I use crushed egg shells to clean my bottles. It’ so effective, that I don’t bother with a bottle brush anymore. In fact, I started doing it because I couldn’t find a brush to fit into some of my bottles.
The eggs come from our own hens and when I cook with them I leave the egg shells in a little bucket on the window sill to dry out. When the bucket fills up, I crush them using the mortar and pestle and then I store them in another bucket under the kitchen sink so they are within easy reach.
I put a couple of tablespoons of crushed egg shell into a one litre bottle with about a cup of water and then shake. The egg shell is abrasive and acts like a scrubbing brush to remove debris. It works well with water only, but you can also add a drop of dishwashing liquid or use soapy water for extra cleaning power.
Once a bottle is clean, I tip the egg shell and water into the next bottle and clean it too. I have cleaned five bottles using the same two tablespoons of egg shell. When I’m done I scatter the egg shells around the garden or in the compost.
I think this is a great way to reuse egg shells and clean any hard to reach place. Give it a try.