Testing Leached Premo

There has been some discussion in clay circles recently about the necessity to leach the new, softer formulation of Premo, and what, if any, effect leaching has on the strength of the clay. I decided to do a test. I took a big chunk of black Premo from a large block dated January 2008. It was quite soupy, so I'm sure it is the new formulation. I conditioned the clay by putting it through my pasta machine several times to make sure the chemicals were evenly distributed, and then I divided the clay in half. I leached one half by rolling it to #3 on my pasta machine (#1 is the thickest) and then put it between sheets of white paper. I rubbed the top sheet gently and then put a light weight on top. Then I let it sit overnight until 10 am the next morning. There was a lot of oily residue on the paper in the morning.

Next I thoroughly conditioned both the leached and unleached clay by putting it through the PM about 30 times. The leached clay cracked when I folded it and the sides were very ragged at first, but at the end of conditioning, it no longer cracked when folded.

I cut strips 1" by 5" at the #1 setting (thickest), #4 setting and #6 setting. It was easy to get a very thin strip of the leached clay, but the unleached was so sticky, it wouldn't roll flat. I got waves in it. Then I thought it would be interesting to see what would happen between the two extremes, so I blended the left over clay and made strips that were half leached and half unleached and then some that had a small amount of unleached clay added in to the leached. (3 and 1/2 parts leached and 1/2 part unleached.) This would simulate what you would do if you felt you had over-leached the clay and added some fresh clay back in. I had enough of this mixed clay to make strips at the #1 and #4 thicknesses. I baked the strips on tiles, at the same time and all at the same level in my convection oven, for 25 minutes at 285 degrees F. I wanted to be sure that the temperature met and slightly exceeded the 275 degree threshold suggested by the manufacturer. The strips were allowed to cool in the oven for several hours while I went to a movie.


First I tried bending the strips by touching one end to the other. I did that 50 times to the leached clay at all three thicknesses and they showed no wear or tear. I bent the thickest strip 50 times in the opposite direction and it was fine. That test wasn't going to show anything useful unless I spent a week doing nothing but bending clay so I tried something more extreme.

I bent about an inch of the strip back on itself and pinched the fold as tight as I could using both thumbs and forefingers. Then I bent it in the opposite direction and pinched the fold again (much tighter than in this photo). I kept doing this until the strip tore, and I tried to apply the same pressure each time. If leaching weakens the clay, I would expect to see a progression: the less leached, the more bends it would take to tear the clay.




Here's what happened:

Number of bends until the strip tore all the way across

First trial at #4 thickness
...Leached................... 1 bend until break
...Mostly leached .....20 bends
...Half and half.......... 5
...Unleached ..............25

Second trial at #4 thickness
...Leached ..................4
...Mostly leached .....25
...Half and half .........22
...Unleached .............33

Trial at #1 thickness (the thickest)
...Leached................... 2
...Mostly leached....... 7
...Half and half .........21
...Unleached.............. 37

Clearly, at both thicknesses, the fully leached clay breaks much faster than the unleached clay when subjected to extreme bending pressure. The thickest strips showed the progression I expected, but the results for the thinner (#4 thickness) strips was strange. The clay that was 3.5 parts leached to .5 parts unleached performed better than the clay that was half and half. I can't explain that. I mixed the clay thoroughly before making the strips. Perhaps the pressure I applied wasn't uniform.

For the very thinnest strips (#6), the leached clay broke after 14 bends and the unleached was still fine after 45 bends. No cracking at all. I didn't have intermediate samples at this thickness.

This bending test subjected the clay to stresses that probably would not be found in real life, so I tried whacking the different strips against the edge of my desk to simulate what might happen during shipping or if an item was dropped. None of them broke or chipped. Tests of objects with different shapes would be necessary to see what might happen during shipping of a sculpture, for example.

So, what does this tell us? Well, leaching makes Premo less able to withstand flexing, i.e., it is more brittle. For something like a cuff bracelet, less leaching is better. But for other objects not likely to be subject to such stresses, leaching may be acceptable. The question remains, how much is OK for a particular application and how do you control the amount of leaching that takes place? Is there a critical point where it goes from acceptable to not acceptable and how do you know where that is?