If the acne hardens, under normal circumstances, if it occurs on the face, then it is possible that the acne has an inflammatory reaction. The inflammation may cause localized inflammatory edema, which may harden when it occurs, or it may be caused by other causes such as folliculitis or other infections of the face, most commonly infectious molluscum contagiosum. If the molluscum contagiosum is secondary to a bacterial infection, it can also cause redness and swelling in the area, along with significant pain and may become hard when touched. The former is an anti-infection treatment, usually oral doxycycline, or minocycline, and topical medication can be applied topically with fusidic acid cream. If the latter is an infection, then you need to scrape the wart treatment, and also need to pay attention to the local cleanliness and disinfection.