentages, even up to 70%. However, since feldspar contains so much alumina, these glazes typically have almost no clay (since its presence would add alumina and destroy the active melting nature). That means they have poor slurry properties (e.g. settling, dusting, flocculating, running). These situations can be fixed using ceramic chemistry by supplying the Na2O/K2O from a low alumina material (eg. a frit) thus enabling an increase in the amount of clay in the recipe. Out Bound Links
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FirebrickA brick capable of withstanding high temperatures without deforming. 'Insulating firebricks' have the additional advantage of acting as good insulators due to the large pockets of air in the matrix of the brick. There are many different kinds of firebricks available, some very expensive. Types are categorized for their heat duty and the types of materials and atmospheres they must come into contact with.FlamewareFlameware is ceramic that can withstand severe temperature changes without cracking (i.e. stove top burners). Ovenware is another class of ceramics, it is not as resistant to thermal shock as flameware.
Ceramic is much more susceptible to thermal shock failure than most other materials because of its brittle nature, lack of elasticity and tendency to propagate cracks. Thus the creation of true flameware requires compromising things like plasticity and vitrification. Non-vitreous flameware bodies can be made using high a proportions of a low expansion material like kyanite, mullite, pyrophyllite or molochite (powder or grog) plastic-bonded with a small amount of clay or organic binder and fire-bonded with a glass producing flux. Of course, if the particles of these materials are altered or taken into solution in the glass bonder (e.g. feldspar) then the low expansion character of their natural state is lost.
True flameware cannot normally be glazed because it is very difficult to make a glaze of low enough expansion not to craze.In Bound Links
Flocculate, flocculation, flocculantThe opposite of deflocculation. The process of making a ceramic glaze or clay slurry that would otherwise be thin and liquid into a gel. This is typically done to improve suspension properties or allow application of slips and glazes without problems of running and dripping. However flocculated slips have a high water content and thus a higher shrinkage. Common flocculants are calcium chloride, vinegar, epsom salts.
Glazes can change their viscosity with storage, when they thicken they are said to 'flocculate'. In these cases slightly soluble materials in the mix (e.g. nepheline syenite) can act to change the viscosity of the slurry.In Bound Links
Fluidity, Melt FluidityMolten glazes exhibit viscosity, that is, a tendency to run or to stay put. This is why matte glazes are referred to as stiff or viscous. The degree of fluidity is often compared using flow testers that have reservoir of glaze feeding onto an inclined runway. Glaze melt fluidity relates closely to a variety of problems like pinholing, crawling, gloss, blistering, etc. Logically, glazes for vertical surfaces will be more viscous that tile glazes, for example, which are applied to horizontal surfaces. Molten glaze viscosity can be understood in terms of molecular silicate chains (which also link across to other chains). The chemistry of the melt determines the rigidity of the structure and therefore the viscosity of the melt. The Potter's dictionary has a very good discussion with diagrams of this under the term 'viscosity'.In Bound Links
FluxOn the theoretical chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of others. Fluxing oxides are in the RO group and include ones like K2O, Na2O, CaO, Li2O, MgO. B2O3 is actually considered a glass former but it is also a flux by virtue of its low melting temperature. Fluxing oxides are active within specific temperature ranges and can actually be refractories if used at lower temperatures. The term can also be used on the material level, fluxing materials lower the melting temperature when added to mixes. Fluxes do not necessarily melt well by themselves. Dolomite, for example, can be dead-burned and used as a heavy duty refractory for ladles and slag furnaces. But in a stoneware glaze it is a strong flux! Understandably, predicting melting temperatures of mixes is complex (involving interactions, eutectics, proportions, premelting and the physical and mineralogical properties of the particles in the mix).Out Bound Links
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Forming MethodRefers to the method by which a ceramic component or object is created or manufactured. Common traditional ceramics forming methods include dusting/die pressing, jiggering/jolleying, slip casting, extrusion, ram pressing, throwing, etc. Forming methods in advanced ceramics also include isostatic pressing, tape casting, injection molding, green machining, hot pressing, hot isostatic pressing, diamond grinding. Choosing an appropriate forming method for a specific object is a big factor in achieving low costs coupled with high quality.Out Bound Links
FormulaA formula is typically used to evaluate the oxide content of fired glazes and glasses. Each value in a formula represents a number of oxide molecules and formulas are typically unified on the fluxes. Formulas do not usually show LOI because they are used to model the fired product and predict properties based on oxide content. A formula can be converted to an analysis by multiplying each oxide amount by the molecular weight of that oxide and then calculating percents.Out Bound Links
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Formula WeightQuite simply, the weight of a formula. Typically, in glaze chemistry, when we refer to formula weight it is assumed we are talking about the weight of the fired formula of a glaze (without LOI and volatiles). However is is possible to also talk about the formula weight of a material (although materials are normally evaluated as analyses). In this case, the weight specified includes the volatiles (e.g. CO2, carbon, CO, H2O, etc) that burn away during firing.Out Bound Links
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Simplistically, LOI is the amount of weight a mate...
