Styptic pencils are made of powdered alum crystals and a waxy binder. The pencils are used to stop bleeding from small cuts and nicks on the skin by constricting the capillaries.
An antihemorrhagic agent is defined as a substance that stops bleeding. While alum is often used in cooking, certain types of alum compounds are also antihemorrhagic agents. Anhydrous aluminum sulfate and potassium alum are two of these compounds, both of which are used to make styptic pencils.
To make a pencil, crystals from one of these compounds are combined with a neutral waxy binder to form the stick. When a person nicks the skin while shaving or suffers a minor cut, the pencil can be used to almost instantly stop the bleeding. The alum in the styptic pencil causes vasoconstriction, which simply means the blood vessels constrict and therefore halt the blood loss.
To use the pencil, one touches the pencil to the abrasion on the skin. There may be a slight stinging sensation, indicating the blood vessels are constricting in reaction to the alum. In addition to using the pencil on nicks received when shaving, it can be used for other simple abrasions such as paper cuts or scraped knees. Styptic pencils are also used to stop dogs' nails from bleeding when they are cut too close. To ensure there is no transfer of germs, one should clean the pencil with a bit of water after each use.