|Monthly Tech-Tip |
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
As kiln temperature increases bodies densify (particles pack closer and closer). As temperature continues to rise, some of the particles begin to melt and form a glass between the others that pulls them even closer. Some of the particles shrink themselves, kaolin is an example (in the raw state particles are often loosely packed in layers, these pull together at temperature rises). These factors result in shrinkage of ware during firing.
Fired shrinkage (shrinkage from dry to fired) is a thus comparative indicator of the degree of vitrification. As a clay is fired higher it shrinks more and more to a point of maximum shrinkage (after which swelling occurs as a precursor to melting). If fired shrinkages are measured over a range of temperatures for a body it is possible to create a graph to get a visual representation of the body's maturing behaviour and range. The shrinkage plotted against temperature produces a line that increases to a maximum, levels out and then drops off. As noted, fired shrinkages are relative within a system, there is no absolute of how much a clay should shrink when fired.
Some special purpose sintered bodies have very low fired shrinkages (because they are packed so tightly during pressing and because no glass develops). However whitewares shrink 7-8% during firing, vitreous porcelains more than 10%, stonewares about 5-6% and earthenwares 3-4% or less. Again, these percentages are not total shrinkage wet-to-fired, but dry-to-fired. These shrinkages are not a product of temperature, but of the amount of flux present in a body to develop particle-bonding glass during firing. Fluxes are available at all temperatures, at higher temperatures feldspar is the most common, at the lowest temperatures frit is used. Dense and strong ware can thus be made at any temperature.
It is very important to consider firing shrinkage when adapting an engobe to fit a body. If it does not shrink the same amount the engobe will be either excessively compressed or excessively stretched on to the body surface. While some incompatibility can be tolerated, an overgraze having an unmatched thermal expansion can be a cause of failure in the engobe-body bond. The firing shrinkage of engobes is normally adjusted by changing the amount of frit or feldspar in the the recipe.
Developing an efficient way to make, fire, measure, boil and weigh test bars is a key to being able to study fired shrinkage of your bodies and body materials. You can use an account at insight-live.com to learn how to do this and log and report your results.
These are fired bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward. Drying shrinkage is 4% at 25% water (it is very non-plastic). The darkness of the fired color suggests higher MnO than our published chemistry shows.
Ball clays are normally refractory, none of these are vitrified to any extent. The cone 10R bar is yellow because it is stained by the soluble salts present in the material. These are very typical of what most ball clays look like.
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.
These bi-body strips are made by rolling two clays together in a thin sandwich. Three porcelains are being compared to a very plastic grogged sculpture body. After drying (top) they curl a little, two toward the sculpture body and one, the most plastic of the porcelains, toward the white. But on firing to cone 8 they curl dramatically toward the porcelain side (because it shrinks alot more). Now imagine one of these porcelains is being used as a engobe on this body.
This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).
Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrinkage at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they do fit well (either during the firing or after the customer stresses your product). The engobe also needs to shrink with the body during drying. How can you measure compatibility? Bi-body strips. First I prepare a plastic sample of the engobe. Then I roll 4 mm thick slabs of it and the body, lay them face-to-face and roll that down to 4 mm again. I cut 2.5x12 cm bars and dry and fire them. The curling indicates misfit. This engobe needs more plastic clay (so it dry-shrinks more) and less frit (to shrink less on firing).
The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.
These DFAC testers compare the drying performance of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.
Large particle kaolin (left) and small-particle ball clay (right) DFAC tests (for drying performance) demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.
These have already been measured to deduce drying shrinkage. After firing they will be measured again to calculate the firing shrinkage. Then they will be weighed, boiled in water and weighed again to determine the water absorption. Fired shrinkage and absorption are good indicators of body maturity.
Left: Dry mug. Right: Glazed and fired to cone 6. This is Polar Ice porcelain from Plainsman Clays. It is very vitreous and has the highest fired shrinkage of any body they make (14-15% total). This is the highest firing shrinkage you should ever normally encounter with a pottery clay.
