I found this online somewhere when I was looking for a way to use up some lemons and limes that I had in the fridge. The original recipe called for using either all lemons or all limes but I wanted to use both so I tried it out and it came out really well! The baking soda in this recipe helps make the peels less bitter and more tender and eliminates the need to soak them overnight - it also helps if you remove as much pith as possible and slice the peels very thinly! The butter will greatly reduce or eliminate the foam that you will have to skim off and is in such small amount that it will not alter the pH enough to make canning unsafe but do not increase the amount as that could raise the pH too much!!