This is a comprehensive test to evaluate the drying and fired shrinkage and the porosity of clay bodies. It is intended that test bars should be fired at a range of temperatures to give a broad picture of the thermal performance.
This is the shrinkage due to drying only. Assuming 10 cm marks on the wet bar, drying shrinkage is simply 100 mm minus the dry length in mm.
IF(V1>0 AND V2>0, (V1-V2)/V1*100, "n/a")
This is the fired component of total shrinkage. Note that dry+fired
shrinkage does not equal total shrinkage because fired shrinkage is based on the dry length not the original 10 cm.
IF(V3>0 AND V4>0,((V4-V3)/V3*100),"n/a")
This is a measure of the clays fired maturity as interpreted from its pore space. The pore space is calculated from the increase in weight a bar experiences during boiling in water.
The distance between the outer edges of two marks on the dried clay bar as measured with a set of calipers. These marks were pressed into the wet bar at exactly 10 cm apart on the outer edges.FLEN - Fired Length (V)
The length between two marks on the fired clay bar as measured with a set of calipers.FWT - Fired Weight (V)
The weight in grams of the clay bar after firing.BWT - Boiled Weight (V)
The weight in grams of the clay bar after boiling for 5 hours and soaking for 19 and being blotted on a towel.CONE - Cone (V)
The Orton cone number to which the bar was fired. Take the highest cone to show deformation and interpret it as follows:
1 oclock n-.4
2 oclock n-.3 For example, if cone 6 is at 3 oclock, then the
3 oclock n-.2 value is 6 minus .2 = 5.8.
4 oclock n-.1
5 oclock n
If the guard cones shows deformation you must increase the figure
Bullers Ring diameterTEMP - Temperature (V)
The temperature in celcius. This can be derived electronically or from a bullers ring.
This test is designed to derive shrinkage and absorption data by drying and firing clay bars according to a detailed procedure. This test does not account for LOI as does SAWL. This test is meshed with the SOLU, DFAC and LDW tests in that it is very convenient to perform all four at the same time. The numbers you measure during this procedure are entered as testdata into a recipe in your account at insight-live.com.
2. Purpose of Test
1.1 This test is designed to measure dry shrinkage, absorption and fired shrinkage properties. Results from this test are repeatable if instructions are followed closely.
1.1.1 DRY SHRINKAGE
As a clay dries the removal of interparticle water causes the mass to tighten up and pack together resulting in shrinkage. Clays of fine particle size and those of high plasticity have high shrinkage. Unfortunately the benefits of plasticity are offset by drying problems. Variation in drying shrinkage is an indicator of changes in a clays plasticity. However comparing the dry shrinkage of different types of clay is not necessarily in indicator of their comparative plasticity since some fine clays are not plastic. Note that higher water content also means greater dry shrinkage.
For typical modelling stiffnesses dry shrinkage for non-plastic clays is around while plastic clays which require care in drying are usually above 7.0%. High shrinkage can be reduced by the addition of an aggregate however this can produce a matrix where micro-cracks radiate outward from each of these larger particles creating a weaker dried and fired product. A low drying shrinkage is important to successfully dry larger items or ware of uneven cross section.
Dry shrinkage is simply the per cent change in length between wet and dry. The SHAB test provides the data for this property as follows:
Wet length - dry length / wet length * 100
or where a 10 cm marks are stamped on the bar it is simply:
100 - mm dry length
1.1.2 FIRED SHRINKAGE
As a clay fires, it shrinks and particles continue to pack together. At some point, they begin to break down and react with each other, fluxes begin to melt and flow, and mineral grains seed the development of more stable forms. The amount of shrinkage during firing is thus an indication of the degree to which the complex "maturing" process has proceeded.
Fired shrinkage figures are valuable both in maintaining a clay body's fired properties and formulating bodies to have a good compromise between shrinkage and fired maturity. A body's fired shrinkage plotted against increasing temperature normally describes a graph which rises to a level plateau, then quickly drops off during expansion and melting. Porcelain has a very high fired shrinkage compared to stoneware clays which are in turn much higher than earthenware materials. This is a result of lower dry density, finer particle size, and higher maturity.
