Introduction

The field of products known as whitewares is considered by the federal government to be a mature field -- which is to say that we know everything about whitewares and whitewares production, so there's no need for them to fund any more research in this area.  Those of us who work in the field of whitewares know it is baloney to think that we know everything about everything.  But federal money continues to fund cutting-edge research in new fields of ceramics -- not whitewares.  So where does one look to learn more about whitewares and whitewares problems?  Good question!  We will try to cover some of the topics here -- at least briefly.

In this paper, we will discuss the different raw materials and their purposes in whitewares.  It won't be a particularly deep coverage -- just fundamentals.  Maybe this brief explanation will bring other, more specific questions to the fore -- and then we can delve deeper into these topics. 

We won't go into any details of specific raw materials from specific suppliers.  Each of you can discuss the details of your own materials with your suppliers' reps.

Silica

Fundamentally, silica, a.k.a. quartz, sand, or flint, after it has been fired, has the overall properties that we desire in whiteware products.  That is why glass products are major competitors of whitewares.  Electrical insulators have been made from glass.  Some cups, saucers, and dinner plates are glass.  Although silica produces the desired fired properties, pure silica has several processing problems:  it doesn't flow well in suspension;  it doesn't hold its shape well after forming;  and it fires at much higher temperatures than desired.

To be sure, pure silica is used to make a variety of ceramic products that require purity, transparency or translucency, and high firing temperatures.  But for normal whitewares such as cups, saucers, toilet bowls, insulators, etc., pure silica products would be too expensive.  Of course, they could be made, but they would command higher prices than good whitewares.

On the positive side, the properties silica provides are desirable -- so we want the overall body composition to contain substantial quantities of SiO₂.  Vitreous or semi-vitreous products of silica plus other ingredients will produce these properties.  We know, for example, that the whiteware bodies have glassy properties because toilet tanks hold water on the inside, unglazed surfaces.  We know also that we can glaze whiteware products to achieve smooth, glassy, beautiful outer surfaces.  Some whiteware bodies are translucent.  So we want silica in whitewares bodies to contribute its properties.  But we need to mix other ingredients with the silica to achieve overall goals.

On the negative side, crystalline quartz in whiteware bodies can cause firing problems.  The low-quartz to high-quartz structural inversion, which occurs at 573^oC, creates many firing problems in whiteware bodies.  On heating, quartz goes through a major displacive phase change at this temperature which produces a relatively large, instantaneous volume expansion.  On cooling, any remaining quartz grains go through a similar large instantaneous contraction at this temperature.  When the proper firing curve is used, however, quartz inversion expansions and contractions can be managed.  If quartz grains are present in the body, they will go through this inversion both on heating and cooling.  There is no way around this.  If low quartz is present, the inversion will occur.  If low quartz is not present, or if it has vitrified at the firing temperature, the inversion will not occur.

In short, silica is present in whitewares bodies for its properties.    ... and because sand is both abundant and inexpensive.   ... and because inversion firing problems are manageable.

Feldspar

The feldspars are present to address the high silica (and ball clay and kaolin) firing temperatures.  Pure silica melts near 1750^oC  (~3180^oF) which is a much higher firing temperature than desirable for whitewares. 

Regarding feldspar compositions, here's a way to remember the general formula for feldspars:  Feldspars start with four silicas:  4 SiO₂ = Si₄O₈.   Then, one of the silicon ions (+4 charge) is replaced by an alkali ion (+1) and an aluminum ion (+3):  NaAlSi₃O₈.  This is a typical formula for a feldspar:  three silica ions, an aluminum ion, and an alkali ion, plus oxygen ions.  The point of this is to show that feldspars, too, are three fourths silica.  Vitreous feldspar properties are very similar to vitreous silica -- but feldspars melt at much lower temperatures.

Both feldspars and silica are non-plastic materials.  Pure feldspar suspensions share poor viscous properties with pure quartz suspensions.  These suspensions can be made, but their rheologies will also tend to be quite dilatant.  Since these minerals are non-plastics, products made from such suspensions won't form and hold their shapes easily.  On the plus side, feldspars vitrify at very low temperatures.  If we tried to make whitewares out of pure feldspar powders, handling and forming would be poor and it would be difficult to maintain product shapes as the feldspars vitrified during firing. 

Depending on the types of products and properties desired, different feldspars will be used.  For electrical porcelains, potassium and/or other non-sodium feldspars are often used.  For particularly low firing applications, lithium feldspars can be used.  Lithium feldspars are also useful when low thermal expansion products are desired.  In some cases, particular feldspars are desireable because they don't contain certain contaminants.  For example, pure nepheline syenite is used in some whitewares bodies because it does not contain quartz impurities.  When the primary concern is to formulate a whiteware body without any quartz impurities, nepheline syenite is an excellent choice. 

Since feldspars are compositionally quite similar to silica, and since there are a wide variety of feldspars containing many different alkali and alkaline earth ions,  feldspars are desirable in combination with silica in whitewares formulations.  The main feature contributed to whiteware bodies by the feldspars is their lower melting points. 

Feldspars are used in whiteware bodies primarily to bring firing temperatures below 2200^oF.

Ball Clays & Kaolins

Since both quartz and feldspar are non-plastic materials, ball clays and kaolins are used in whitewares to provide the plasticity and the desired rheological and forming properties.  Ball clays and kaolins are grouped together in this article because they are compositionally similar.  Kaolins contain much purer, whiter, larger, and more well-crystallized forms of kaolinite than ball clays.  Ball clays contain less-well-crystallized, finer particles with more carbonaceous impurities than the kaolins.  Many ball clays are kaolinitic, two-layer silicate minerals.  Some ball clays are illitic, three-layer silicate minerals. 

The rheological properties ball clays and kaolins provide to body slips are shear-thinning forming properties with their associated yield stresses.  These plastic properties are needed to mix, handle, and store whiteware slips, to form the wide variety of ware shapes, and then to hold the shapes of the wares after forming.

Different ball clays and kaolins are selected to provide continuous, broad particle size distributions over the whole range of particle sizes in the whitewares slips.  Also, the wide variety of ball clays and kaolins allows control of body slip surface areas to be set over a wide range.  Ball clays have higher surface areas;  kaolins have lower surface areas.  The proper selection and proportioning of materials of these two materials allows batch to batch control to a consistent intermediate surface area.

Selection, use, and control of specific ball clays and kaolins allows control of whiteware body particle size distributions, surface areas, impurities, and forming properties.  Particle size distributions, surface areas, and methylene blue indices should all be used to select particular ingredient materials.  Then, these same three properties should be continually measured and controlled as daily body batches are formulated and mixed.     

Summary

Fundamentally, the reasons for the use of silicas, feldspars, ball clays, and kaolins in whiteware bodies are:

▪  The properties contributed by silica are the properties generally desired in whiteware bodies. 

▪  Feldspars are fluxes.  Although silicas, ball clays, and kaolins have high melting points, feldspars lower whiteware firing temperatures into manageable ranges below about 2200^oF. 

▪  Ball clays and kaolins provide the necessary and desirable rheological and forming properties. 

Selection of particular raw materials and formulation of their percentages in body compositions depend upon specific material properties, types and percentages of impurities, particle size distributions, surface areas, and methylene blue indices required to achieve desired body properties.  Body compositions are company and product specific.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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Copyright © 2006  Dennis R Dinger

103 Augusta Rd, Clemson, SC 29631   (864) 654-5731

consulting@DingerCeramics.com

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All Rights Reserved.