Slow cooker
Pour tomatoes into slow cooker. Swirl 1/4 cup water in each emptied can and pour into the slow cooker. Add sugar, vinegar, onion, garlic, salt, celery salt, mustard powder, black pepper, cayenne pepper, and clove; whisk to combine.
Cook on High, uncovered, until mixture is reduced by half and very thick, 10 to 12 hours. Stir every hour or so.
Stove top
Pour tomatoes into heavy pot. Add water to cans/jars and add to pot. Add sugar, vinegar, onion, garlic, salt, celery salt, mustard powder, black pepper, cayenne pepper, and clove; stir to combine.
Bring to boil and then lower heat to simmer. Keep a close watch and stir often to avoid burning. Reduce to about half or preferred consistency.
(cont)
Use an immersion blender for about 20 seconds to smooth the texture of the ketchup.
Cool completely.
Taste and adjust salt, black pepper, and cayenne pepper as desired.