Wash and trim the rhubarb, then cut it into 1/2-inch pieces. You should have about 8 cups.

In a large, non-reactive pot or Dutch oven, combine the chopped rhubarb, granulated sugar, fresh lemon juice, and lemon zest. Stir well to coat the rhubarb with sugar.

Let the mixture sit at room temperature for at least 2-3 hours, or preferably overnight in the refrigerator. This draws out the juices from the rhubarb and helps dissolve the sugar.

Bring the rhubarb mixture to a boil over medium-high heat, stirring occasionally. Once boiling, reduce the heat to medium-low and simmer, stirring frequently, until the jam thickens. This can take 30-45 minutes. If using, add the butter now to reduce foaming.

To test for doneness, place a small spoonful of jam on a chilled plate and let it sit for a minute. Push the jam with your finger; if it wrinkles, it's ready. Alternatively, use a candy thermometer; the jam is set when it reaches 220°F (104°C).

Carefully ladle the hot jam into sterilized canning jars, leaving 1/4 inch headspace. Wipe the rims clean, place lids, and screw on bands until fingertip tight. Process in a boiling water canner for 10 minutes (adjusting for altitude if necessary).

Remove jars from the canner and let them cool undisturbed on a towel-lined counter for 12-24 hours. Check seals; sealed jars can be stored in a cool, dark place for up to 1 year. Unsealed jars should be refrigerated and consumed within 3 weeks.