How can I clean the brush that I am using for burnt pots? It is black and greasy..
What sort of brush is it?
If it's like this one:
then the only way I have found it to soak it in almost neat washing up liquid (add a bit of water if you need to thin it down a bit), mash it around then rinse it all out, and finally soak it in bleach if it's still stained. This is really time, labour and materials intensive and likely not worth it for a cheap brush.
I have had much more success with the kind of metal-coated scrubby things, which you can simply wash off afterwards, but they are more expensive to buy in the first place.
I toss a number of my cleaning tools (sponges, brushes, etc) into the dishwasher anytime there's a bit of extra space. If done regularly it keeps the grime buildup from happening too much.
If yours is REALLY greasy, you might want to try heating some soapy water in a pot and simmer it for a little while. (This will get the pot quite greasy too, so be ready to clean that as well). You can even boil it if the plastic is the right kind. Heat will help dissolve the grease.