Vitrification is the solidification of a melt into a glass rather than a crystalline structure (crystallization). Glass, clay bodies and glazes vitrify, but in ceramics use of the term focuses most on clay bodies.
Some bodies cannot be fired to even near zero porosity
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).
Test bars of different terra cotta clays fired at different temperatures
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.
What often happens when already vitrified ware is refired?
Bloating. These teapots have been refired to cone 6.
Is Lincoln 60 really a fireclay? Simple physical testing says...
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.
The difficulty of vitrifying the base of heavy stoneware
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!
A novel way to compare degree of porcelain vitrification
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.
The best firing temperature for this body?
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.
Some iron clays bloat before reaching zero porosity, others do not
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.
How much does the size of a piece change when it is bisque fired? Glaze fired?
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
A porcelain mug warps under the weight of its own handle
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.
Cone 6 iron stoneware cross section close-up with glaze
The glaze is well melted, but the interfacial zone with the body is wider than terra cotta but much narrower than for porcelain. The body is developing glassy phases as does porcelain and stoneware and its color has changed from red to brown. However it is possible to add a frit and glass-bond the particles at cone 02 (at much higher cost of course). Not surprisingly, glazes must be more closely tuned to match the thermal expansion of the body for lower temperatures (since they are not stuck on as well).
What are the two key causes of firing warpage in porcelain?
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.
Iron oxide goes crazy in reduction
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.
The difference between vitrified and sintered
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.
You will not believe the secret of translucency at cone 6
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 strange vitrification profile of a talc body
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.
Stoneware from your terra cotta body? Is is very possible.
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.
A highly fluxed body, when over fired can do this!
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.
A vitreous sculpture clay. Vitreous enough for functional ware!
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.
Two bars ready for pyro-plastic comparison test
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.
Cross section view of the inside and outside glazed walls of a porcelain vessel
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.
Cone 04 terra cotta cross section close-up with glaze
The glaze is well melted, but the interfacial zone with the body is very narrow. It is basically just stuck on the surface. The body is not developing any clearly visible glassy phases as does porcelain and stoneware, so not surprisingly, its strength is much lower than vitrified clay bodies at higher temperatures. However it is possible to add a frit and glass-bond the particles at cone 02 (at much higher cost of course). Not surprisingly, glazes must be more closely tuned to match the thermal expansion of the body for lower temperatures (since they are not stuck on as well).
The foot ring on the left is plucking, the right one is not. Why?
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.
Particle size and LOI determine behaviour of over-fired bodies
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.
Which is stronger: Cone 10R mug or cone 03 mug?
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.
Can a cone 03 porcelain be better than a cone 10R one? Yes!
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.
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