Preheat the oven to 350 degrees F. As soon as the oven has reached 350 degrees F, turn it off. Place the lavender on a baking sheet and let it dry out for about 2 hours. When dry, remove the flowers from the stalks and set aside.
For the next stage you need to make sure that the bowl you are using and your hands are completely dry - otherwise the bomb will start fizzing. Mix together the bicarbonate and acid powder in a glass bowl. Add the vegetable oil, lavender oil and 1 teaspoon dried lavender flowers. Mix everything together with a metal spoon.
Place a biscuit cutter on top of a sheet of parchment paper. Put the mixture into the biscuit cutter and press down with the back of the spoon. The oil now needs to evaporate so the bomb can set as a dry, hard block - leave for a minimum of 30 minutes and preferably overnight.
Store in tin foil to keep out the moisture.
Note: If you are making this with kids, you can add 1/2 teaspoon edible glitter into the mix to create an even more dramatic effect.
Courtesy James Wong