|Monthly Tech-Tip |
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
An oxide is a combination of oxygen and another element. There are only about ten common oxides that we need to learn about (most glazes have half that number). CaO (a flux), SiO2 (a glass former) and Al2O3 (an intermediate) are examples of oxides. CaO (calcium oxide or calcia), for example, is contributed by calcium carbonate, wollastonite and dolomite.
The heat in the kiln "decomposes" materials into their oxides. This decomposition breaks the bonds that held the oxides together in their materials creating a liquid "melt" (or mixture) of unbonded oxide molecules (ready to form a new compound, normally a glass, when the kiln cools). This phenomena occurs because the molecular bonds that hold oxides together are stronger than those that hold oxides one-to-another.
There are direct links between the oxide chemistry of a glaze and its fired presence, each oxide imposes specific properties (according to its concentration) on the fired glass. Its effects can be modified by interactions. Glaze chemistry is about looking at mixtures of raw material powders as if they were already the fired glaze. It does an inventory of all the oxides being contributed by all the materials, discards the ones that will gas off during firing (e.g. CO2, SO4, H2O) and tries to visualize what glass will be like.
A glaze "chemical formula" expresses relative oxide molecule numbers in the fired glass (a "chemical analyses" expresses the concentration of each oxide, by weight of molecules, in a material). Oxides are divided into three categories that recognize their functions. There is a correlation between the amount of oxygen in each class and the contribution that class of oxide makes. Fluxes are designated RO, intermediates R2O3 and glass formers RO2. When the links between how a glaze fires and its oxide formula are well understood, the glaze formula can be manipulated to produce the desired changes in the fired properties.
"Target Formulas" (or limit formulas) characterize the balances of oxides in a type of glaze (e.g. low temperature, crystalline, matte). However limit formulas are general guides only, they express what is likely to melt into a usable glaze.
Glazes for lower temperatures have more flux. We see that in unity formulas as lower SiO2 and Al2O3. Some of the properties that oxides contribute can be quantified. Thermal expansion is an example. Na2O, for example, has a very high expansion. SiO2 is low. We know the thermal expansion of each oxide, thus it is fairly simple to tabulate their expansions, according to percentage, to predict the thermal expansion of the glaze (high expansion glazes craze). When the SiO2:Al2O3 ratio is high (e.g. 12:1) the glaze will likely be glossy (matte when low). If MgO is 0.3 or higher (in a unity formula) the glaze will likely be matte. If Al2O3 is low the glaze melt will be runny. If SiO2 is also low it will melt at a low temperature.
Recipes show us the materials in a glaze but formulas list oxide molecules and their comparative quantities. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get the oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. You can view glazes as recipes-of-materials or as formulas-of-oxides. Why use formulas? Because there is a direct relationship between the properties a fired glaze has (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and the oxides it contains (links between firing and recipe are much less direct). There are 8-10 oxides to know about (vs. hundreds of materials). From the formula viewpoint materials are sources-of-oxides. While there are other factors besides pure chemistry that determine how a glaze fires, none is as important. Insight-live automatically shows you the formulas of your recipes and enables comparing them side-by-side. Click the "Target Formula" link (on this post at digitalfire.com) to see what each oxide does.
Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
Cations having high charges and small radiuses (and thus high field forces like boron and silicon), are network formers. Network modifiers have small charges, large radiuses and a big coordination number for oxygen ions. While considering ions as rigid spheres is an over-simplified way to describe reality, it has still proven useful to describe characteristics of each. For instance lithium has a ionic radius smaller than sodium and so it can locate into smaller cavities. The ionic field force of lithium is also stronger than sodium and it is essentially non-directional, thus it more easily produces crystals of a separate phase. Alkaline earth elements locate into cavities of the network as well, but they have double charges and thus act like a bridges between two oxygen ions (preventing the three-dimensional network from being fully destroyed). Moreover bonds between alkaline earth ions and oxygen are stronger than alkaline so we observe neither a rapid decrease in viscosity or a significant increase of the thermal expansion coefficient. It is notable that for similar molar percentages, frits containing magnesium crystallize more easily than frits containing calcium.
Aluminum, titanium and zirconium are classified as intermediate glass formers because they have a strong four-way coordination for oxygen ions, like silicon. Thus, for these oxides, we do not observe any interruption of the three-dimensional silicate-based network. For a better understanding consider more details about aluminum, boron, zirconium and titanium.
Aluminum: Usually aluminum shows a four-way coordination when it acts as a glass former, tetrahedrons are linked to four oxygen atoms while the local excess of negative charge is counterbalanced by an alkaline cation placed close to the aluminum ion. Thus additions of aluminum to a glass help to stop alkaline ions from breaking the three dimensional network of the glass. This produces the characteristic lower melt fluidity and tendency to crystallize and also reduces the thermal expansion and thermal stability. One downside to alumina is that it contributes to a higher viscosity of frit batches during melting (in the furnace tank) making homogenization more difficult. Usually the percentage of aluminum oxide is in the range 4 – 12%.
Boron: Boron is a basic component of frits yet its characteristics are so peculiar that it cannot easilly be compared to other elements. Boron, like aluminum, exhibits a four-way coordination when forming a glass network (being in the center of a tetrahedron of oxygen ions). This is possible only when the molar alkaline percentage is less than 30-40% because above this limit boron has three-way coordination, forming triangles.
Another peculiar characteristic is that boron is not just dispersed as tetrahedrons or triangles in the network of silica tetrahedrons. Rather it forms boric groups, containing from 3 to 5 boron atoms and the groups are randomly dispersed in the glassy matrix. However single BO3 triangles and BO4 tetrahedrons are always present. For quenched frits the presence of these groups is likely minimal but we can presume they form again when glazes containing the frit are fired (there are experimental evidences demonstrating this).
Boron oxide is an important component of low melting frits because it increases fusibility without a proportional increase in thermal expansion. Moreover boron oxide and sodium borate, due to their low melting point, are useful during smelting of frits because they form a glassy matrix early and act as catalysts in the melting and dissolving of other materials.
Zirconium - Titanium: Their influence on surrounding oxygen ions is very strong and scarcely directional so their solubility in frits is poor. Their solubility in glass and actions as glass formers are proportional to temperature. In quenched frits they remain in the dissolved in the glassy matrix but when we fire them again (within a glaze), these oxides easily precipitate crystal compounds.
|Oxides||MgO - Magnesium Oxide, Magnesia|
|Oxides||Na2O - Sodium Oxide, Soda|
|Oxides||K2O - Potassium Oxide|
|Oxides||B2O3 - Boric Oxide|
|Oxides||SiO2 - Silicon Dioxide, Silica|
|Oxides||KNaO - Potassium/Sodium Oxides|
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
|Oxides||CaO - Calcium Oxide, Calcia|
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
In ceramics, raw material chemistry is expressed a chemical analyses. This is in contrast to fired glaze chemistries which are expressed as oxide formulas.
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization
Glaze chemistry is the study of how the oxide chemistry of glazes relates to the way they fire. It accounts for color, surface, hardness, texturem, melting temperature, thermal expansion, etc.
Refers to a group of ceramic fluxing oxides that contribute similar properties to fired glazes. They contrast with the alkalis which are stronger fluxes.
In ceramics, the chemistry of fired glazes is expressed as an oxide formula. There are direct links between the oxide chemistry and the fired physical properties.
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
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