Well you can scrap off the ashes out of it and save if you want and use it for other things.
You should be careful on washing your cauldron, the material that is made of can rust and so on.
After scraping out the ashes, you can put water and get a rag and wash it or you can use a magical wash to wash it. Magical washes are used to wash your tools and workspace. Some magical wash are used for protection, getting rid of negative energy and so on.
Lemons and aconite can be used for magical washes, note: aconite is poisons, so make sure not to swallow and keep it away form open cuts and keep away form small children and pets.