This series represents a mini course in dental ceramics for the beginner, and persons seriously interested in gaining a basic working knowledge of dental ceramics are advised to take the time to start at the beginning.  If all five pages are read in order,  the reader will gain a good understanding of just what dental ceramics really are, how they differ from each other other and how different forms of porcelain are utilized in various applications. 

Introduction--glass vs. refractories

The two major components found in ceramics are a refractory crystalline structure and glass.  A refractory substance is one which does not melt at normal kiln temperatures, and glass has no coherent internal structure of its own. 

The first two pages in this course are essential reading since many of the terms used on pages devoted to more advanced dental ceramics are defined on this page, and the next one in the series.  This page deals with pottery, which was the first, and remains the most common ceramic.  All ceramic science springs from discoveries made by potters centuries ago.  The second page in this series deals with glass, which is the second major component in ceramics.  While the material presented on subsequent pages is designed to stand alone, a real understanding relies on knowledge presented on the first two pages. 

The pages in this course are as follows:

The name porcelain is said to have been coined by Marco Polo in the 13th century from the term porcelino, which is the Italian name for the  cowrie shell (also called the Venus shell).  The cowrie got its name because of its resemblance to a "little pig", which is the real meaning of the term "porcelino".  Polo referred to the cowrie shell to describe Chinese porcelain to fellow Europeans because of the shell's thinness, translucency, hardness and strength.  To quote a tenth century European reflecting on the porcelain he encountered on his journey through China:

"A ceramic so white that it was comparable only to snow, so strong that vessels needed walls only 2-3 mm thick and consequently light could shine through it.  So continuous was the internal structure that a dish, if lightly struck would ring like a bell.

This is porcelain!"

The definition of ceramics

The term ceramic covers various hard, brittle, non metallic, heat-resistant and corrosion-resistant materials.  They are made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature. The non metallic materials in question (for the purpose of common and most dental ceramics) are aluminum oxide (alumina) and silicone dioxide (silica). 

All ceramic bodies contain, at minimum, a refractory (non meltable) skeletal structure made from sintered (fused) particles of a metallic oxide, most frequently aluminum oxide.  Most ceramic bodies also contain varying amounts of glass, which is infiltrated between the sintered refractory particles.   Pottery was the first, and still is the foremost ceramic.  Pottery is made from clay, and contains both of these components.

Clay bodies are further subdivided into three groups: earthenware, stoneware and domestic porcelain.  Each classification, from earthenware to porcelain contains increasing percentages of glass and decreasing percentages of alumina.  Dental porcelain is a further subdivision of domestic porcelain. It is impossible to understand  dental porcelains and their associated cores without first understanding the art and science of ceramics, and this begins at the potters wheel.

Pottery

The earliest attested precursors of ceramics are fired clay figurines made in the area of modern Czechoslovakia 27,000 years ago. The first fired clay vessels appeared in Japan around 14,000 years ago.  In the Fertile Crescent and China, pottery appeared by around 10,000 years ago, and later in Amazonia, the African Sahel zone, the US southeast and Mexico.  In each case, the discovery of the technology took place independently, and not by diffusion from other cultures.  (The reference for these dates appears in "Guns, Steel and Germs" by Jared Diamond.) 

The first ceramics were in the form of pottery, and that pottery was in the form of low fired earthenware.  Pottery is made by forming clay into a desired shape, allowing it to dry, and heating it in a very hot oven, called a kiln, at a sufficient temperature, and for a sufficient period of time until the clay particles fuse together.   The process of heating a clay body until the particles fuse together is called firing.

Clay is a specialized form of mud.  Not just any mud is suitable as a ceramic clay.  Clay requires three specific constituents to qualify as a good ceramic medium, and for the most part, you will find these three constituents in all potters' clays.  These "big three", are feldspar, quartz and kaolinite.

Potters clays also contain water which reduces the friction between the clay particles, but also allows the clay particles to bind together.  Water lends the clay plasticity so that it can easily be formed into shape by hand.  A clay composed only of the big three (kaolinite, quartz and feldspar) lacks plasticity because it is a little like a very fine wet sand.  It can absorb only minimal water before it becomes too "sloppy" to hold its shape.    This would be known as a "short clay".  In short clays, there is a very fine balance between too much water, and too little.  Most potters will tell you that short clays are difficult to work with.  This is because even the addition of the water on the potter's hands affects the workability of the clay.  Porcelain clays are short clays because ideally, they are composed almost exclusively of the big three. 