Fired shrinkage is the percent change in length or width of a test sample from dry to fired. The SHAB test provides dry and fired length data for calculation of this property as follows:
Fired length - dry length / fired length * 100
Absorption is an indirect indication of the pore space within the fired clay. Since pore space gradually closes up during the firing process, absorption indicates the extent to which the heat has developed the matrix, that is, the "maturity" of the body.
Stoneware clays most often do not reach zero pore space before bloating and melting. Large particles create more pore space than can be filled by fluxes and evolving mineral species. Functional ware can tolerate 1% or more pore space without leaking water. High porosity can still be tolerated by using a good fitting glaze that seals the surface. Wall tile makers require resistance to sagging during firing, so density is secondary. However, porcelain-type products require high strength and density, thus zero absorption is often important.
This property is a measurement of the percentage increase in weight that a test bar experiences during a normal five hour period in boiling water (followed by a 19 hour soak). The SHAB test definition provides weight data before and after boiling, so the absorption is calculated:
Wet weight - dry weight / wet weight * 100
2.1 Test bars are made from each production run and from each body or clay material tested for R&D or quality control work.
3.1 SHAB - Shrinkage-Absorption
3.2 QC - Quality Control department
4.1 Responsibilities are transferred between departments as the test
proceeds. These are outlined in section 5.0.
5.1. Create a FORESIGHT Record
5.1.1. If this test is being done to evaluate a trial or test mix, then a database record should be assigned in the FORESIGHT Results or Recipe areas and a description of the objective for the mix entered.
5.1.2. If this test is being done as part of a quality control program for production then the FORESIGHT database record need not be assigned till the specimen and associated run information is recieved.
5.2. Take or Make the Sample (Pugmill Operator or Clay Mixer)
5.2.1. If the test is being done on a production material: Midway through the run take a slug of clay which is of representative stiffness. If the test is being done on a lab mix make sure that it has been prepared in a standard way (i.e. slurried and dewatered on a plaster batt and thoroughly kneaded) and is of the correct stiffness.
5.2.2. Put the slug in a plastic bag and mark the bag with appropriate identification. Mark the time and date the sample was taken or prepared on the bag.
5.3. Make the Bars (QC)
5.3.1. Take approximately 800-1000 grams of the pugged material and knead it for 30 seconds; change direction once midway through the process. 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 3/8" (9.5 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 until it is 5" wide. Peel it up again and without flipping it over turn it 90 degrees again and roll until the rolling pin runs on the metal rods.
5.3.3. Carefully lift the slab of clay from both ends and place it on a bar board lengthwise. Trim the slab to the board width by using the edge of the board as a guide. Using the bar width gauge, cut bars for firing at 6 or 7 different temperatures of 25 mm (1 in) width.
5.3.4. If LDW or DFAC specimens are being made do them now and place them on the free space at one end of the bar board.
5.4. Mark and Stamp the Bars
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 three groups of digits left to right separated by at least one space as follows:
18.104.22.168. The ID#
The number of hours since the specimen was taken or slurried and dewatered
5.4.3. Press the stamp into the middle of the first bar, increment the specimen digit(s) and stamp the next one. Proceed until all bars are done. It is not necessary to begin numbering the specimens at 1. If, for example, bars are to be fired at cones 6-12, then numbering the specimens 6-12 is better.
5.4.4. Press the metal or metal marking template into the first bar. Be careful to avoid lateral movement that could smudge the mark. Remove it with a twist to produce a crisp impression. Dip each end of the template into a cup of talc powder, tap it so excess powder falls free and stamp the next bar. Continue till all bars are done. Do not press too heavily, 2 mm deep is sufficient.
5.5. Dry the Bars
5.5.1. Within a few minutes of making the bars place them into an air-circulating drying chamber for at least 4 hours until they are thoroughly dry on all sides. If necessary weigh a bar, dry longer and weigh again to see if 4 hours removes all the water.
5.5.2. Using a fettling knife trim every corner to round it and remove any burrs or irregularities.
5.6. Measure the Bars
5.6.1. Measure each bar using calipers capable accuracy to 0.1 mm. Using a ceramic pencil write the length in mm on the back or side of the bars. If the bars are too fragile or the clay is not abrasive enough for the pencil to make a mark then do one of the following:
22.214.171.124. Record the data on a Testdata record sheet as printed by FORESIGHT. Record the ID number, specimen number and length for each bar.
