Which tool is used to spread printing ink evenly before printing?

The brayer is a hand-held roller that is specifically designed for applying ink to a surface evenly, which is crucial in achieving a consistent print quality. In techniques such as linocut or woodblock printing, the brayer helps to spread the printing ink across the raised surfaces of the block, ensuring that the ink adheres evenly and uniformly. This even distribution is essential for creating clean, sharp prints that accurately represent the artist's design.

Other tools listed, like burins and carving knives, play roles in the carving process of printing blocks and are not intended for ink application. A ruler has a different purpose, mainly used for measuring or drawing straight lines, which does not apply to spreading ink. Hence, the brayer is the essential tool needed for the task described in the question.