In order to make their clays more plastic and workable, manufacturers add other minerals such as ball clay or bentonite.  These materials have chemical configurations which allow their constituent particles to break down in water to nearly molecular size.  This increases the surface area available to retain water, and produces a much more plastic clay.  (NOTE: Manufacturers of dental porcelain frits add sugar and starch to their porcelain powders for the same reason.)

The three essential constituents of an idealized clay: feldspar, quartz and kaolinite:

• Feldspar --Feldspar is a substance which comprises approximately 60% of the upper 8 miles of the earth's crust.  Feldspars are all naturally occurring glasses.    Although manufactured glass has no internal crystalline structure, feldspars do.  They are all naturally occurring crystalline rocks.   When any form of glass is allowed to cool very slowly, this allows time for crystals to form, a process known as devitrification.  Since feldspar, like any rock that was formed in the earth's crust, was allowed to cool over a period of millions of years, it had plenty of time to devitrify.  Thus feldspars are really just crystalline rocks made of naturally devitrified glass.

  When we study glasses on the next page, you will see that the three components in each of the formulas below represents one of the three basic constituents of glass; A flux, a stabilizer, and a glass former.  There are twelve naturally occurring types of feldspar (and numerous combinations).  Their formulas are all similar and can be inferred from the three provided here:

  

  Clays very high in feldspar melt into a glasslike consistency, and flow, like a thick liquid at high temperature.  A clay containing too much feldspar would be unsuitable as a potters clay since objects made from it would simply melt into a puddle in the kiln instead of maintaining the desired shape.  Most potters clays contain no more than 15% feldspar, but porcelain clays may contain up to 25%.  The other 75% is made up of refractory materials.  Some glazes, on the other hand, contain up to 100% feldspar, since the purpose of a glaze is to melt and flow over the surface of the clay body. 

  Feldspars melt at about 1150 degrees C.  The feldspathic glass they produce surrounds the refractory clay particles and fills up the pores between them.  Due to the free fluxes they contain, feldspathic glasses will also bind to the surfaces of the refractory particles thus helping to bind the ceramic body together.   The more feldspathic glass a ceramic body contains, the denser the fired body will be. Each of the three components of feldspar is discussed below.

  • Fluxes --The Na2O, K2O and CaO in the above formulas  are called alkaline metal oxides because they are strong bases when added to water.  These oxides are used as fluxes.  Fluxes have very active molecular structures at high temperature, and they attach to and combine with the surface molecular structure of otherwise hard crystalline materials, causing the surface molecules in the crystals to "dissolve".  This exposes deeper layers of the crystal to the dissolving action of other flux molecules and so on until the entire crystal melts away.  In other words, fluxes cause crystalline structures to melt at lower temperatures than would otherwise be possible, a bit like water melts a cube of sugar at room temperature.  Without fluxes present, none of the other constituents in the ceramic body would be able to melt at normally attainable temperatures, and the fabrication of pottery would have been beyond the reach of prehistoric peoples.  Fluxes are a major constituent of glass, and they are discussed in more detail on the next page.

     

  • Aluminum oxide -- (Al2O3). Aluminum oxide exists in two separate forms within clay and porcelain bodies. 

    • When chemically combined in molecular form with the other constituents of feldspars (see formulas above), aluminum oxide acts as a stabilizer, and is a part of the glass melt.  Aluminum atoms can bond with silicone via a shared oxygen atom and can thus be an integral part of the amorphous silicone matrix.  In this form, it does NOT affect the transparency of the glass. 

    • However, aluminum oxide is also added to clays as a separate constituent in the form of kaolinite.  Because of the large amount of flux contained in the feldspar, some of the kaolinite also melts into a glass, like the feldspar itself.  But the byproduct left over when the kaolinite melts is a precipitate of pure crystalline aluminum oxide called alumina. The alumina crystals remain unmelted and scattered throughout the glass melt, and in this form, aluminum oxide causes the glass to become cloudy or opaque.  The alumina crystals contribute to the refractory substructure which supports clay and porcelain bodies.

