Saggars are ceramic containers which used to be used to protect the pots from carbon ash and other contaminates from the fire.
These days potters use them to make sure as many “contaminates” as possible come into contact with the pot and in fact “colour” or patina the pot.
I find the best surface treatment for this kind of firing is a burnished pure white clay.
I bisque to just over a thousand degrees Celsius and then load the pots into the saggar with sawdust wood off cuts (from my local cabinet makers trash can) salt, seaweed off different types, copper wire, regular wire, steel wool, copper sulphate and anything else that catches my attention.
I then close the saggar with kiln shelves and fire to about 700 degrees Celsius, varying the oxidation and reduction periods. I then sometimes open the kiln a couple of hours later and dump some more salt into the saggar before closing again and letting the kiln cool down.