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Wendt Pottery Clay Standards Page 2014
Check this page regularly for research updates as they occur. Last update 01-01-12 MW
See photos of the equipment and procedure at the bottom of the page.
Test from 12-01-03
I cored Seattle Pottery's SP750 Columbia White cone 8-10 directly from the cut face of an undisturbed pug several months old. The core required 19.35 PSI to cause it to deform 0.3000 inches. I then wedged another piece from the middle of the same pug and cored it. After wedging, a weight of 15.75 lbs resulted in 0.3000 inches of deformation. Slugs 1" in diameter by 1" long weighed exactly 28 g and weighed 22.7 g after air drying until no further weight loss was noted.
I cored Seattle Pottery's SP760 Pine Lake White cone 4-10 directly from the cut face of an undisturbed pug several months old. The core required 19.25 PSI to cause it to deform 0.2335 inches. I then wedged another piece from the middle of the same pug and cored it. After wedging, the same weight of 19.25 lbs resulted in 0.4560 inches of deformation. This is nearly twice as much after wedging using the exact same weight. This is what most people expect.
I cored my medium Helmer body which had been in storage for several months. The core taken directly from the pug required only 13.55 lbs to achieve a 0.3000 inch deformation. Surprisingly, after it was wedged it deformed only 0.1810 inches under a 13.55 lb load. This is NOT what one expects. It is what the clay feels like to me, though. I notice the Helmer body gets stiffer as I wedge it rather than softer! Explanations?
I next returned to the original format proposed: to take all samples to the same 30% deformation and thus compare the stiffness of each as a function of weight needed to deform them 30%.
I cored Seattle Pottery's SP850 Kenzan Porcelain #1 White cone 8-10 directly from the cut face of an undisturbed pug several months old. The core required 14.85 PSI to cause it to deform 0.3000 inches. I then wedged another piece from the middle of the same pug and cored it. After wedging, a weight of 10.4 lbs resulted in 0.3000 inches of deformation. Slugs 1" in diameter by 1" long weighed exactly 28 g and weighed 23 g after air drying until no further weight loss was noted.
Conclusions: Clays lose moisture in storage, even in a heavy, high quality plastic bag. This is obvious to users. Also obvious is that as clay sits undisturbed, it takes up "set" so that the undisturbed pug feels very firm.
Wedging really does reduce the force needed to change the shape of the clay. The amount of force needed to alter the clay depends on moisture content AND the amount of non-plastic material in the clay body.
As the amount of non-plastic material in a body increases, the softening effect of wedging becomes measurably less.
From this, it is clear each clay will perform differently. More later.
Below is the test plunger being machined so it measures exactly 1 square inch and is true and flat with respect to the shaft. The wooden frame is easy to build, has a very low coefficient of friction with metal and costs next to nothing. The most expensive part is the digital vernier caliper. Mine is accurate to 0.0005".The view with the caliper shows a round slug used to calibrate the device. It is exactly 1" in diameter. The caliper is brought into contact with the top plate and the reset button is pressed to get a true zero each time.
Lathe work assures the plunger will have an exact area of 1 square inch. Even though the cores are less than 1 square inch initially, by the time they deform 30%, they contact the entire face of the plunger and so give a true force based measurement anyone anywhere can test and repeat easily.
Full view of the machine with weight on top. Since the first test gave me the general range of values that clay deforms under, I have replaced the large bucket with a steel weight of 5 lbs and will have additional weights made at the machine shop to allow quicker testing. Only the final 5% of deformation requires the smooth application of force to get a true reading without overshoot. I found that overshoot happens because clay "creeps" under a load.
This creep feature means that the reading continues to change with time if the load is higher than the true force value for the body being tested. That is probably why the standard penetrometer is useless in this application.
Calibrating the platen to 1".
Once the unit is built, the cores are taken from a pug of clay using the Monoject 60 cc syringe with the end cut out. The slugs are next cut exactly to 1 inch height to assure all the samples tested are identical. For ease of use, the slugs are capped top and bottom with thin plastic bag stock. Notice the slug in the picture spread out to slightly more than the diameter of the plunger, giving a true value of the total weight it is able to support with one square inch of top area.
The reason I think this test method is more meaningful to potters is we already do this test with our hands. When we mix clay, we take a piece and squeeze it to see if it feels right. When I want batches to match, I swap samples from hand to hand. Now consider the tester... each batch that I mix can be tested after 15 minutes of mixing by pulling a core, putting it under the plunger and recording the tester value. If the clay requires more force than the target value, I can add a few pounds of water, mix again and then test. Likewise, if the clay is softer than the recorded value, I can add more of the dry blended body to stiffen it up until it matches. With practice, I am sure the repeatability of the process would go up.
Cutting the samples to 1".
30% deformation, note the plastic sheets.
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