I had a go at making some clump foliage flock out of some budget kitchen sponges from the supermarket. Generally, I'd call the experiment a success, but there are improvements that could be made.
I have a cheap little blender that I bought specifically for model-making and the like, so that I don't have to be spending hours getting tiny fragments of unmentionable stuffs out of our kitchen blender. It's not large, nor is it powerful, but it's sufficient for my purposes so far.
I tore the sponge into strips and then soaked them in water to give them a bit of inertial resistance to the blender blades. If they're dry, they just bounce around inside the blender, and bounce right off the blades without taking much useful damage — the added water gives them more mass, and the blades can tear through them.
It takes a little while to get everything rendered down into sponge fluff, but after about five or ten minutes of shaking the blender and pulsing it and so forth, the resulting slurry is decanted into a fine-meshed sieve and as much water as possible squeezed out.
Then acrylic paint — I just used a house-paint test pot — is squeezed and kneaded through. I found I didn't need a huge amount of paint.
Then the excess paint is again squeezed out, and here I may have gone too far — the final colour is a bit lighter than the paint I chose, and maybe if I'd left a bit more paint in there it would have stayed darker. Also, the dried, finished mass is more friable than I expected; though that's not necessarily a bad thing, more paint left in it would probably have helped it to clump more and perhaps shed fewer little bits.
I spread out the wet mass to dry on a sheet of baking paper in the hot-water cupboard overnight, and it was bone dry when I got it out the next day.
I'm not unhappy with the results at all. I already have some commercial clump foliage in dark shades that I've been working my way through for ages, so having a much lighter lot is advantageous at the moment. However, I do want to try another batch and see if I can make the end colour a bit more predictable.
Is it worth the trouble? Maybe. A commercial bag of clump foliage isn't all that expensive, but it does only come in a fairly limited range of tones. If I make my own, I can make it pretty much any colour I want — if I can just come to grips with staining it predictably.