126.96.36.199. Write the data on the bars using an ordinary pencil then take them to the computer and enter the data into FORESIGHT.
188.8.131.52. Measure the bars and type the information directly into the computer. This is the best option as it eliminates a step and acts to visually link each bar with its associated shrinkage.
5.7. Accumulate Dry Bars
5.7.1. As sets of bars are measured bring them to a pre-fire holding area.
5.7.2. Make a stack for each temperature to be fired and maintain these stacks carefully and in order as new bars are added.
5.7.3. Where the cone number to which a bar is fired differs from its specimen number, write the intended cone on the bar in pencil. If the bar is to be fired by an automatic firing device, write the program number to be employed on the bars or on a label beside each stack of accumulating bars.
5.8. Fire the Bars
5.8.1. Prepare the kiln by first placing three bar supports approximately 3-4 cm apart on the kiln shelf. Do not position them directly across from each other; offset them by at least 1 cm. This is done so that bar shrinkage occurs at an angle to the setting and is less likely to upset the stack.
5.8.2. Place three test bars on edge across the bar supports and position them in a similar manner. Use the remaining bars to build up criss-crossed layers to a maximum of 6. Make more stacks as needed.
5.8.3. Place a set of cones (guard, firing and guide) and a buller's ring in the kiln.
5.8.4. Fire the kiln so that it maintains and even rate of rise and finishes in approximately 6-8 hours for cone 10, 3-4 hours for cone 06. If the kiln contains bars which are made from heavily bentonitic or ball clay, add 2-3 hours at the beginning of the firing and hold the kiln around 150C to prevent blow-up.
5.8.5. Unload the Kiln
184.108.40.206. Remove the buller's ring and measure it in the gauge. Write the measurement on the ring.
220.127.116.11. Remove the cones and interpret the cone number for the firing (according to IMC Cone Interpretation Guidelines) and write it on the buller's ring.
18.104.22.168. Remove the bars from the firing and take them with the buller's ring to the scale.
5.9. Measure the Bars
5.9.1. Weigh each bar to the nearest hundredth of a gram and write the weight on the back.
5.9.2. Measure the distance between the outsides of the length marks to the nearest tenth of a mm on each bar and write the distance on the back of the bar.
5.9.3. Record the length and weight data for each bar using one of the following:
22.214.171.124. Write the ID#, specimen number, weight and length on a Testdata collection sheet as printed by FORESIGHT.
126.96.36.199. Enter the information into FORESIGHT using the Testdata Entry Dialog. This method is the preferred one since it removes one step reducing error and provides an opportunity to relate the numbers collected with the identify of each bar as displayed by FORESIGHT. Problems may be noted and mental notes made.
5.10. Accumulate Fired Bars
5.10.1. As sets of bars are weighed, measured and recorded collect them in a fired bar holding area.
5.10.2. Take the bars to the next step either when enough have been accumulated or special circumstances require test results sooner.
5.11. Boil the Bars
5.11.1. Get the bars from the fired bar holding area and place them on edge in rows and layers in a boiler. Place them in such a way that each layer is dense enough to support well the layer above it which is placed perpendicularly. It is helpful to place the bars in neat rows so that data can collected later by specimen groups rather than randomly from a haphazard stack.
5.11.2. Fill the boiler with distilled water.
5.11.3. Boil the bars in a steady but not violent manner for 5 hours, topping up the water periodically as it boils down.
5.11.4. Soak the bars for 19 hours.
5.12. Weigh the Boiled Bars
5.12.1. Remove the bars in groups of 10 or 20 and blot them on a towel.
5.12.2. Weigh each bar and write the weight figure on the back of the bar below the dry weight if still visible.
5.12.3. Record the wet weight data using one of the following:
188.8.131.52. Write the ID#, specimen number and weight on a Testdata collection sheet as printed by FORESIGHT.
184.108.40.206. Enter the information from the bars directly into FORESIGHT using the Testdata Entry Dialog. This method is the preferred one since it removes one step reducing error and provides an opportunity to relate the numbers collected with the identify
of each bar as displayed by FORESIGHT. Problems may be noted and mental notes made.
5.13. Accumulate Fired/Boiled Bars
5.13.1. As sets of bars are weighed and recorded collect bring them in a boiled-bar holding area. Organize them in stacks of common ID# and order the stacks by ID#.