     

  • Silica --Silica is silicone dioxide, the SiO2 portion of the feldspar formulas shown above.  Like alumina, silica also exists in two entirely separate forms within clay and porcelain bodies.

    • When chemically combined with flux and aluminum oxide as it is in feldspar (see formulas above), it exists as a molecular component in an amorphous melted gel called glass.  In this form, it is called a glass former, and is discussed in more detail on the next page in this series.

    •  It also exists as as unmelted crystalline particles of quartz scattered throughout the glass melt. This form is discussed below and is part of the refractory substructure which supports clay and porcelain bodies.

   

• Quartz (clay constituent)--Quartz is pure, crystalline silica.  This is usually in the form of fine particles of flint, chert or sand.  Unlike the silica in feldspar, the silica in crystalline quartz is not combined with flux molecules, and consequently it does not melt when fired in a potter's kiln.  The quartz remains as separate, unmelted particles dispersed throughout the glassy phase  produced by the melting of the feldspar.  Quartz is part of the refractory crystalline structure in ceramic bodies, and helps the body to retain its shape in the kiln while the feldspathic glass melts around it.  Quartz melts by itself at approximately 1713 degrees C.  By comparison, iron melts at around 1510 °C, and steel melts at around 1370 °C.  (The highest temperature reached by even very efficient potter's kilns is about 1450 °C.  Most dental ceramicists fire their work between a range of about 850 °C to about 1100 °C, and potters work between about 1000°C and 1320°C .)

  A refractory (quartz in the form of silica is one example) is any ceramic constituent that will not melt at normal kiln temperatures.  While the quartz particles remain unmelted, the availability of the alkaline metal ions (fluxes) from the feldspar encourages bonding of the outer layers of the refractory quartz particles to the surrounding feldspathic glass matrix.  The presence of the free flux molecules in the melt also helps to fuse together (sinter) all the refractory particles in the clay body, including the alumina, quartz, and the unmelted kaolinite particles.   The fusion of these refractory particles creates a sturdy "skeletal" structure throughout the clay body, and helps to strengthen it and maintain the original shape formed by the potter.  This is an important point, and its importance becomes more obvious when we address the internal structure of dental porcelains.

  Silica has the chemical formula of SiO2.  Even though the chemical formula of silica shows only two atoms of oxygen associated with each silicone atom, silicone actually forms bonds with four oxygen atoms when in combination with other silica molecules.  It does this by sharing oxygen atoms with two adjacent silica molecules.  Thus, it forms tetrahedral crystalline structures.  The tetrahedrons are bonded together via shared oxygen atoms at each apex of the tetrahedron, as in the diagram below.  This describes crystalline silica, but the addition of alkaline metallic oxides in the form of fluxes can cause the ordered crystalline structure of quartz to become disordered, as it does in feldspar glasses.  This is the basis of glass formation.

• Kaolinite (clay constituent) --Although it is a component of all ceramic clays, kaolinite is found in nature in a relatively pure form known as kaolin (China clay). It derives its name from the Chinese term for "high ridge", the place where the Chinese first discovered this purest form of kaolinite.  The Europeans had been using kaolinite in much less purified forms for centuries in order to make stoneware pottery, but when exposed to this new, pure white, translucent form, the demand for Chinese porcelain went through the roof.  Since the Chinese recipes were kept secret, the European quest for porcelain probably sparked the world's first case of industrial espionage.  The chemical formula for kaolinite is:

  Al2O3 · 2SiO2 · 2H2O

  Kaolinite has a crystalline structure, and it contains silicone, just like feldspar and quartz, the other two common constituents of clay.  In pure form, kaolinite melts at 1770°C, but  in clay form, because the clay contains highly fluxed feldspar, the melting point drops to between 1200°C and 1450°C. Technically, kaolin is a hydrated aluminum silicate.  Note in the illustration above, there are two layers to the structure.  The upper layer is composed of aluminum oxide  (Al2O3) which is also called the gibbsite layer.  The lower layer is composed of silica (SiO2).  In this case, you are looking at a vertical "slice" through the crystalline structure.  Actually, this structure continues out from your computer monitor toward your face, and also behind the monitor.  These crystalline "slabs" also stack, one on top of the other to form a three dimensional crystal lattice.   

