|Monthly Tech-Tip |
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
In the glass industry, “vitrification” is the solidification of a melt into a glass rather than a crystalline structure (crystallization). By this definition ceramic glazes vitrify. However, in traditional ceramics, use of the term "vitrification" focuses on what clay bodies do when fired to increasing density in a kiln. It is a process. Bodies do not have specific vitrification points and different ceramic industries define the term "vitrified" differently. As clay is fired hotter and hotter, it reaches a point where, if cooled from there, will produce ware of sufficient density and strength as to be useful for the intended purpose (it is thus said to have been "fired to maturity").
In some applications vitrification is considered necessary to attain the needed strength or appearance. The state of being vitreous for a buff stoneware might be 1.5% porosity. For a hard porcelain, zero percent is considered vitreous. A terra cotta body may be considered vitreous at cone 1 where it has 3-5% porosity. However actually firing a terra cotta to vitrify can require much more attention to the process and bring more problems (e.g. warping, blistering) than a manufacturer can actually handle in a practical way. For this reason, semi-vitreous (higher porosity) is often considered sufficiently strong and durable, especially when the glaze on the ware fits well, is hard and durable. In fact, many potters use poorly fitted or melted glazes on a vitreous body, producing ware that is actually less functional than if they had a better glaze on a less vitreous body.
That being said, generally vitrified commercial bodies have better glaze interfaces and fit. However some bodies that appear vitrified (because porosity is low) are actually over-fired. Black burning, manganese-stained bodies are an example. For some bodies, decomposition of some particles happens while the glaze is in the melt state. These are obvious as bubbles in transparent glazes, But opaques, although appearing smooth, can be Aero-chocolate-like inside. The thin glass layer hiding bubbles immediately below the surface can rupture producing tiny “sink holes” as the surface wears or experiences stress. Glazes that melt late are thus valuable to minimize entrained bubbles.
When a porcelain is fired to higher and higher temperatures it densifies and shrinks. The porosity curve plotting this flattens toward horizontal as it falls to the zero point. Depending on the body formulation, this zero-porosity density can maintain over several cones as the body softens and approaches melting. Subsequently, as melting ensues, bubbles will develop in the matrix, as materials decompose, and the ware will actually expand in size. In a translucent porcelain, the body is fired to a point beyond where the porosity curve hits zero, but not so far along the curve that warping cannot be managed or ware becomes brittle. With stonewares containing coarser particles, the situation is different, they often cannot be fired to zero porosity without bloating, perhaps the minimum achievable porosity is 1%. For these, the porosity curve dwells at the minimum over a shorter period and rises more abruptly on over-firing, they are normally considered vitrified before the curve hits minimum. Low fire bodies have an even more volatile vitrification curve, so volatile that firing is not normally taken anywhere near the low curve point.
It is not possible to calculate the vitrification temperature of a body from the chemistry. It is a matter of physics and testing. Using an account at insight-live.com you can manage a project, making a series of mixes, firing test bars of them at a range of temperatures and comparing their porosities and fired shrinkages. From the data you will be able to create a recipe that achieves the state-of-vitrification your application needs.
On highly vitrified bodies (e.g. translucent porcelains) it is possible to produce a food-safe and functional surface (surfaces that will not stain and can be effectively cleaned). It is best to use bodies made from high quality clays that have been wet-processed (and therefore water washed). Mechanical polishing may be required.
The centre set-of-test-bars shows Plainsman F-95, a cone 6 sandy coloured stoneware, fired from cone 8 down to cone 4. Because of a change in a low-fire red clay in the recipe it has become much more refractory (10%+ porosity at cone 6 instead of the normal 4-5%). We had made hundreds of boxes for a custom order (for these we are unable to exercise the same diligence in testing). Two approaches were tried make it fire more dense. For the left set-of-bars we prepared some plastic Gerstley Borate (it exhibits workability like a clay) and wedged that with the F-95 in a ratio of 5:95 (by weight). This greatly improved maturity, and color, bringing cone 6 porosity down to 6.5% (this is still too high, likely 7% would be needed). For the right set-of-bars we wedged in Plainsman L210 in a ratio of 15:85. That lowered the cone 6 porosity to 2.7%, too far (also darkening the color). So it appears that a 10:90 mix should be about right.