5.13.2. If bars are needed for visual examination and comparison be sure that they are returned and placed in the stacks from which they were taken.
5.14. Glue the Bar Sets on Boards
5.14.1. As the prescribed firings are done for each set of bars of common ID#, take one of each set to the computer. In FORESIGHT perform a search for the ID# written on each bar to determine its identify.
5.14.2. In FORESIGHT set a filter for its name to show records of other tests which should be glued on the same board (results should be grouped on boards according to type). For example, if the clay is P300 porcelain, filter on "P300" and browse the
recipe database and display the last screen full of records. Use the information shown to determine which archive board has room and should be used to hold the new set of bars. Write the board number on the edge of the clay bar.
5.14.3. If there is no existing board appropriate for the bars, take a new one. Assign it the next board number by setting FORESIGHT to use the LOCATION index in the Recipe or Results area. Go to the end of the database, note the board number and use the
next one. You can also assign a new number by going to the board storage area and looking at the last board, however this is not always reliable as someone may have the board out temporarily.
5.14.4. Determine the board number for all the remaining bars.
5.14.5. Place all the bars on edge to display the board numbers written there. Sort them in order from lowest to highest board number.
5.14.6. Take the bars to the board archive storage area and pull out all the needed boards and get some new boards if needed.
5.14.7. In the gluing area take each fired bar, get its companions and glue them onto the appropriate board. Write the name of the clay body on the board above the bars for future quick reference.
5.14.8. If a board is full, take a new one and assign it a number as described above.
5.14.9. Take all the boards (including those which were full and a new one started) to the computer.
5.15. Record Notes
5.15.1. Take each board and look up the FORESIGHT record for those groups of bars just glued.
5.15.2. Compare the new set of bars to previous ones on the board (or the accompanying full board if a new one was started) and make appropriate notes if there are visual differences.
5.15.3. Print a test report in the FORESIGHT Results area. Compare the results with standards defined for the body and previous tests and make notes of any pertinent observations.
5.15.4. Take appropriate action based on the results.
5.16.1. Bar Making and Drying
220.127.116.11. Cutting wire.
18.104.22.168. Needle tool to write on wet or dry specimens.
22.214.171.124. Large wooden rolling pin and canvas covered rolling board.
126.96.36.199. Gauge for cutting bars to width made from a 25 mm (1 in) square piece of wood 15 cm (6 in) long. The wood should be light and porous so it will not stick to the clay.
188.8.131.52. 3/16 inch (9.5mm) diameter rods to act as thickness gauges for rooling clay to correct thickness.
184.108.40.206. A rotary date-stamp style stamp capable of imprinting 14 digits of any letter or number.
220.127.116.11. A metal marking template. When pressed into a clay bar it will make two crisp impressions 4 mm square and 2 mm deep whose outer edges are exactly 10 cm apart. You will have to have this make or make it yourself. Either way make sure it is accurate and square.
18.104.22.168. Bar boards made from 1/4 inch plywood 114 mm (4.5 in) wide by 400 mm (16 in) long.
22.214.171.124. A cup of talc powder.
126.96.36.199. Drying cabinet which maintains a temperature of 80-100C and has an air-circulating fan and a damper which controls the amount of fresh air entering the chamber.
188.8.131.52. Vernier calipers accurate to 0.1 mm.
184.108.40.206. Scale capable of 1/10 gram accuracy or better yet, 1/100 gram.
220.127.116.11. Ceramic pencil for writing on dried specimens (preferably a dark color like black).
18.104.22.168. Water boiler with timer. An electric slow-cooker with adjustable dial to fine tune the temperature works best.
22.214.171.124. Computer equipped with FORESIGHT Ceramic Database software.
126.96.36.199. Orton cones. These are a heat measuring device used to determine when to terminate a firing.
188.8.131.52. Buller's rings. These are a device which are used to provide information on the amount of heat which a firing received based on the amount of shrinkage of the ring.
184.108.40.206. Test kiln capable of firing comfortably to all temperatures at which test bars will be fired.
220.127.116.11. Bar supports which form the bottom layer of each stack of bars during firing. These should be made from insulating firebrick and made the same dimensions as a dry test bar.
