For decades now I have been filling up my 3'x8' back pan (3'x12' arch) with sap at the end of the season and leaving it until the end of July or the start of August. I then empty it and with some scrubbing and rinsing bring the pan back to an "as new" condition. The pan is soldered stainless. I think it is lead free solder. After sitting in the pan for those months, the sap takes on a strong vinegar smell and I'm pretty sure one could make good edible vinegar with that but I haven't tried it because my wife got some mother from a friend years ago and makes vinegar from that. Only two or three gallons a year though. She gets me to set aside partially boiled sap of between 20 and 25 degrees Brix and then adds the mother to that. I dump on the ground most of what has been in the pan for months but I do save a few five gallon pails of it to use for front pan cleaning the following Spring. I have two front pans so the usual procedure is to place a pan on a palette in the brook after it has had 20 or 30 gallons of syrup go through it. After 2 or 3 days in the brook with plenty of water running through it, I drain it out, bring it down to the garage, put it on a pair of sawhorses and pour 5 or 10 gallons of the "vinegar" in it. After a few hours the nitre will loosen up but if I'm not in a hurry I'll let it sit for a day or two or three. A gentle scrub and a good rinsing is all that is then needed to have it properly cleaned and ready to use again. The "vinegar" is saved for the next pan. Both my front pans are soldered stainless. I should note that I add additional supports under the old arch before I fill the back pan with sap - IT IS HEAVY! Some years I don't end up with enough sap in the buckets to fill the pan so I will use what I have and then top off with water - this seems to work as well.