Here is an example of how a profile having no inherent strength can warp during firing (the one on the left is just bisque fired, the one on the right is fired beyond zero porosity to achieve translucency). Two key factors contribute to this failure: This porcelain is highly vitreous. This shape is vulnerable to warping. If the lip were flared out, for example, it would have much more strength to stay round. If the porcelain was less vitreous it would warp less. Of the two factors, which contributes more to the warping for this specific piece? The shape.
This 1 gallon heavy crock was fired to cone 6 (at 108F/hr during the final 200 degrees) and soaked 20 minutes (in a electric kiln). The bare clay base should be the color of the top test bar (which has gone to cone 6). Yet, it is the color of the bottom bar (which has gone to cone 4)! That means the base only made it to cone 4. The vertical walls are the right color (so they made cone 6). It may seem that this problem could be solved by simply firing with a longer hold at cone 6. But electric kilns heat by radiation, that base will never reach the same temperature as the sidewalls!
Materials are not always what their name suggests. These are Lincoln Fireclay test bars fired from cone 6-11 oxidation and 10 reduction (top). The clay vitrifies progressively from cone 7 upward (3% porosity at cone 7 to 0.1% by cone 10 oxidation and reduction, bloating by cone 11). Is it a really fireclay? No.
Bloating. These teapots have been refired to cone 6. This is just a standard porcelain body (feldspar, kaolin, silica, ball clay) that vitrifies fully at about cone 7. The second firing is obviously vitrifying to an equivalent of more than that.
Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.
Bloating in an over fired middle temperature high iron raw clay (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But between cone 6 and 7 (top bar) it is already bloating badly. Such clays must be fired at low enough temperatures to avoid this volatility (if accidentally over fired). This clay only reaches a minimum of 1% porosity (between cone 4 and 5), it is not possible to fire it to zero porosity. This is because of the particulate gas-producing particles (it is ground to 42 mesh only).
This cone 6 brown functional stoneware has been fired across a range of temperatures. Cone 4 is too porous. From cone 7 it is expanding and density is not improving, it will likely warp or bloat. Cone 7 is losing the red color, there is no room for over-firing (by accident). The porosity at cone 6 is so much better than cone 5 and color is still stable. Therefore, cone 6 is the one we want.
An example of a cone 10 porcelain that is over mature. It contains too much feldspar and is vitrifying so much that it is beginning to melt. The weight of the handle is pulling the lip into a oval shape, even though the hourglass shape of the piece should offer stability.
The one on the left weighs 176 grams. The right one is thrown and weighs 376 grams. Of course the body needs to vitrify well to be strong enough to produce a durable product. But not vitrify so much that the weight of the handle pulls the rim to a oval shape during firing.
Some terra cotta clays can be used to produce stoneware by firing them a few cones higher. Terra Cottas are almost always nowhere near vitrified at their traditional cone 04-06 temperatures, so they can often stand much higher firing. However, clear glazes do not usually work well in higher firing since products of decomposition from the vitrifying body fill them will micro bubbles, clouding the surface. In addition, the body turns dark brown under clear glazes. But with a white glaze, these are not a problem. This is Plainsman L210 fired to cone 2. The glaze is 80% Frit 3195, 20% kaolin and 10-12% zircopax, it fires to a brilliant flawless surface.
This body is made from approximately 50:35:15 ball clay:talc:silica:silica sand. These test bars are fired from cone 2 to 9 oxidation (bottom to top) and 10 Reduction and from them the porosity and fired shrinkage can be measured (shown for each bar). Notice that the fired shrinkage is pretty stable from cone 2 to 8, but accelerates at cone 9 oxidation. But in reduction this stage has not been reached yet. The same thing happens with porosity, the cone 9 bar is dramatically more dense than the cone 8 one. But in reduction, it is still porous.
