Digitalfire Ceramic Glossary

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• Abrasion Ceramics

  Fired 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.

• Acidic

  In 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.

• Alkali

  Alkalies 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
  

  • (Glossary) Alkaline Earths

    The fluxing oxides of MgO, CaO, SrO and BaO. These...

• Alkaline Earths

  The 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.

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  • (Oxides) SrO - Strontium Oxide, Strontia
  • (Oxides) MgO - Magnesium Oxide, Magnesia
  • (Oxides) CaO - Calcium Oxide, Calcia
  • (Glossary) Alkali

    Alkalies are the strong base fluxing oxides of Na2...

  • (Oxides) BaO - Barium Oxide, Baria

• Amorphous

  Without 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.

• Analysis

  An analysis (or percentage 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.
  
  An "analysis" compares oxides by the weights of their molecules, not the numbers of molecules. It is important to note that an analysis comparison between two glazes can look quite different from a Mole% comparison since oxide molecule weights differ greatly.
  
  The analysis format is best suited to showing how much of each individual oxide is in a mix. For example, feldspars are used as a source of flux, although they also provide SiO2 and Al2O3 , so a buyer wants to know how much flux each brand has. A percentage analysis figure shows this, whereas a formula figure does not. An individual item can be extracted from an analysis (e.g. 10% K2O) and it is meaningful. However, an individual item in a unity formula is only significant in the context of other amounts in that formula.
  
  While an analysis can be expressed in only one way, it does provide flexibility in allowing the inclusion of organics, water, and additives which are burned away during firing. For example, if a material loses 10% by weight on firing, we can just say LOI (Loss on Ignition) is 10%. However, it would be difficult to express this 10% loss in a formula. Remember the formula is ideal to express the mix of oxides in a fired ceramic and thus there is no LOI. It is no surprise then that the analysis has become a standard used to express the make-up of raw glaze and clay materials on manufacturers data sheets.
  
  Here is how we would convert the unity formula above into an analysis:
  

  Oxides Formula Weights                  Percent
  
  ------------------------------------------------
  
  K2O     0.27 x  94.2 =  25.43 / 353.78 =  7.19%
  
  CaO     0.59 x  56.1 =  33.10 / 353.78 =  9.36%
  
  MgO     0.09 x  40.3 =   3.63 / 353.78 =  1.03%
  
  ZnO     0.05 x  81.4 =   4.07 / 353.78 =  1.15%
  
  Al2O3   0.41 x 101.8 =  41.74 / 353.78 = 11.80%
  
  SiO2    4.09 x  60.1 = 245.81 / 353.78 = 69.48%
  
  ------------------------------------------------
  
  Formula Weight         353.78

  Out Bound Links
  

  • (URLs) Laboratories who can can do a chemical analysis on a material
    http://digitalfire.com/services/database.php?list=...
  • (Glossary) Mole%

    The Mole Percent (Mole%) calculation type has beco...

  • (Glossary) Unity Formula

    A unity formula is just a formula that has been re...

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  • (Glossary) LOI

    Simplistically, LOI is the amount of weight a mate...

• Apparent porosity

  The relation between the volume of a mass and the volume or water absorbed when the mass is immersed.

• Ash Glaze

  A 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
  

  • (Materials) Wood Ash
  • (Library) Add a Native Material to the INSIGHT MDT, Build a Glaze From It

    This chapter in the lessons section of the book (a...

  • (Hazards) Plant Ash Toxicity
    The hazards of using plant and wood ash in ceramic...
  • (Materials) Volcanic Ash

    Pumicite

  
  Pictures
  Woodash glaze cone 6 vase
  
  
  A wood ash glaze at cone 6
  
  
  Example of a variegated wood ash glaze at cone 6 oxidation.