Place celery, thyme, parsley, tarragon and saffron in a large saute pan, add 4 cups stock from original stock pot and put on high heat for 30 minutes, stirring every 5 minutes. Add black pepper. Turn down heat and simmer 30 minutes, stirring every 5 minutes. Press entire contents of saute pan through sieve into stockpot of 8 cups previously made stock.