|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.
Key phrases linking here: firing shrinkage, fired shrinkage - Learn 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 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.
Terra cotta bodies are more volatile in the kiln than stonewares. They mature rapidly over a narrower range of temperatures, that process is accompanied by dramatic changes in fired color, density and fired strength. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP (a quarry material), cone 02 finds it at maximum density (and fired shrinkage). At cone 06 (1830F/1000C) it is porous and shrinks very little. But as it approaches and passes cone 03 (1950F/1070C) the color deepens and then moves toward brown at cone 02 (where it reaches maximum density and stoneware strength). However, past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). This is typical of most terra cotta clay materials. The good news: With careful selection of temperature this clay can produce strong and durable pottery at a very low, and economical, temperature.
These are the fronts and backs of dust-pressed and fired gradient bars. They were done by Luke Lindoe in the I-XL brick lab to assess the firing history on two clay samples from Montana. After final drying, the bar width at each line is carefully recorded. They are fired horizontally in a furnace capable of reproducing linear thermal gradients along the length of the bar (equally spaced thermocouples enable control in each micro-zone). After firing, the widths are re-measured. The data produces a graph of fired shrinkage vs. temperature. Bars can also be visually inspected side-by-side for differences (color being the most obvious but also surface character). This method of comparatively assessing the effect of temperature on a clay test bar is popular in the brick industry (e.g. when a new mining of clay is being compared with a previous one). However, the SHAB test, although requiring more effort, provides more information and is more accurate (e.g. for pottery and porcelain).
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.
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.
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).
Plainsman Clays, for example, publishes dry and fired shrinkage data for their clay bodies. The former is the shrinkage from wet-to-dry. The latter is the shrinkage from dry-to-fired. You cannot add the dry and fired numbers together to get the total because the two are based on different starting points. Consider this example: 6.25 dry shrinkage + 6.66 fired = 12.9 whereas the actual total shrinkage is 12.5%. Shown is the way to calculate the total shrinkage correctly if you only have drying and fired values (thanks to Tom Hittie for deriving this for us). Of course no one is going to bother actually doing this calculation! So just remember that the actual total is a little less than adding the two together.
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
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.
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).
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.
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.
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. Is this technique practical? Yup! The entire monoporosa wall tile industry is based on it!
What really is Barnard Slip?
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.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
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.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.
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.
Case Study: Testing a Native Clay Using Insight-Live.com
SHAB Shrinkage and absorption test procedure for plastic clay bodies and materials
Simple Physical Testing of Clays
Learn to test your clay bodies and clay materials and record the results in an organized way, understanding the purpose of each test and how to relate its results to changes that need to be made in process, recipe and materials.
|By Tony Hansen|
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