Combine zucchini, onion, pepper flakes, and celery in a non-reactive bowl. Sprinkle with pickling salt and cover with water. Let stand for one hour. Drain vegetables in a sieve, pressing out excess moisture. Set the vegies aside.
Combine sugar, vinegar, celery seed, mustard seed, dry mustard and cloves in a large stainless steel or enamel pan. Bring to a boil over high heat. Add vegies. Return to a boil, reduce heat and boil gently, uncovered about 45 minutes or until sauce thickens.
Remove hot jars from canner and ladle relish into jars within 1/2 inch of rim. Process 10 minutes for half-pints and 15 minutes for pint jars, in a boiling water bath.