  The gibbsite layer is firmly bound to the silica layer by the shared oxygen atoms, and each hydroxyl group in the silica layer is weakly bound to the hydrogen atoms in the gibbsite layer of an adjacent layer.  This causes the crystalline structure to resemble a vertically stacked set of hexagonal plates (the image to the left).  These plates can support pressure when applied to the top of the stack (compression), but do not do so well when pressure is applied to the sides (shear) since the plates tend to slide over one another due to the weakness of the hydroxyl  bonds.  The orientation of these plates is heavily influenced by the pressure of the potters hands as the clay forms on the wheel under them, and the durability of stoneware pottery owes much to this fact.

   

  The application of heat does some interesting things to the crystalline structure of the kaolinite.  As the temperature increases toward the fusing (melting) temperature of the feldspar with its load of flux molecules, some of the hydroxyl groups attached to the silica layer in the kaolinite are driven off and combine with the hydrogen atoms attached to an adjacent gibbsite layer.  This produces water, which volatilizes and abandons the arena.  This process destabilizes the bonds between the the gibbsite and silica layers, allowing the silica and aluminum oxide to react separately, on their own.  The presence of alkaline metal ions from the flux in the feldspar disrupts the tendency of the free silica radicals to form stable crystals, and instead forces the formation of an amorphous glassy gel (glass) instead.  The flux molecules accelerate the process.  The more flux in the medium, the more of the outsides of the kaolinite crystals are "eaten away" and the more glass is formed.  In the case of porcelain, in which as much as 25% of the clay is in the form of feldspar with it's heavy load of flux molecules, quite a lot of the silica in the kaolinite melts into glass.  

  The leftover gibbsite layer which has lost its hydrogen atom becomes refractory crystalline aluminum oxide, also known as alumina. Not all of the kaolinite will melt and quite a bit of the original kaolinite remains behind as plate-like crystalline inclusions in the glass gel matrix.  Therefore, when the clay body melts at high temperature, it consists of the following constituents:

   

  Feldspathic glass formed from the melting of the feldspar Glass from the kaolinite--- the debonded silica layer from the melted kaolinite Refractory Alumina crystals---the debonded gibbsite layer from the melted kaolinite Refractory kaolinite particles in the form of flat plates Refractory Quartz particles

  The two forms of glass mix together and form one fairly homogenous melted body throughout the structure of the ceramic body.  The alumina, kaolinite and quartz particles are refractories, and these unmelted particles bond together in a process called sintering to form a brittle skeletal structure which allows the ceramic body to retain its shape throughout all the phases of firing, and gives the finished body a great deal of strength. 

Greenware

When the potter has finished "throwing" his or her pot, she lets it dry out.  When it is dry, before firing, the "ceramic" body is in a very fragile green state, and at this stage is called  greenware.  In its green state, the body has not yet actually been converted into a ceramic.  It is, rather, a fragile pile of microscopic rocks held together only because they have been forced into their most compact form by the potters hands.  When the greenware is totally dry, the potter places the unfired body into a kiln for a low temperature firing known as a biscuit bake.  During this low fire process, little if any feldspathic glass is produced.  The term greenware will crop  up again when we discuss ceramic dental cores.

Sintering

While the refractory constituents of the ceramic body do not melt, temperature increases well short of the melting point of the feldspar cause the outer molecular layers of these hard particles to become quite active.  The molecules on the surface of these particles begin to move very rapidly, and this causes all the unmelted particles (including the refractories as well as the still-crystalline feldspar) to become "slippery".  The outer layers of the particles begin to act a bit like they are coated with a liquid, and the surface tension of the "liquid" tries to minimize the surface area by drawing the particles closer together.  This causes quite a lot of shrinkage in the ceramic body.  As the various particles draw together, their surfaces begin to bond at the points of contact, and they remain this way as the ceramic body cools.    This process is known as sintering, and it is responsible for the formation of the coherent skeletal internal structures that characterize pottery and domestic porcelain.  Sintering is also an important process in the fabrication of dental porcelain and ceramic dental cores.

