Drying Factor - DFAC
This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com. Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.
This test is designed to measure a clay's ability to resist cracking during drying. Generally, bodies having a high drying shrinkage will score more poorly on this test. However clays that have a higher drying strength will score a better DFAC result. This test is designed such that a typical body of good plasticity will crack the disk. The crack width and number of cracks provide a way to score one body against another, not just in drying performance, but in plasticity. In fact, DFAC score can be a good way to monitor plasticity changes over time.
5.1. Create a FORESIGHT Record (QC)
5.2. Take or Make the Sample (Pugmill Operator, Test Sample Mixer)
5.2.1. (Same as SHAB test 5.2.1 to 5.2.2)
5.2.2. Deliver the sample to QC.
5.3. Make the Disk (QC)
5.3.1. Take approximately 500 grams of the pugged material and knead it for 30 seconds. Roll the kneaded lump into a cylindrical mass about twice as long as its diameter.
5.3.2. Place the lump on a canvas board straddled by two round metal rods of 3/16" (4.7 mm diameter). Position the lump so that the center axis runs away from you and use a rolling pin to roll it to about 1/2 as thick. Peel it up and without flipping it over turn it 90 degrees and roll half as thick again. Repeat until the rolling pin runs on the metal rods.
5.3.3. Carefully lift the slab of clay from both ends to free it from the canvas board and turn it over.
5.3.4. Use a divider or compass to cut a 12 cm diameter disk. Turn it over again and place it on one end of a bar board (use the other end to hold the SHAB bars).
5.4. Mark the Disk
5.4.1. Using FORESIGHT determine the ID# for the run or mix begin tested.
5.4.2. Prepare the stamp by setting it to stamp the ID# and a specimen number of 1.
5.4.3. Press the stamp into the disk in two places (opposite each other across the centre point) to produce clear easily-readable letters.
5.4.4. If multiple disks are being made (e.g. to assess drying performance for various custom stiffnesses in the run), then assign successive specimen numbers to each, and stamp or write these on each sample.
5.5. Dry the Disk
5.5.1. Within a few minutes after rolling the disk place it in an air-circulating 90-100C drying chamber. Put a 60 mm metal disk (or can filled with sand or plaster) on the center of each disk, leaving an even ring of exposed clay all around. Leave the samples in the drier until they are completely dry.
5.5.2. Accumulate dried disks in preparation for evaluation. When ready bring them to the computer for the next step.
5.6. Evaluate and Mark the Dry Disk
5.6.1. In FORESIGHT look up the Recipe or Results database record corresponding to the disk so it is displayed on the screen.
5.6.2. Break the disk into two halves if it has not cracked in two already.
188.8.131.52. Break the most poorly formed half to create a quarter circle with a visible stamped number in tact if possible. If this cannot be done write the number on the quarter disk with a ceramic pencil or scratch it in clearly with a needle tool so it can be read after firing.
184.108.40.206.1. Use a pencil to write the clay type on the specimen so it can be iddentified and included in a firing appropriate to its firing range.
220.127.116.11.2. Assign a drying factor as follows and enter it into FORESIGHT using the Testdata Entry Dialog:
(Note: The IMC Magic of Fire book has a chart which graphically shows each of the following)
There are four major crack types:
The drying factor has four digits which are:
Digit 1 Crack type
I.E. B624- means:
Crack type B, 6 cracks, a little less than 2 mm wide main crack, 4 cracks coming in from outer edge, not counting main crack.
If D-type cracks are hairline then designate it as a C-type. The other digits will still alert the evaluator that concentric cracks were present.
18.104.22.168.3. If the SOLU test is not being performed discard the specimen, otherwise:
22.214.171.124.3.1. Enter a value into the computer or onto a log sheet for the degree of discoloration of the dry disk.
126.96.36.199.3.2. Accumulate quarter-disks for firing and further evaluation of soluble salt discoloration of the fired surface as described in the SOLU procedure.
188.8.131.52.3.2.1. Accumulate these samples in the dry bar holding area. Maintain a stack for each temperature (e.g. low, medium, high). When enough have been accumulated fire them or else include them in routine firings as opportunity arises. Fire them lying flat with solubles side up.
184.108.40.206. If the SOLU test is not being performed discard the specimen, otherwise:
220.127.116.11.1. Mark the half-disk as described in the SOLU test procedure.
18.104.22.168.2. Accumulate half-disks for bisque firing, glazing and evaluation of possible interaction of soluble salts with the glaze.
5.7. Record Notes
5.7.1. Print a test results report for each mix tested, compare and evaluate data as appropriate and enter notes for records where appropriate.
5.7.2. Report to management or production if needed.
5.8.1. Disk Making and Drying
22.214.171.124. The same tools are needed as for the SHAB test with the following exceptions
126.96.36.199.1. Bar width guage, metal marking template and talc powder are not needed.
188.8.131.52.2. The thickness gauge must be 3/16 inch (4.75 mm).
184.108.40.206.3. A compass or divider which is suitable for cutting the 12 cm disk from the soft rolled clay is also needed.
5.8.2. Evaluation of Dried Disks
220.127.116.11. Vernier calipers accurate to 0.1 mm.
18.104.22.168. Ceramic pencil for writing on dried specimens (preferably a dark color like black).
5.9.1. Do not breathe clay dust unnecessarily.
5.10.1. Clay Preparation
22.214.171.124. Same as for SHAB test.
5.10.2. Making Disks
126.96.36.199. It is important to roll the clay exactly the same way each time to get consistent and comparable results.
188.8.131.52. Use a dry canvas board and peel the clay up frequently during rolling to make sure it does not stick to the board, making the back side of the specimens excessively rough and stretching the clay, thereby affecting the integrity of the test.
