Okay, I AM an animal lover, so please do not be offended by this post.
I am also an eco lover, and want to use it up, wear it out, recycle, etc. etc.
So here's the story: I had an ancient can of hide glue (remember LePage's?)
It's also called mucilage, animal glue, or rabbit-skin glue.
Here's how to do crackles like the ones above:
Paint a base coat in acrylics...I went light, but you could reverse the contrast if you wish. When dry, cover with a layer of animal glue. While that is still slightly tacky, gently apply a coat of contrasting acrylic paint. I used dark green, lightly stroking it over the entire surface. That is all there is to it! The crackles will appear later.