Place cut rhubarb in a large pot.
Add sugar, salt and pepper.
Simmer for 30 minutes to an hour, stirring regularly.
Add butter to stop foaming. A thin slice from a butter stick should be enough.
Check the set of your jam. This can be done either with a candy thermometer reaching 220°F, or by placing some jam on a cool plate to see if it holds up.
Ladle hot into sterilized jars.
Water bath jam for 5 minutes.