Bells Magnesium Sulphate Paste is a traditional drawing ointment for boils and carbuncles.Boils and carbuncles are usually caused by a type of bacteria called Staphylococcus aureus which infects 1 or more hair follicles. A boil can appear when bacteria enters the skin through cuts and scrapes. The immune system responds to this infection by sending white blood cells to kill the bacteria. A build-up of dead bacteria, dead blood cells and dead skin cells forms pus inside the boil. A carbuncle develops when the bacteria infection spreads further beneath the skin to create a cluster of boils.