In fact, even though the outer layers of the refractory particles behave as if they were coated with a slippery liquid, sintering begins prior to the actual formation of any liquid phase at all.  The kaolinite in a clay body that would ordinary melt at 1200 °C sinters at temperatures as low as 600 °C.  Sintering appears to happen not so much because of melting, but because of diffusion of the rapidly moving atoms between the neighboring refractory particles.  Potters make use of this characteristic in a low temperature firing called a biscuit bake.  If the fusing temperature of a particular clay is about 1250 °C, then potters generally use a biscuit (sintering) temperature in the vicinity of 1060 °C. 

Once the greenware has been fired at low temperature, the clay particles sinter together producing the first stage of ceramic formation.  Note that in the diagram below, the center image (sintered) corresponds to the potter's biscuit bake.  Note also that at each successive stage of firing, the spaces between the particles is reduced and thus the size of the clay body has also shrunk in proportion.  The shrinkage has thermodynamic consequences, because as the clay particles jumble closer and closer together, the "pile of rocks" becomes thermodynamically more and more stable and thus less prone to fracture or collapse.  This translates into less and less tendency to slump or distort during handling or firing, and consequently,  stronger microscopic structures less prone to fracture.

Fusing

Once the potter has biscuit fired (sintered) the greenware, the body is no longer fragile and can more easily be handled for further processing.  It is not yet fully fired, however, and most of the feldspar remains in its crystalline form. 

Now the potter applies a watery mixture of feldspar or highly fluxed silica particles over the surface of the ceramic body.  This is called a glaze coat.  Once the glaze coat has dried to a powder, the ceramic body is placed back into the kiln and fired to a higher temperature during which both the glaze coat and the feldspar particles in the ceramic body melt into glass.  This second firing is called the glaze firing, but it could also be termed the "fusing firing" because fusion happens even in the absence of the glaze coat.

During this high temperature firing, the glass formed from the melting feldspar particles within the body flows into the pores between the sintered refractory particles, which jumble a bit closer together during fusion, but remain in approximately the same positions they occupied after the sintering phase.  The glass attaches to the sintered refractory particles and further fuses them even more tightly together.  Remember that the alumina, quartz and unmelted kaolinite particles remain, even during the high temperature firing, as a sort of skeleton that maintains the original shape of the ceramic body. 

The presence of the refractory alumina and silica particles is extremely important because without this refractory skeleton, the ceramic body would distort, slump, or even melt into a puddle.  Furthermore, any finished ceramic body without this internal refractory structure would be composed exclusively of feldspathic glass, and would be extremely prone to fracture due to any shock (see crack stoppers on page 3).  During this firing, the glaze coat also melts and forms a thin glass coating which flows over the entire surface of the body.  The glaze fills in any surface porosities and makes the ceramic body waterproof.

Triaxial blends

While potters' clays contain many more materials than the three discussed above, the most important are always feldspar, kaolinite and quartz.  The exact proportions of these three minerals determines the characteristics of the ceramic in question.  One of the most common ways of discovering how various mixtures of any three materials will behave is to run an experiment called a triaxial blend.  Potters use this type of experimental protocol all the time to determine how a particular glaze will look or act on their clay body when three components are mixed in controlled amounts and applied to a series of tiles made from the clay they intend to use.  The numbers in the table below are the percentages of components A, B and C which will be used in the blend.  The tiles in the image beneath it are the results of the triaxial blend experiment.

The tiles on the three apexes of the triangle show the colors of the pure glazes.  The tiles between them show the colors of the mixtures corresponding to the percentages in the table above.  In this case, we are looking only at the colors that result from specific mixtures of each axial glaze, however, the same experiment could be carried out to see if any given mixture exhibits crazes, or devitrification (clouding due to the formation of crystals in the glaze), or any number of other characteristics that might be of interest.  Note also that this sort of experiment is not limited to three axes.  It is theoretically possible (although very difficult) to run tetraxial and pentaxial blends, or any number of axes for that matter.  (Thanx to Diana Spiller for providing all the work involved in producing the images above.)

