Vacuum all around the fireplace after you have removed all the ashes inside the firebox with a fireplace shovel. Use one of the attachments to remove loose ash from the sides and bottom of the fireplace's brick surfaces.
Put 2 oz. of dishwashing detergent into the bottom of a bucket. Add 2 oz. of kosher salt.
Pouring in a little water at a time, mix the soap and salt into a creamy paste.
Use a cloth to apply the paste to the bricks that have been covered in ash. Let the paste stay on the bricks for 30 minutes.
Scrub the bricks with a stiff-bristled scrub brush, working the paste into the bricks and removing the grime and ashes.
Rinse the fireplace brick with water after all the ashes have been removed.