6/7: Shear Stress

Last time in this conditioning series, I talked about non-Newtonian materials and how their flow rate (also known as viscosity) changes when you apply pressure to them. There’s another thing that affects their flow rate, and that is the rate at which you apply pressure.

That makes sense. You squish something harder, it flattens faster.

But what happens when you take a slice of Fimo Professional and shove it through your poor little pasta machine? It shatters into annoying crumbs.

You see, polymer clay changes viscosity when you apply pressure. That’s why we condition it. But then an interesting thing happens. When some non-Newtonian materials encounter too much stress too quickly, they fracture and break apart. Even if it’s a liquid. If the RATE of stress is too high for that particular material, it will shatter.

That’s why the same slice of Fimo that shatters in your pasta machine will soften nicely if you slowly work it in your hand.

Of course, smashing your clay with a hammer will certainly get it conditioned faster, especially if you don’t mind making tons of crumbs. But if you don’t want your raw clay shattering when you condition it, go more slowly!

This very much becomes a factor when you’re extruding your clay. If you’re getting cracks on the edges, go more slowly. (Or change the viscosity by adding clay softener.)

Okay, one more article in this conditioning series. In my next article, I’ll share my observations about the flow characteristics of various brands and how that affects how you work with the clay.