Minecraft wiki entry for Glow_Ink_Sac, associated text: Glow ink sacs can be used on signs to make their text brighter in low light levels. This also adds an outline around the text on the sign, the color of which is based on the color of the sign's text. The text does not emit any light, it is only more visible in darkness, similarly to the eyes of Spiders and Endermen. Normal ink sacs can be applied to revert glowing text back to default.