184.108.40.206. If the stamp does not produce a clean imprint press a little harder. If the clay lacks enough plasticity to make a readable imprint then write carefully with a needle tool.
220.127.116.11. When cutting the circular disk pay attention that the divider or compass does not change setting. Also, before cutting a disk, verify that the compass or divider is set at 12 cm.
18.104.22.168. It is important that the disks suffer minimal edge cracks as a result of rolling and cutting as these will affect the final drying factor.
22.214.171.124. If you must write on a disk with a sharp pencil or needle tool do it so that it is clearly legible for later data entry. Write in small letters to minimize the chance that a crack will start at the written label.
126.96.36.199. Stamping the ID# on each disk twice is helpful because if a disk cracks across one the other will likely still be readable and its section can be used for possible SOLU evaluations.
188.8.131.52. When breaking one of the disk halves to make the quarter disk for further use in the SOLU test, make sure both exposed and non-exposed surface areas are preserved.
Reason for Re-issue
DFAC - Drying Factor (V)
There are four major crack types:
SOLD - Salts on Dry DFAC (V)
The drying disk will develop soluble salts around the out exposed edge. Rate these as either Nil, VLight, Light, Medium, Heavy.
SOLW - Salts on Wet DFAC (V)
Treat the same as dry disk.
A typical DFAC drying disk of an iron stoneware clay
The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
How to interpret the crack in a DFAC drying disk
Drying disks used for the DFAC test are 12cm in diameter and 5mm thick (wet). A crack pattern develops in almost all common pottery clays as they shrink during drying. This happens because the center portion is covered and stays soft while the perimeter dries hard. This sets up a tug-of-war with the later-drying inner section pulling at the outer rigid perimeter and forcing a crack (starting from the center). If the clay has high plasticity and dry strength it can pull so hard from the center that cracks appear at the outer dried edge to relieve the tension. Or, it can create cracks that run parallel to the outer edge but at the boundary between the inner and outer sections. The nature, number and width of the cracks are interpreted to produce a drying factor that can be recorded.
Example of a lab drier with heating elements, fan and temperature indicator
These are invaluable for creating a consistent environment in which to perform drying and shrinkage tests.
Albany Slip DFAC dried disk
This shows the soluble salts in the material and the characteristic cracking pattern of a low plasticity clay. Notice the edges have peeled badly during cutting, this is characteristic of very low plasticity.
A DFAC drying test disk of a terra cotta pottery clay from St. Ignacio, Sinaloa, Mexico
This clay is used by potters in the Mazatlan area. The wideness of the main crack and number of edge cracks indicate high shrinkage and high plasticity. The clay does has some coarser grains that help channel water out. Soluble salts are fairly low. Local potters are able to use this successfully in spite it its tendency to crack.
Grog does not always have the intended effect
These DFAC drying performance disks show that minor additions of grog do not reduce the fired shrinkage of this medium fire stoneware much. Nor do they improve its drying performance. In this example, a 10% addition has not reduced shrinkage appreciably nor has it improved drying performance. The 20% addition has reduced the shrinkage and narrowed the crack, but it is still there and resembles the zero-grog version.
Examples of DFAC disk, SHAB bar, LOI bar for clay testing
By preparing these three tests you can measure many properties of a clay body. These include drying shrinkage, fired shrinkage, porosity, drying performance, soluble salts content, water content and LOI.
High drying shrinkage of Plainsman A2 ball clay (DFAC disk)
This test shows the incredible dry shrinkage that a ball clay can have. Obviously if too much of this is employed in a body recipe one can expect it to put stress on the body during drying. Nevertheless, the dry strength of this material far exceeds that of a kaolin and when used judiciously it can really improve the working properties of a body giving the added benefit of extra dry strength.
The white one feels smoother, but it is actually far coarser. Why?
Large particle kaolin (left) and small-particle ball clay (right) DFAC drying disks 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.
An example of a DFAC drying test of a bentonitic clay
This disk has dried under heat (with the center part protected) for many hours. During that process it curled upward badly (flattening back out later). It is very reluctant to give up its water in the central protected section. Obviously it shrinks alot during drying and forms a network of cracks. When there are this many cracks it is difficult to characterize it, so a picture is best.
Do grog additions always produce better drying performance?
This DFAC drying performance test compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise there appears to be no improvement. While the grog addition reduces drying shrinkage by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Another issue is the jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.
Cutting the disk for a DFAC drying factor test
The plastic clay has been rolled to 3/16 inch thickness (using the metal rods and a rolling pin). The disk is cut to 12 cm diameter.
DFAC disk under a heatlamp
The heat lamp dries the out edge in minutes (this photo makes it appear hotter than it really is). The center section of the disk is protected by the glazed bowl and takes an hour or more to dry. This sets up stresses that cause the disk to crack. The nature and size of the cracks enable establishing a drying factor value for the clay.
Particle size drastically affects drying performance
These are DFAC drying performance tests 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.
Lab testing a clay for its physical properties
SHAB test bars, an LDW water content sample and a DFAC drying disk about to be put into a drier. The SHAB (shrinkage-absorption) bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI from these measured values.
A bentonitic clay that takes a long time to dry
I finally gave up trying to dry the inner section of this DFAC test. During that test the inner part of the disk is shielded from the air flow or heat lamp. This sets up a shrinkage gradient that encourages cracking of the sample. But with some clays drying can be so slow that it can take a days. Serious cracking and high drying shrinkage almost always accompanies this phenomenon.
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