Violet Beauregarde's Blueberry Lavender Jam

There was a glut of cheap Blueberries in the supermarket just after Christmas so I bought up a number of boxes and hastily froze them for future Jam-making.
I really wanted to try using Lavender in a recipe, after someone gave me a couple of large bunches in the summer. It fragranced the Kitchen-Witch's area of la cuisine all summer and has been in a jar ever since.
Then, we watched Charlie and the Chocolate Factory again and ,seeing Miss Beauregarde turn into a blueberry, I just had to make jam. Here it is:

Miss Beauregarde' s Blueberry Lavender Jam

2 ½ lbs of blueberries

½ lb raspberries

1 lb of sugar

3 tbsp dried lavender

Juice and zest of one large lemon, preferably organic [Keep the pulp of the lemon for tomorrow's recipe!]

1/4 teaspoon of butter or soft margarine (this prevents foaming)

On The Night Before Jamming: Measure out the sugar and place in a large bowl. Measure the dried lavender and place in the centre of a square of cheesecloth and tie up tightly in a sachet. Bury this sachet in the sugar and cover well, letting it sit overnight. This helps infuse the lavender essence into the sugar, which lends to a beautiful, full flavour in the jam.

On The Day Of Jamming: Wash the blueberries and raspberries and place them into a large saucepan or stockpot, Don't use an aluminium pan. Mash the berries with a potato masher. Add the rest of the ingredients, including the lavender sachet, and stir. Let the mixture macerate for about 10 minutes. Place a small plate in the freezer, as this will be used for testing later.

Turn heat to medium-high. The mixture will bubble and froth vigorously. Skim the foam off the top with a slotted spoon, if necessary (the butter should help prevent foaming; stirring regularly will keep it from foaming too much.) The boil will subside to larger bubbles, but still bubble vigorously. Be sure to stir frequently, to keep the jam from sticking to the bottom.

After about 20 minutes, begin testing the jam by placing a small amount on the cold plate. Allow 30 seconds to pass and then run your finger through it to see what the cooled consistency will be. Boil for a few minutes longer if desired for a thicker jam. (Because of the high amount of pectin, the jam sets up pretty well, so turn off the heat when it still seems just a little too liquid)

Remove the lavender sachet and discard, then ladle the jam into hot, sterilized jam- jars* and seal leaving 1/4 inch of head space. Wipe the rims of the jars clean before applying the lids. Screw on the lids to finger-tight. Work quickly.

Store in a cool, dry place.

Makes about 6 half-pint jars.

*To sterilize the jars, place then jars on a baking tray and put them in the oven for 10 minutes on 100ºC.

Put the lids on a draining board and fill with boiling water. Tip it out before placing lids on jars. The buttons on top lids usually pop down as the mixture cools, providing a nice seal.