The brine from the fridge - did you heat that up before you poured it in the bowl? It was prepared and left over from another batch - same recipe? Had it had rind sitting in it and left over after canning, or was it extra, never used?

The brine was NOT reheated before adding it. It was prepared and left over from another batch of the same recipe but did NOT have rind sitting in it. The old brine had been boiled with the rind from the first batch.

I still have to boil the rind and brine for an hour today so I'm not so worried about bacterial contamination. I can't imagine much surviving that. I just want the acid content to be high enough to be safe.

If it hadn't been used before then I wouldn't worry about bacterial contamination (or possible dilution from water seeping out of the first batch of rind, which is what you were getting at when you asked about acidity ). It should have been heated (per the recipe) before adding to the rest, not added straight from the fridge b/c it would cool off the hot brine, but it's already done, if you stirred it in well I'm sure it was fine and the recipe does call for refrigerating overnight anyway.

I don't know how much spilled, if 1C was enough to replace, if you haven't boiled the rind yet today you might want to borrow 3C of sugar from a neighbor and mix it with 1C of water and 1C of vinegar (1/3 recipe) to make sure you have enough to can. Worst case, if you find you don't have enough syrup to cover, just process what you can and stick the rest in the fridge, it should last until you can get some more sugar and then just make refrigerator pickle version out of it.

I decided to take your advice and make a 1/3 recipe of brine and go forward.

The recipe is a bit vague at NCHFP, though. Do I remove the lemon slices before boiling for one hour on day two? Do I remove them after the boil? Do I leave the lemon slices in the finished product? Do I leave the spice bag in the brine while doing the one hour boil?

What I've done in past years with good success is to boil both the spice bag and lemon slices for the one hour and then discard everything except the cinnamon sticks that go into the jars.

What does everyone thinK?

Here is a link that might be useful: NCHFP Watermelon Rind Pickles

I've never made that recipe (I tried the one in The Joy of Pickling once and didn't care for it). I can't tell about the lemon slices - I think it's our call but they'd probably be in tiny pieces after boiling that long, it would be hard to pull them out. I really don't know if the spice bag gets pulled out after the overnight soak or whether it stays in for the final boil - again, I don't really think it matters.

The recipes in the Joy of Pickling call for the lemon slices to be in the spice bag and removed with them.