(Denver, CO USA)
I have a barrel style composter. I've added grass clippings, coffee grinds, egg shells etc... It didn't get watered while we were on vacation and now I have large hard clods of compost along with other materials. I've been turning and watering. How do I break down these clods?
I think there's a good chance these clods will stay clods without some radical intervention.
I've had hard clods form with grass clippings before and they seem impervious to any breakdown. You think these formed because the compost wasn't watered while you were away. My guess is it is a combo of the grass clippings in particular being wet, and then no tumbling and dry conditions making the material into those hard bricks you are faced with now.
It might seem like a good idea to water the compost very well now. The problem with that, especially in a tumbler, is you will likely end up with things too wet and anaerobic stinky conditions developing.
The clods do eventually break down but I don't think it's going to happen in your tumbler. I would empty the clods out of the tumbler. You could experiment with these two methods then.
Take some of the clods and soak them until they fall apart. Then either spread them out in a thin layer on top of your soil and let the worms finish the job... or dry them out till they are just damp and put them back in the composter.
As a second experiment you could just wet some of the clods down and pile them up in some out of the way spot of your garden, maybe even bury them.
The material that didn't turn into hard dry clumps can stay in the tumbler and you can carry on.
I do wish I had a magic solution to this but I have been there and they are pretty tough cookies those clods.
Good Luck and Carry On...