Three cone 6 mugs. All have zero porosity. Why is the middle one so translucent? Three reasons. 1. It has 10% more feldspar than the one on the left and reaches zero porosity already at cone 5. 2. It employs New Zealand china clay while the one on the left contains high-TiO2 #6 Tile kaolin. But this is also true for the one on the right. The third difference is the key. 3. The center one contains 4% Veegum T plasticizer (while the other two use standard bentonite). This is surprising when I tell you one more thing: The mug on the right also contains 3% Ferro Frit 3110. That means that the frit does not have near the fluxing power of the VeeGum!
The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature (because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely). That is what vitrified means. But it has a high fired shrinkage, poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder (VeeGum), all that cements it together is sinter-bonds between closely packed particles (there is no glass development). Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, low thermal expansion and maintains its strength and hardness at very high temperatures.
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
Cone 6 iron bodies that fire non-vitreous and burn tan or brown in oxidation can easily go dark or vitreous chocolate brown (or even melting and bloated in reduction). On the right is Plainsman M350, a body that fires light tan in oxidation, notice how it burns deep brown in reduction at the same temperature. This occurs because the iron converts to a flux and the glass development that occurs brings out the dark color. On the left is Plainsman M2, a raw high iron clay that is quite vitreous in oxidation, but in reduction it is bloating badly. When reduction bodies are this vitreous there is a much great danger of black coring.
When porcelains mature in the kiln they progress toward vitrification, getting softer. This simple test enables anyone to quantify the degree to which a porcelain is likely to warp. Bars of plastic clay almost never dry straight, so the measurement (in mm) to which they deviate from straight is recorded and the bar is mounted with the hump upwards. After firing the mm of firing deviation-from-straight are added to the dry value to derive a total pyro-plastic deformation measurement. This can be recorded as an absolute value for comparison with other clays or temperatures.
The unglazed surface of the left piece has a sheen, it is a product of glass development during firing to cone 6. That body is a 50:50 mix of a cone 8 stoneware and a low fire earthenware red (a material that would normally be melted by this temperature). Together they produce this dense, almost zero-porosity ceramic. The unglazed surface on the right looks more like plaster, and it is absorbent, about 5% porosity. It is a mix of the same stoneware but with 50% ball clay. The refractory ball clay assures that the stoneware, which was already inadequately vitreous, is even more so. As you can imagine, the left piece is far stronger.
Want to make this incredible porcelain and glaze yourself? Read on. The mug on the left is a cone 10R (2350F/1290C) porcelain (#6 Tile kaolin and Nepheline Syenite) with G1947U clear glaze. The other is a fritted cone 03 (1950F or 1065C) porcelain (NZ Kaolin, Ferro Frit 3110) with G2931K clear glaze. We call the body/glaze/firing system "Zero3" (google it or use the links here). The Zero3 porcelain is blue-white instead of grey, the glaze is crystal clear, underglaze colors are so much more vibrant. The Zero3 mug was fired in 3 hours (cold-to-cold). It also withstands thermal shock better, it is as strong or stronger and much more translucent. How is this possible? The magic of the frit, it melts so much better than nepheline. The recipes and method are linked here. It is the most expensive body you will ever make. But from it you will create the highest quality ware you have ever made using the most plastic body you have ever thrown! Follow the instructions carefully.
Porcelains look much more glassy and melted than you might expect when viewed close up (this is cone 6 Polar Ice from Planisman Clays). The development of the glassy phase within the body creates a very good bond with the glaze. Actually it is a bonding zone where the glaze has melted into the body enough to create a transition rather than just a point of contact. The degree to which this transition develops determines the integrity of the bond. Of course, with porcelains it is far better developed than with stonewares and terra cottas.
These two mugs are made from the same material: Ravenscrag Slip plus 20% Ferro Frit 3134. The one on the right has been bisque fired to 1550F. The one on the left has been clear glazed and fired to cone 03 (1950F). That means that this body vitrifies well below cone 03, likely well below cone 06. Thus strength, maturity, vitrification are not a matter of temperature, they are a matter of how much flux is available in the body to mature it to a dense, strong matrix.
The mug on the left is high temperature Plainsman P700 (Grolleg porcelain). The other is Plainsman Zero3 fired at cone 03. Zero3 has a secret: Added frit which reduces the porosity of the terra cotta base (therefore increasing the density) dramatically. How? The frit melts easily at cone 03 and fills the interparticle space with glass, that glass bonds everything together securely as the piece cools. Although I do not have strength testing equipment right now, I would say that although the P700 mug likely has a harder surface, the Zero3 one is less brittle and more difficult to break.
