The Time Bath Bomb is an invention created by Doctor Proctor. As explained in Bubble in the Bathtub / Time Travel Bath Bomb, it was created by Doctor Proctor to go with the Time Travelling Bath. Without the powder, the bath wouldn't work at all!
In the book, it is said to be powder which you pour into the bath and stir quickly, much like a real-life bath bomb. According to Doctor Proctor; it is strawberry flavoured.
Since there was only a limited amount of the powder, the Bath Bomb is a conflict in the book along with the bath itself. However, Raspa has her own amount of the Bath Bomb too.