Stains can and do influence the degree of vitrification of a porcelain. Some stains will make a porcelain more refractory (decreasing fired shrinkage), others will make it more vitreous (increasing the firing shrinkage). Obviously, the greater the percentage of stain the greater the effect. Stained porcelains having differing fired shrinkages will stress at boundaries in accordance with the degree of difference in their fired shrinkages. In this piece, you can see how the boundary between the red (more vitreous) and green (less vitreous) porcelains is the point-of-failure. The only solution is to adjust the porcelain recipe to move the fired maturity in a direction that counterbalances the effect of the stain. For example, you could employ three recipes (regular, more vitreous, less vitreous) and use the indicated one for each stain added.
Plainsman Clays publish dry and fired shrinkage data for their clay bodies. Dry shrinkage is, of course, the shrinkage from wet to dry. Fired shrinkage is not, however, the total from wet to fired. Rather it is the shrinkage from dry to fired. And you cannot just add the dry and fired numbers together to get the total because the fired shrinkage value is based on the dry length, not the original (in this example, 6.25 dry shrinkage plus 6.66 fired equals 12.9 whereas the actual total shrinkage is 12.5). It is not a huge difference but this is the way to calculate it correctly if you only have drying and fired shrinkage. Thanks to Tom Hittie for deriving this for us.
A batch of fired test bars that have just been boiled and weighed, from these we get dry shrinkage, fired shrinkage and porosity. Each pile is a different mix, fired to various temperatures. Test runs are on the left, production runs on the right. Each bar is stamped with a code number and specimen number (the different specimens are the different temperatures). The measurements have all been entered into our group account at insight-live.com. Now I have to lay out and photograph each pile and upload the picture into the code-numbered record. Upon doing so I compare color and tests results to make decisions on what to do next (documenting these in insight-live).
These are the fronts and backs of dust-pressed bars. After final drying the width at each line is carefully recorded. They are fired horizontally in a furnace able to reproduce linear thermal gradients along the length of the bar. Thermocouples monitor the temperatures along the bar, so the temperature reached at each line is known. After firing the widths are re-measured, this produces a graph of fired shrinkage vs. temperature. Clays can be visually inspected side-by-side and differences or changes in maturity are immediately obvious.
It seems impossible but that is what happens with this one at cone 03. This is a native material that was found on the banks of the South Saskatchewan river near Hayes, Alberta (and brought to me for testing). Even when fired to maturity (around cone 2) it still has 10% porosity! This specific sample has even been ball milled for hours and it still does not shrink. And it still feels sandy on the potters wheel. It also has incredible dry strength, the highest I have ever seen. Yet its drying shrinkage is still less than 7% (that of a typical plastic pottery clay). Plus it has very high plasticity. This behavior defies logic, I have found a good explanation.
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.
It is about the recipe and the "magic material" that makes it work. And about the firing temperature. This is Plainsman BGP, a terra cotta, mixed with 30% dolomite. Note the "DSHR" column in the SHAB test data (third last column): The drying shrinkage still averages over 7% even with the 30% dolomite, so BGP is very plastic. Notice the "FSHR" (fired shrinkage) column, it is negative for the first five test bars fired at cone 05-01, that means the bars grew in size! But notice the shrinkage hits 0% at cone 1 (bar #6). By cone 2 the trend has reversed to 0.3% shrinkage. The #6 bar is appears to be vitrifying, the color is darkening and it is strong. But notice the last "ABS" column (water absorption), it is 18.7%! This body was intended as a high-porosity ceramic at the lower ranges (it has 25% porosity at cone 05), but the dolomite is also slowing the densification as it goes through the vitrification process. Without the dolomite the top bar would be melting! By cone 1 its firing shrinkage would be 7% and the porosity would be zero.
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.
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 ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
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.
It Starts With a Lump of Clay: How to Assess a Native Clay
The Physics of Clay Bodies
Learn to test your clay bodies and recording the results in an organized way and understanding the purpose of each test and how to relate its results to changes that need to be made in process and recipe.