Place tomatoes in a large bowl and pour boiling water over the tomatoes until they are covered. Leave for 5 minutes and then drain off the liquid and wait until they are cool enough to touch. Peel tomatoes.
Chop off skin of pineapple and remove core.
Cut tomatoes and pineapple into small cubes (about 12/-1 cm by ½ - 1 cm)
Place in a saucepan with 1 cup of the sugar.
Place rest of sugar in a baking dish, place in oven and turn heat to 160 degrees celsius (fan-forced) or equivalent.
Gently heat the pineapple, tomato and sugar mixture until the sugar dissolves.
Turn up heat to medium/high and boil for 20 minutes.
Add heated sugar from oven to the mixture and stir in quickly to dissolve.
Continue to boil steadily on medium heat, stirring regularly, for 30 minutes.
A couple of minutes before taking the jam off the stove, add the tartaric acid and stir to combine.
Pour into sterilised jars and seal immediately.