Put 4 16-oz canning jars and separate lids and tops in a large pot (e.g. pasta pot), cover with water and bring to the boil. Maintain boil for 10 minutes, turn off heat and leave the equipment in the hot water while you make the jam.

Remove grapes from their stem and separate the skins by hand-squeezing.

Put the skins into a food processor and pulse gently - don't fully puree them. Move them to a saucepan with 1/4 cup of water, and bring to a simmer for 10 minutes.

Bring the de-skinned grapes to a simmer in a separate saucepan and cook until the pulp is fully liquid - about 10 minutes. Pour through a sieve into a bowl and scrape to remove seeds and any tough flesh.

Combine the pulp and skins in the larger of your saucepans and add the sugar, butter and lemon juice. Bring to a boil, stirring all the while, and simmer until jam is thick and beginning to set. This may take 35-50 minutes.

You can test the jam by smearing a little of it onto a chilled plate and putting in the freezer for a minute. If it does not run after the chilling, it is ready.

Remove the sterilized jars from the water using a canning jar lifter. Pour out any water but do not dry them (and don't touch them! they will be very hot and you need to maintain the sterility of the glassware). Ladle the jam into the jars, leaving 1/4 inch headspace. Wipe any jam from the rims, add the lids and screw tops. Hand-tighten (do not over-tighten).

If any of the jars are not filled to the top, you should use it straight away and eat fresh. We recommend you keep this jar in the fridge once it's fully cooled.

Return the filled, sealed jars to the hot water pot and boil for another 10 minutes. Turn off heat and let the jars rest in the water for 5 minutes. Remove using the jar lifter and leave them on the kitchen counter for at least 6 hours or overnight.