These two unglazed porcelain tiles appear to have a similar degree of vitrification, but do they? I have stained both with a black marker pen and then cleaned it off using acetone. Clearly the one on the right has removed better, that means the surface is more dense, it is more vitreous. In industry (e.g. porcelain insulators) it is common to observe the depth of penetration of dye or ink into the matrix as an indication of fired maturity.
These are four terra cotta body disks that have been fired to cone 10 reduction. The fluxing action of the iron has assisted to take them well along in melting. Notice that one is hardly bubbling at all, it is Redart clay that has been ground to 200 mesh (the lower right one is a body mix of 200 mesh materials also containing it). The upper left one is bubbling alot more. Why? Not just because it is melted more (in fact, the one on the lower left is the most melted). It is a body made from clays that have been ground to 42 mesh. Among the particles are larger ones that generate gases as they decompose. Yes, the particles in the others do the same, but their smaller size enables earlier decomposition and expulsion of smaller gas amounts distributed at many more vents. Some bodies cannot be fired to a point of zero porosity, they will bubble before they get there.
These are translucent porcelains, they are vitreous. The firing is to cone 10. The one on the left is a cone 6 body, and, while it survives to cone 10 it does warp. But more important, it is much more vitreous (more melted). The plucking problem makes it quite difficult to get a good foot ring. The other, which has only slight plucking, is also quite vitreous (high in feldspar). The plucking problem on both can be solved by simply using a better kiln wash. What is better? More refractory, and therefore having a powdery, non-stick surface. Spend more money on your kiln wash, base it on calcined alumina or zircon.
Both of these bodies have a range of particle sizes (from 42 mesh and finer). The glaze on the cone 04 section (close-up on the left) is very well melted, but its interfacial zone with the body is narrow, it is basically just "stuck on" the surface (so it must be tuned to match the thermal expansion of the body to prevent crazing/shivering). The body is not developing any clearly visible glassy phases, meaning that any strength it has is purely a product of sintering. On the right, the interfacial zone of glaze-with-stoneware-body is wider. The body has incipient melting (tiny feldspar particles are fusing and beginning to dissolve into the surfaces of surrounding ones). The color has changed from red to brown.
The chocolate brown burning super-plastic base clay (to which 20% coarse grog is added) matures at cone 6. Yet this is a cone 10R body. The grog stabilizes the fired matrix enough that it stands up in the kiln. And it fires to a dense product that can withstand any weather. Any porosity that can be measured is only from the grog. A number of manufacturers around the world make bodies like this, some can have almost double the grog this one has. These employ engobes (a brown and a blue, applied at the leather hard stage - L3954N) on the insides, enabling a smoother glaze surface.
The particles in low temperature bodies are not glass-bonded, they only have sinter bonds. Broken edges will only be sharp if there is a glaze. They tend to break off in pieces rather than shatter.
Plainsman Clays did 6 weeks of mining in June-July 2018 in Ravenscrag, Saskatchewan. We extracted marine sediment layers of the late Cretaceous period. The center portion of the B layer is so fine that it must have wind-transported (impossibly smooth, like a body that is pure terra sigillata)! The feldspar and silica are built-in, producing the glassiest surface I have ever seen at cone 6 (2200F). Despite this, pieces are not warping in the firings! I have not glazed the outside of this mug for demo purposes. I got away with it this time because the Ravenscrag clear glaze is very compatible (similar thermal expansion). But with other less compatible glazes they cracked when I poured in hot coffee. This mug was the beginning of an exciting project the sieve out +325 mesh particles any make many more pieces.
This is a grog clay with 25% Christy Minerals STKO22S grog (20 mesh one size). This piece is about 8 inches tall fired at cone 10R. This body is a Redart, Ball clay base that totally vitrifies to a chocolate brown. But with the added refractory grog it is fairly stable in the kiln and is much more vitreous than other grog bodies. Because it is such a plastic smooth base and because the grog is only one size, this is actually throwable. And it is very resistant to splitting during hand building.