18.104.22.168. Silicone sealant for gluing bars. While you can glue bars with hot glue, over a period of time they will release.
22.214.171.124. Mounting boards for fired bar archival. They should be heavy cardboard or pressed board and be big enough for at least six sets of bars.
5.17.1. If you are firing the test kiln, do not look into it without a dark visor and make sure it is in a well ventilated room.
5.17.2. Do not breathe clay dust unnecessarily.
5.17.3. Use asbestos-free gloves to unload kilns which are hot to prevent burns.
5.18.1. Clay Preparation
126.96.36.199. Be careful not to knead air into the material when preparing to make bars.
188.8.131.52. If the clay will not knead without severe splitting consider the addition of 1% or more white firing bentonite to augment its plasticity. Reslurry the clay, add the bentonite and pour it back on a plastic batt to dewater.
5.18.2. Making Bars
184.108.40.206. It is important to roll the clay exactly the same way each time to get consistent and comparable results.
220.127.116.11. 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.
18.104.22.168. When cutting the bars be sure that the first cross-cut is done against the edge of the bar board. This cut will act as an accurate perpendicular reference for placing the width guide to cut remaining bars.
22.214.171.124. Be sure that during all cuts the knife is held vertical to get a square cut. This is important because the bars must sit on edge during firing.
126.96.36.199. When cutting the bars to width, flip the bar width guide end for end and upside down so that it does not become too wet and stick to the bars thus disturbing them as it is lifted off.
188.8.131.52. It is important that the bars not have edge cracks if they will be used later for strength testing. Edge cracks usually indicate a clay which lacks plasticity.
184.108.40.206. The length marks should not be impressed too deeply as the pressure can stress the bar.
220.127.116.11. 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.
18.104.22.168. All information should be written on the bars before the 10 cm length marks are impressed. The bar should not be moved or stressed in any way after the length marks are impressed. This causes internal discontinuities and stresses that affect the bars firing and drying shrinkage and tendency to warp.
22.214.171.124. The bars must not be subjected to excessive dry drafts or very high heat while drying to avoid warping them or building in stresses which will affect shrinkage.
126.96.36.199. It is important that bar edges be trimmed so that burrs and chips do not fall off during processing and throw out any weight measurements.
188.8.131.52. When bars are stacked in the dry-bar holding area be careful not to stack dark colored bars on top of light ones thereby discoloring them.
184.108.40.206. Do not fire the test bars by laying them flat on a kiln shelf. Bar supports are employed because bars laid directly on the kiln shelf suffer a heat-sinking effect from the slower-to-heat shelf. Even if they are placed on edge on the kiln shelf
they fire more mature along the upper portion and the bars will often bend.
220.127.116.11. It is important for the bars to be thoroughly dry before they are put into the kiln as some fine grained clays will explode if fired too quickly.
18.104.22.168. The bars must be fired using a consistent rate of temperature rise. 22.214.171.124. The bars should be fired right next to a set of cones or rings if fired in a large kiln.
126.96.36.199. The Buller's ring must be fired on edge in a holder to allow heat access from all sides.
188.8.131.52. Be very conscious about entering the locations of all specimens which are glued onto boards. The ability to find them again depends on this.
7. Reference Documents
Test Definition Report,
Test Procedure Report,
Data Collection Fill-In Form Report.
1 Initial Draft
2 Spelling and Grammer check and minor revisions.
3 Adjustment in data collection process due to better data entry
facilities in the software.
The clay was wedged thoroughly, rolled to 3/8 thickness (using the metal rods as gauges) and cut to 12cm long by 2.5cm wide bars. Code and specimen numbers were stamped on each bar (these are needed to enter data into Insight-live). The disk is being cut from 1/8" thickness. Notice how the clay tears as cut, this is an indication of low plasticity.
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.
The weight data from these fired test bars is being collected for the SHAB test in Insight-live (they have just been boiled for five hours and soaked for 19). Compiling this type of data for hundreds of simultaneous tests is possible because Insight-live takes care of all its organization.
SHAB test bars, an LDW tester for water content and a DFAC test 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 batch of fired test bars, organized by temperature, have already been weighed (the weight is written on the side of each bar). Now they will be measured and the SHAB test data (shrinkage/absorption) entered into each recipe record (in an account at insight-live.com). From this data Insight-live can calculate fired shrinkage and fired porosity, enabling you to compare the degree of vitrification of different materials and bodies. This is especially good for quality control purposes.
