|Monthly Tech-Tip |
One can look at a ceramic material from a mineral, physical or chemical standpoint. Each viewpoint is appropriate depending on the context, understanding this is a key to exploiting materials properly.
Nomenclature refers to the process of naming and creating a set of terms and symbol conventions. The subject is of concern to us because there are several ways of presenting the chemistry of materials and glazes.
As we have seen, ceramic materials are made up of elements (from the periodic table we took in high school science). Their names are familiar and their symbols are well known (for example, Si is silicon and Al is aluminum). However, to satisfy electrical charges they possess, these elements oxidize (combine with atoms of oxygen) at the first opportunity (for example, the rusting of bare metal is a form of oxidation).
It is either impractical or impossible to process ceramic related metals or elements into powders because the huge surface area of a powder guarantees quick oxidation. Once a ceramic element has oxidized, it becomes stable and we cannot easily decompose it (we can't fire it high enough to get it completely apart). Some elements will take on varying numbers of oxygen molecules and these forms vary in stability. For example, the metal Fe prefers to oxidize to Fe2O3 but it will also take on the less stable forms of FeO and Fe3O4. 'Less stable' means that given the right heat, time, and atmosphere, they will convert in a kiln.
From the viewpoint of making glazes, it is convenient to consider ceramic materials as simple 'sources of oxides' although, in many cases, they do not exist as such in the raw material. We justify this by the fact that during melting two things can happen with a material to make oxides available for reorganization and rebuilding by the kiln God:
During normal oxidizing, firing oxides are liberated as the melt develops. When the melt is complete, the oxides reach equilibrium (they "swim around" in the fluid glaze melt undergoing no further change). We can rely on typical ceramic temperatures and atmospheres to maintain glaze melts in which oxides do not further decompose into their basic elements.
We should note that the natural state of oxides in a glaze melt can be disturbed by a reducing atmosphere: oxide molecules which expose them-selves at the surface of the melt lose some of their oxygen to oxygen-greedy CO (subsequent oxidizing can change them back).
This oxide viewpoint allows technicians to most often ignore elements which become volatile and boil away as gases during the glaze melting, and simply classify them as LOI. For example, many clay materials contain H2O in their crystal structure which is lost during the early stages of firing. The term "decomposition" is fitting for this process. For example, as gypsum is heated it passes through three discrete decomposition temperatures where water molecules are driven off.
When ceramic oxides are quickly cooled and frozen in the kiln, a glass is produced. A glass is fundamentally different than its more cantankerous cousin, the crystal. When typical glaze melts cool there is no time for molecules to arrange themselves in an orderly lattice to solidify the crystal-line structure they might prefer. This is very unlike situations in nature when rocks can cool over a period of decades or even centuries. Since glasses don't usually exhibit the volatile physical properties (i.e. expansion, phase changes) of crystalline materials, they tend to display predictable characteristics that are a compromise of the constituent oxides.
Now we can better appreciate why frits are so nice, even user friendly! They are powdered glass, not powdered crystal; they are like storehouses of oxides; they are stable, reliable, and predictable. Thus, a frit manufacturer presents them as an "inventory" of the oxides. A popular Ferro frit is offered as 46.5% SiO2 , 23.1% B2O3 , 10.3% Na2O, and 20.1% CaO.
However, raw materials must be considered from a mineralogical viewpoint. Most minerals have a well understood lattice; that is, scientists have described the geometry of their molecular structure. This structure is usually fundamental to a material's physical presence.
Corundum, sapphire, and ruby, for example, have the same chemistry, in that they are all alumina, but they have very different mineralogies and this provides the key to understanding their physical properties. When you receive a bag of raw material, it is much more than just a powdered collection of oxides like a frit. Each microscopic granule is often a crystal which duplicates on a small scale any properties that we can measure from large chunks of the material.
Thus, ceramic chemists working on the material level arrange the chemical symbols to emphasize and explain this atomic structure. Properties like hardness, sudden volume changes during heating, solubility, plasticity, and chemical changes during firing can all be rationalized in terms of the chemical structure of materials. For example, silica added to a glaze reduces its expansion, added to a body it increases the expansion. Why? Because the silica dissolves in the glaze to form low expansion silicates with other oxides and then cools to form a 'docile' glass. In the clay body, the crystals of silica mineral do not decompose but act as an aggregate (like rocks in concrete) and they impose their natural mineralogical high expansion on the fired body.
Consider the material borax. To a materials scientist at a borax company it is Na2B4O7 · 10H2O. This format does not emphasize its chemistry, but suggests something about its mineralogical structure. Borax has an incredible number of interesting properties that make it useful for many non-ceramic purposes. The scientist seeks to explain these properties primarily in terms of its crystal structure and could use the tools in an account at insight-live.com to document the material's physical properties.
To a frit maker, borax is 16.3% Na2O, 36.5% B2O3 , and 47.2% LOI. It is simply a 'warehouse' with two oxides in stock and another inseparable one (LOI) that must be 'taken on all shipments but guaranteed to be lost during shipping'. In other words, borax supplies Na2O and B2O3 to the frit, but the LOI portion is lost during firing.
However, let us move further down the line to the glaze chemist using the frit. He sees these oxides in terms of what they will do in the final fired glaze. To him or her, the borax in the frit is Na2O · 2B2O3. He doesn't evaluate glaze properties on their crystal structure or mineralogy because they don't have one. Rather, he must evaluate fired properties based on 'oxide demographics', that is, proportions of oxide populations and their associated contributions to properties. He combines materials from many different places to source the needed oxides. He/she could well work in an account at insight-live.com, it knows the chemistry of the frit (and if it does not he can add it). He just needs to enter recipes and it will show the chemistry. By showing recipes side-by-side he can fine-tune glaze recipes to adjust any property related to chemistry.
So remember, while most materials are just white powders, there is a lot more to understanding them than meets the eye. We have to be willing to put on a variety of hats and be able to present them in a way that complements the aspect being considered. "Each microscopic granule is often a crystal which duplicates on a small scale properties we can measure from large chunks f the material."
|By Tony Hansen|
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