Mortar rakes will have either a gritted or a fluted design.
Both are designed to remove mortar from between bricks.
When attached to a power tool, the mortar rake rotates at a high speed and grinds (gritted design) or cuts (fluted design) away at the mortar.
Gritted design
The granules on the sides of a gritted mortar rake grind down the mortar, scraping it away from the brickwork leaving a clean surface to be repointed.
Fluted design
With a fluted mortar rake, the rotating action means the dust and debris is channelled along the flutes and away from the brickwork.
The blades on the side of the tool, that make up the flutes, cut through the mortar as the tool spins. The flutes are designed as pathways for the mortar debris to pass through.