The three (plus 1) classes of clay (aluminous silicates)

The triaxial blend above shows only five divisions between each axis, but theoretically, it would be possible to divide the blends into much finer gradations, even an infinite number.  The triaxial blend image below does exactly that, and by doing so, it is possible to show the approximate composition of the three major classifications of aluminous silicates, which range from the those containing the least glass to those containing the most.

Earthenware--Clays in this category fire at a relatively low temperature and are very porous.  Earthenware vessels were probably the very first form of pottery to be manufactured over 14.000 years ago.  In order to be able to contain liquids, these bodies must be glazed, otherwise, the liquid permeates the body.  Red clay flowerpots are a good example of earthenware goods, and anyone who has ever handled this type of pottery knows that it is not especially strong.  From the diagram above, one can see that this type of clay contains little feldspathic glass to bind the particles together and to fill pores between the sintered alumina and silicone particles.  On the other hand, the red color comes from iron oxide which, along with other metallic oxide "contaminants" found in these naturally occurring clays acts like a flux to lower the fusing temperature of what little feldspar the clay contains.  In fact, the paucity of feldspar in earthenware pottery clays means they need only a single, low temperature sintering firing.  There is no need for higher temperature firing because there is so little feldspathic glass to fuse.  Thus glazes on these bodies can be formulated to fuse at very low temperature, only slightly above the sintering temperature of the clay itself.  This is one of the reasons that earthenware products are so inexpensive.

Stoneware--Stoneware is a hard, strong and vitrified ware which fires above 1200 degrees centigrade.  It tends to have low porosity, which is a defining characteristic of stoneware.  More important, however is that higher firing temperature means that the glaze can be applied to a previously sintered body and both glaze and body mature in a second high temperature fusion firing at the same time.  This creates a well integrated glazed surface.  Note its position in the triaxial diagram above.  It contains more feldspar than earthenware, and this accounts for the hardness and higher density, since there is more feldspathic glass to bind the alumina and silica together, and to fill voids between them.

Domestic porcelain--Domestic porcelain is the type that is manufactured from china clay by potters.  From the triaxial diagram, one can see that it contains a great deal more feldspar than ordinary stoneware, and for this reason, there is a lot more feldspathic glass in the clay body.  The large amount of glass in the mix has the advantage of reducing porosity to nearly zero.  This in turn produces a very dense, hard and translucent glassy body so that vessels made from porcelain clay can have very thin walls, through which light can shine, and can quite literally "ring like a bell" when struck.  There is so much feldspathic glass in the body that there is often no need for a separate glaze layer (depending on the purity of the kaolinite) as there is with earthenware and stoneware.  It is important to note, however, that porcelain still retains a refractory matrix to fortify the body, strengthen the glass, and help it to retain its shape.  The large amount of glass also has disadvantages for the potter.  Porcelain clays are very "short", and difficult to throw (To throw clay is to form it on a potters wheel).  The clay is very prone to slumping while being fired.  The glass wants to flow at high temperature because there is much less refractory material to act as a skeleton to support the original shape of the ceramic body.  The firing temperature must be precisely controlled in order to fully vitrify the glass while preventing it from slumping. 

Dental feldspathic porcelain--When dental porcelain was first formulated, in the early 1900's, it had about the same general composition as domestic porcelain.  Kaolin is a hydrated aluminum silicate, and it is opaque.  Even very small quantities in the mixture caused the porcelain to lack adequate translucency.  Thus ,by 1938, little or no kaolin was left in the porcelains chosen for dental use, and for a long time, dental porcelain was manufactured exclusively with feldspathic glass and finely ground quartz.  (By the 1960's, however, the aluminum oxide had been added back in.  It's an interesting story, so keep reading.)  The quartz particles still remain unchanged during firing.  The purpose of the aluminum oxide and quartz is to strengthen the glass by reducing the distance a crack can propagate within the body before it runs into a hard particle which stops it from progressing.  It also acts as a skeletal structure to reduce slumping during firings.  Dental feldspathic porcelains are considered in detail on the third and fourth pages of this series. 

Ceramics 2--Glass and Glazes-->

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