A B C D E F G J K L M N O P R S T U V W All
New version of this page available at digitalfire.com: Click here to go now
Words beginning with F
Firebrick
A 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.Flameware
Flameware is pottery that must withstand severe temperature changes without cracking (i.e. stove top burners). 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. Ovenware is not nearly as resistant to thermal shock as flameware.
Flameware bodies are typically made from as high a proportion as possible of low expansion minerals like spodumene, kyanite, mullite, and pyrophyllite. Theoretically 100% of these minerals is the ideal but for practical purposes clay and other flux content is also needed.
Flameware cannot normally be glazed because it is not possible to make a glaze of low enough expansion not to craze.Flocculate, flocculation
The opposite of deflocculation. The process of making a ceramic glaze or clay slurry which would otherwise be thin and liquid into a gel. This is typically done to improve suspension properties or allow application 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.Flux
A substance that lowers the melting or softening temperature of the mix or compound in which it is present. The degree of melting and interaction that occurs depends on the particle size of the powders present and the melting temperature of the individual particles. It also depends on whether material particles present are premelted and whether they soften or melt suddenly. Fluxing oxides are those of the RO group and include ones like K2O, Na2O, CaO, Li2O, MgO. B2O3 is actually considered a glass former but it is also regarded as a flux by virtue of its low melting temperature.Formula
A 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.Frit
A 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 homogenous 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.
Much more information with complete interlinking to many related Copyright 2003 http://digitalfire.com, All Rights Reserved Please support http://ceramicmaterials.info to improve this library |  |
INSIGHT is ceramic chemistry |