- Preheat the oven to 140°C Wash two or three jam jars in hot, soapy water, then rinse and put in a warm oven to dry, or wash on a hot dishwasher cycle, then keep warm. Put two plates in the freezer ready to test for setting.
- Put the rhubarb, sugar, vanilla and orange juice in a large, wide-based pan and heat gently, stirring occasionally, until the sugar has dissolved.
- Increase the heat and bring to the boil. Boil rapidly for 10 minutes, then test for setting. To do this, put a teaspoon of jam on one of the chilled plates, then push it gently with your finger. If the surface wrinkles, the jam is ready. If not, boil again for 2 minutes, then retest. Repeat until it's ready.
- Skim any scum off the surface and remove the vanilla pod. Leave the jam for 15 minutes, then pour into the jars. Cover with jam pot covers, following the instructions on the pack.