Step 1

Add the blackcurrants to a thick-bottomed pot and simmer on low for about 10 minutes until they begin to release their juice.

Step 2

Add sugar and lemon juice (if using).  Simmer for about 30 minutes, stirring frequently to prevent boiling over.  It will foam heavily, make sure the pot is no more than 2/3rds full when you start.

Step 3

When the jam stops foaming and begins to thicken, cook for another few minutes before pouring into canning jars.  If you're using a thermometer, the jam should be roughly 218 to 220 degrees F at sea level.  Be sure to adjust for altitude!  See note above.

Step 4

Load into half-pint canning jars leaving 1/4 inch headspace.  Attach 2 part lids and process in a water bath canner for 10 minutes.  Or, alternatively, store in the refrigerator and use within a month or two.