FritA ceramic glass that has been premixed from raw powdered minerals and then melted, cooled by quenching in water, and ground into a fine powder. Huge quantities and varieties of frits are manufactured for the ceramic industry every year by dozens of different companies.
Although the fritting process is expensive there are many advantages to using frits in glazes, enamels, etc.
-To render soluble materials insoluble
Often very useful oxides (i.e. boron) are contained in high proportions in raw materials that are either slightly or very soluble. These normally cannot be used in glazes because they have adverse effects on the slurry's fluidity, viscosity, thixotropy, or make it difficult to achieve or maintain the desired specific gravity. In addition soluble compounds are absorbed into porous bodies during glazing and this compromises the body's resistance to bloating and warping and the glaze's homogeneous structure. Fritted mixes containing these materials renders them insoluble and inert.
-To improve process safety of toxic metals
Some materials contain undesirable and unsafe compounds. The fritting process drives these off. Many other materials are unsafe in the workplace and fritting decreases their toxicity for ceramic production workers. Lead is a prime example. Lead frits decrease the process toxicity of raw lead compounds. Barium is another example. However the fritting process has no effect on whether or not a fired glaze will leach or not. This is a function of its chemistry, unbalanced and unstable glaze formulas are just as likely with frits as without. The primary safety benefit for frits is thus for workers who use frits in manufacturing.
-To reduce melting temperature and improve melt predictability
Since frits have been premelted to form a glass, remelting them requires less energy and lower temperatures. Frits soften over a range of temperatures (in contrast to crystalline raw materials that melt suddenly) and lend themselves very well to production situations where repeatability and ease-of-use are necessary.
-To avoid volatilization of unstable substances
Most raw ceramic materials contain sulfur or carbon compounds as well as H2O. These vaporize at various temperatures as materials decompose and are driven off as gases during firing. This volatilization activity has a detrimental effect on the glaze surface and matrix. The fritting process drives off these compounds and glazes are thus much more defect free.
-To achieve homogeneity
Other than dissolution and very localized migration, fired raw glaze melts do not mix well to create an evenly dispersed oxide structure. The fritting process employs mechanical mixing to assure a completely homogeneous glass that will exhibit the intended properties.
-To achieve oxide blends that are difficult or impossible with raw materials.
Many glaze formulations cannot be achieved with insoluble raw materials (i.e. high borax, high sodium). Frits employ soluble materials to make almost any combination possible.
-Improve the quality of decoration
Over and underglaze colors work better with frits than raw materials because the former are cleaner, less reactive, melt evenly, and have a more closely controlled chemistry. This means colors are brighter by virtue of compatible chemistry, by better glaze clarity. Edges of colors also tend to bleed less and color quality is homogeneous rather than variegated (although variegating materials can be introduced to introduce this quality if desired).
The Frit market is driven by large customers who need certain formulations and by the prepared glaze industry. Availability of smaller quantities of frits are generally determined by what industry is using. Since the Frit market changes with time, so does the availability of frit types.
Some frit companies, such as Fusion Ceramics, freely supply the chemical analysis of their frits. Others such as Ferro are more guarded and either provide no chemistry or approximate analyses (although they were more forthcoming with this data in the past). The latter practice makes little sense since it partially defeats the whole purpose of using frits, namely, having control. It also works against the general trend of using ceramic calculations to take control of glaze properties.Out Bound Links
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Ceramic Frits
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