Most jams we do contain equal weights of fruit and sugar, however this rule should be adjusted based on the sweetness or tartness of the fruit. This jam lasts for several months, but should be kept refrigerated. Once the jam has cooked with the pectin, the consistency of the finished jam can be tested by spreading a spoonful on a white plate and placing it in the refrigerator to chill quickly. If the jam seems a bit too thick add a little water to it, if it is too thin, mix a bit more pectin and sugar, then add to the mixture and cook another 3 minutes to thicken further.
Trim away the stem ends of the strawberries. Place 1 cup of the sugar in a mixing bowl and mix with pectin, set aside. Cut off the ends of the rhubarb and use a paring knife to strip away the strings. Cut the rhubarb crosswise into 1/2 inch pieces. You should have 2 generous cups of rhubarb.
Place the rhubarb in a Copper-Core casserole with water. Place the pan over medium heat. When the liquid comes to a simmer, cover and cook for about 3 to 4 minutes. Stir the rhubarb. Much of the rhubarb should break down, but there should still be some chunks remaining. Stir in the strawberries and the 4 3/4 cups of sugar (without the pectin).
Stir over medium heat until the sugar dissolves. Once the sugar dissolves, increase the heat to high, and keep your eye on the pot, as the mixture will bubble up. Stir and reduce the heat as necessary to keep the mixture from bubbling over. Cook for about 10 minutes, stirring often. The strawberries will soften. Whisk in the sugar-pectin mixture and bring to a simmer, whisking constantly. Stop whisking and simmer over medium heat for 3 minutes. Whisk and then simmer for an additional minute. Turn off the heat, then add the butter and lemon juice. The butter will help to prevent any impurities from forming on the top of the jam. Refrigerate in small airtight containers.
Makes approximately 3 pints of jam