[Sizzle]

The other day I was washing some bisqued pieces and I noticed several sizzled like cooking bacon when I got them wet. The pieces made with the Hawaiian red clay in particular sizzled more than the rest. The clay has a rough texture - I believe it has quite a bit of sand in it. It’s a cone 5 commercial clay. 
 

Any ideas why it makes this noise?

 

[Clay Recycling Advice Sought]

What about mixing in a little bleach to kill or at least to tame the algae?