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 an ID and specimen number (the different specimens are the different temperatures) and the measurements have all be entered into our group account at insight-live.com. Now I have to take each pile and assess the results to make decisions on what to do next (documenting these in insight-live).
SHAB (Shrinkage, Absorption) test bars ready to unload. These are measured for length after drying and firing and for weight after firing and boiling. This data is plugged into my account at insight-live.com and it calculates shrinkage and porosity numbers. If you fire bars of a clay to a range of temperatures you can characterize key properties of a clay very effectively.
A 10cm mark is made in the plastic bar during preparation. Talc powder is put on the marker so it pulls cleanly away from the clay leaving a crisp mark. These bars are made from the same clay and will be fired at multiple temperatures. Each one needs to be measured.
This body is used in the sanitary ware industry in China, the supplier sends this report with each shipment. The chemistry and assorted values for porosity, shrinkage, particle size are provided. The factory receiving this report accepts it as gospel and goes into production. However engineers at the plant need to think twice about such reports. These tests are being done at one temperature, they say nothing about what that body is doing above and below that temperature. Is it being employed in a volatile range of the porosity or firing shrinkage curves? Zero porosity bodies of this type are best when fired to a point near where the porosity curve descends to reach the x-axis. However that curve remains at zero while the shrinkage one tops out and reverses direction. At some point the porosity curve sharply rises. Only by firing and testing at a range of temperatures in your own lab can you where your body is on the curve.
Some simple equipment is all you need. It is amazing how much you can learn from characterizing a body or clay material. You need a gram scale accurate to 0.01 grams (very inexpensive at your ceramic supplier). A set of callipers (again, not expensive these days). Some metal sieves (search "Tyler Sieves" on Ebay.com). A stamp to identify samples. A plaster table or slab. A propeller mixer. And, of course, a test kiln. And you need a place to put, and learn from, all the measurement data you will be collecting. An account at insight-live.com is perfect.
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.
The Redart clay bars (left) are fired at cones 06, 04, 2, 4 & 5 (top to bottom). The Plainsman Blue Grey Plastic bars (right) are fired at 06, 04, 03, 02, 2 & 4. The SHAB test procedure (used to make these) gives us the firing shrinkage and porosity at each temperature, these are direct indicators of the fired maturity. Notice how much the fired color changes with increasing temperature. The fired maturity is pretty similar but the BGP is a little browner in color. It is also much more plastic (the drying shrinkage quite a bit higher).
These bars have been fired at cones 4, 2, 02, 04 (top to bottom) using the SHAB testing procedure. We can measure fired shrinkage and porosity in each to get an indication of their fired maturity. The Redart (left) is much more vitreous and reaches almost zero porosity by cone 4 whereas the Lizella still has 11% porosity at cone 4. Lizella also has a much higher drying shrinkage (because it is way more plastic).
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.
These are part of the procedure for the SHAB test. The length of the bars is entered into a recipe record in your account at insight-live.com. When Insight-live has these numbers it can calculate the drying and fired shrinkages.
This type of stamp is deal for stamping mix and ID information on SHAB (and many other test types) clay test bars. Set up the run or recipe number on the left and the specimen number on the right.
Data for hundreds fired clay test bars was logged into a portable Epson custom programmed HX-20 computer and uploaded to a Radio Shack TRS-80 Model III where it was stored first on cassette, then floppy disk, then a loop tape. That data was later migrated to the Digitalfire DOS 4Sight lab record keeping system (as SHAB specimens) where it lived for more than 27 years (expanding to more than 200,000 tests) until being imported to an insight-live.com account in 2014.
|Media||Entering Shrinkage/Porosity Data Into Insight-Live|
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
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.
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
|Glossary||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
ASTM-C373 Water Absorption
|Tests||Dry Strenth (Round Bars)|
|Articles||Firing Clay Test Bars
Being able to make good consistent test bars and fire them in a consistent and proper way is a basic requirement of getting valid results for shrinkage and porosity measurement.
|Articles||How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
|Articles||Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
|Articles||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.
Tests conducted on bodies made from materials, as opposed to the materials themselves.