We tested four different clays (brought in by customers). One is from BC and three from Alberta. These fired sample bars show rich color, low soluble salts and high density and strength at very low temperatures. L4233 (left): Cone 06 to 3 (bottom to top). Reaches stoneware-density at cone 02 (middle bar). Plasticity is very low (drying shrinkage is only 4.5%). But, it is stable even if over-fired. L4254 (center bottom): Cone 04,02,3,4 (bottom to top). Very fine particled but contains an organic that is gassing and bloating the middle two bars. L4243: Fires lighter and looks stable here (cone 02,01,1,2 shown) but melts suddenly less than a cone above the top bar (well before vitrification is reached). L4242 (right): Hyper-plastic, with 12% drying shrinkage! Already melting by cone 02 (third from top). Achieves almost zero porosity (porcelain density) at cone 04 (#2 bar). Even when mixed with 20% kaolin and 20% silica it still hits zero porosity by cone 1. What next? I'll mix L4233 (left) and L4242 (right), that should produce stoneware density at cone 02 (about 1% porosity).
Terra cotta bodies are typically fired between cone 06 and 04. That being said, many, like this Plainsman L215, develop richer color at cone 03 and fire much stronger. Glazes, of course, melt better and micro-bubbles pass through easier at cone 03. But this happens only if the body has not begun to decompose (and therefore generate a lot more gases of decomposition). Notice that crazing is beginning on the one of the left. Apparently the improved body:glaze interface and the development of better vitrification reduces the problem. Cone 03 is somewhat of a sweet-spot for this specific body, however firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 for ongoing success (because cone 02 is too high for this body, glaze will blister). Do you know what terra cotta actually is, if not click the link to learn more (this is a big topic).
The porcelain on the right (Plainsman Polar Ice), breaks much more like glass, to razor sharp-knife edges. But it is not a glass, it is a zero-porosity, dense matrix of glass-bonded crystals. The whiteware on the left (Plainsman M370) has 1% porosity, and although vitreous and quite strong, clearly it is not in the same class as the one on the right.
Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. Less exposed bare clay on the base would improve the situation somewhat, however it would be much better to choose a body that vitrifies sufficiently dense so that it does not absorb water (or fire to a higher temperature). There is a not-so-obvious issue here also: Although this piece did not explode in the microwave, it got incredibly hot. Amazingly, through all of this, the glaze has not crazed. It is G3879.
Producing a zero-porosity cone 6 stoneware is not as easy as you might think. People expect stonewares to be plastic and fit glazes well. That means there needs to be lots of ball clay and silica in the recipe. These are refractory materials and they don't leave much room for the material that produces the vitrification: Feldspar. If the body does not need to be white there is another interesting approach: Use a red terra cotta material to supply plasticity and maturity. In this case I have made a 50:50 mix of a red-burning, super plastic, low fire clay (Plainsman BGP) with a refractory white-burning ball clay (Plainsman 3C). The result vitrifies to a zero-porosity, beautiful light tan body that even glistens in the light. One problem: Fired shrinkage. This body shrinks almost 17% from wet to fired.
A very fine particled low fire red burning terra cotta clay (Plainsman Redearth) fired at cone 2,3 and 4 (top to bottom). Notice the cone 4 bar is beginning the melting process (signaled by the fact that it is expanding). Yet it is not bloating as this type of raw clay normally would. The cone 2 and three bars have reached zero porosity also. Other clays that fire to very similar color begin to bloat long before they reach zero porosity.
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
A firing issue in ceramics where the foot rings of vitreous ware stick to the kiln shelf. Removing them leaves sharp fragments glued to the shelf.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
This term generally refers to the industry that produces the non-metallic objects we use every day (like porcelain, tile, glass, stoneware).
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
A highly sought-after property in porcelain, fired close enough to melting to take on the glass-like property of passing light. Translucency implies tendency to warp during firing.
A term used in ceramic to express the degree to which an item is safe and stands up to everyday use. Functionality embodies strength, hardness, resistance to acid attack and thermal shock, etc.
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
Clay Body Porosity
In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Man-made ceramic surfaces are among the most abrasion resistant materials known. Products made to abrade others are also made from bonded ceramic grains.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.