This page has been deprecatedPlease click the Glossary heading above for the new section.
Fast Fire Glazes
Fast 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. Fast fire glazes can also be formulated to form a crystal network early in the firing (from CaO or MgO) that is porous and stable to above 1000C (after which it collapses and melts 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, magnesia, calcia and silica.
Out Bound Links
Feldspar Glazes
Quite 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
In Bound Links
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 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.
Out Bound Links
In Bound Links
Flashing
A fired visual effect on bare clay surfaces in fuel burning kilns (especially wood). Clay surfaces that have been flashed have been subjected to a thermal history of variations in flame, ash, kiln atmosphere and even imposed vapors (like salt and soda). The degree to which these forces have varied determines the visual variation across the surface of the ceramic. Historical ceramics often had flashing simply as a consequence of the lack of control of the process of clay preparation, forming, drying and firing. In recent years there has been a focus on the reproduction of this rustic look, various methods seek to reproduce the process, others only the final product. A popular method is the application of slips having a makeup likely to react with the atmosphere or flame in the kiln. Slips of high alumina content, for example, are likely to react with an atmosphere containing ash (since the ash can be high in silica and soda). Likewise, a slip high in fine silica and alumina is likely to react with fumes of soda. Slips containing some iron will exhibit differing coloration where differing amounts of flame has touched.
Pictures
Flashing effect on a cone 10 wood fired sample.
Flocculate, flocculation, flocculant
The 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 Fluidity
Molten 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'.
Out Bound Links
In Bound Links
Pictures
Example of how iron turns to a flux in reduction firing and makes the glaze melt much more fluid.
Example of a cone 10 transparent that is running severely on a flow tester, but does run on actual ware. The glaze is cutlery marking (therefore lacking hardness). This, the running and likely leaching are due to extremely low SiO2, Al2O3 content.
Flux
On the theoretical chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of others. Fluxing is about interaction, temperature range and relative amounts: when both the flux and the oxide mix with which it interacts are present in the correct proportions in the right temperature range, the melting temperature of the mix is much lower than either of the two separately. Fluxing can also be about atmosphere, iron for example, is a strong flux in reduction yet very refractory in oxidation. Examples of fluxing oxides for high temperature glazes are K2O, Na2O, CaO, Li2O, MgO, ZnO (CaO and MgO are not active at lower temperatures). In glaze chemistry, each of these oxides is an individual with its own optimal percentage and interaction with silica and alumina. Fluxing oxides make up a minor part of the glaze, they interact with the SiO2 glass former and Al2O3 (and other fluxes). If used in this way, CaO, for example, reacts strongly with stoneware and porcelain glazes to lower their melting temperature. But this same oxide, as the material calcium carbonate, is refractory in a 75:25 mix with bentonite (where the conditions for interaction to produce a glass are not present).
Colorants can also be powerful fluxes. Copper, cobalt and manganese all melt very actively in oxidation and reduction, whereas iron is very active in reduction.
When the term flux is used on the material level, it is referring to the fact that the chemistry of the material contributes a significant amount of one or more of the fluxing oxides. These materials do not necessarily melt well by themselves. Dolomite, like calcium carbonate, is a stoneware glaze flux. But it does not melt by itself (it can be dead-burned and used as a heavy duty refractory for ladles and slag furnaces). Talc in small percentages in middle temperature clay bodies acts as a strong flux, whereas in large percentages it is refractory also.
B2O3 is a very low melting oxide, the ceramic industry depends very heavily on it. But B2O3 is not a flux, it is a low melting glass (it does not depend on percentage and interaction to activate, it works across the entire temperature range used in traditional ceramics). Almost all frits contain at least some B2O3.
Fluxing oxides in frits melt much better than in raw materials. MgO is an excellent example. Glazes that employ frit to supply the MgO melt much better than those employing dolomite or talc.
Understandably, predicting the effects of a flux addition to a glaze (e.g. melting temperature) is very complex (involving interactions, eutectics, proportions, premelting and the physical and mineralogical properties of the particles). For this reason, ceramic chemistry is applied much more in a relative sense than absolute.
Out Bound Links
In Bound Links
Pictures
At cone 10 CaO is a powerful flux. 10% calcium carbonate added to the under-melted bubble-laden celadon on the left transforms it into an fluid super-gloss with no bubbles.
Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and dry shrinakge vary widely. Materials normally acting as fluxes are refractory.
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).
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.
Foot Ring
Footrings, as opposed to flat bottomed containers, lift the piece off the table and enable glazing all of the bottom. While foot rings add extra effort to the finishing stage at fabrication, they make it easier to glaze the ware (articles can be dipped and quickly sponged to remove the glaze). On shallow foot rings are possible in machine made items whereas hand made pieces can distinguish themselves with much deeper rings.
Pictures
An example of a foot ring in a cone 10 reduction mug (tooled at the leather hard stage). It has channels to drain water in the dish washer.
Forming Method
Refers 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
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.
Out Bound Links
In Bound Links
Formula Weight
Quite 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
- (Glossary)
Formula
A formula is typically used to evaluate the oxide ... - (Glossary)
LOI
Simplistically, LOI is the amount of weight a mate...
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 (especially for tile) 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. This being said, some frit formulations require crowding the line solubility line, they are thus slightly soluble and over time can precipitate crystals into glaze slurries.
-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 more 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).
-Frits make it possible to create chemistries that result in phase separations during cooling. This produces special effects, matteness, opacity and specific mechanical properties that the homogenous glass does not have.
-Frits make it possible to create chemistries that can be fired much more quickly than raw glazes because they melt late (allowing body gases of decomposition to pass before the melt is created).
-The particle sizes and surface areas in highly fritted glazes can be more tightly controlled because only one species of particle is present. Particle dynamics are responsible in part or whole for certain glaze properties and effects.
-To produce a material that has a wide softening range (as opposed to a sudden melting temperature)
The Frit market is driven by large customers (especially tile) 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. This practice partially defeats a key purpose of using frits, namely, having control of chemistry. Infact, the lack of chemistry is a key disadvantage of using certain frits. For example, the frit manufacturer might recommend substituting part of one frit for another in a recipe to solve a specific problem (like crazing). The problem with this is that the new frit might have a chemistry that is hostile to the pigments being used, the degree of gloss, the hardness, resistance to devritification, etc. Without the chemistry the new frit can be a bit of a pandora's box. Lack of frit chemistry information works against the general trend of using ceramic calculations to take control of glaze properties.
Out Bound Links
In Bound Links
- (Glossary)
Borosilicate
A silicate is an SiO2-centric solid (crystalline o... - (Typecodes)
1: FRT - Frit
- (Project)
Frits
The number of different frits in the world can be ... - (Glossary)
Flux
On the theoretical chemistry level, a flux is an o...
Pictures
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).
Example of how a frit softens over a wide temperature range
|
