Quick - plum jam help!

[WhatsWrongWithYou]

It's boiling ATM but I'm not sure whether I should be removing the scum on the surface - or is this needed to help with setting? TIA.

Don't you pop in a knob of butter to disperse the scum?

Probably too late now, but I skim off the scum, then drop a knob of butter in and stir it around the surface. That seems to remove 99% of the scum.

I did skim it off and I think it's worked fine, left it cooling in the jars while I'm out. Thanks - will remember the butter for next time .