Abrasion CeramicsFired ceramic has the potential of being extremely hard and resistant to abrasion and wear. Special abrasion resistant ceramic products are made from highly specialized materials and fired to exacting requirements. Calcined alumina, for example, can be cast and fired to very high temperatures to produce surfaces with exceptional resistance to abrasion. Likewise, aggregates and bonding frits are employed by the abrasives industry to make all kinds of abrasive products (products designed to abrade others).
The hardness of pottery is mainly dependent on the development of aluminum silicates during firing (i.e. mullite crystals). This requires temperatures high enough to melt fluxes to allow them to dissolve quartz and other minerals. It also requires time to allow these processes to complete. Likewise the glaze components most resistant to abrasion are refractory alumina and silica, and the higher you fire the more you can get into your glaze and yet still get it to melt.
The abrasives industry concerns itself with making hard and angular particled sands that it then bonds using resins and frits.AcidicIn ceramic chemistry oxide compounds (minerals in geology) are considered basic (RO, R2O group) or acidic (RO2). The greater the proportion of oxygen the more acidic a mix is. Since SiO2 contains the most oxygen, silica rich rocks are the most acidic. Feldspars contain alot of RO2 and R2O, thus they are among the most acidic rocks.AlkaliAlkalies are the strong base fluxing oxides of Na2O and K2O. Feldspars are by far the most common alkali sourcing materials, thus the term 'alkali feldspars'. However frits are also an excellent source and sometimes the only alternative to meet the chemistry requirements (e.g. low alumina and high Na2O). Glazes high in these oxides generally are glossy and bright and have high thermal expansions.Out Bound Links
- (Oxides)
 Na2O - Sodium Oxide, Soda - (Oxides)
K2O - Potassium Oxide
In Bound Links
Alkaline EarthsThe fluxing oxides of MgO, CaO, SrO and BaO. These are not as active as the alkalies but have lower thermal expansions. They also tend to create matte glazes when adequate amounts are present.Out Bound Links
AmorphousWithout a regular structure. Amorphous minerals do not have a repeating crystalline matrix. Glass, for example, is amorphous because it is cooled in the kiln quickly enough that no crystals have an opportunity to form.AnalysisAn analysis is typically used to compare the oxide content of materials. An analysis is expressed in percent and each number represents the amount by weight of the oxide. An analysis will often show LOI as one of the items, this represents the volatile portion of the material that is lost during firing. An analysis can be converted to a formula by dividing each oxide amount by the molecular weight of that oxide.Apparent porosityThe relation between the volume of a mass and the volume or water absorbed when the mass is immersed.Ash GlazeA glaze that employs organic (e.g. paper, wood) or volcanic ash as a source of oxides (e.g. silica, alumina, soda, calcia). Many books deal with the preparation of the organic ash batches (these materials are caustic) and provide example recipes. The chemistry of ash types and batches varies alot. The best approach is to accumulate a large batch, mix it, have an analysis done and use ceramic chemistry calculations to compare the chemistry of the ash with a target formula for the intended temperature. Then add the oxides the ash is lacking to make it into a glaze. In this way the maximum amount of ash can be employed in the recipe and the most variegation will be achieved. These is an example in the INSIGHT software manual (and a matching video at Digitalfire) on doing this.Out Bound Links
Pictures
A wood ash glaze at cone 6
Example of a variegated wood ash glaze at cone 6 oxidation.
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