Calcium carbonate scaling occurs in the chip feed system and the cooking vessel of continuous digesters in pulp mills, and it arises from the reaction of calcium from wood chips with carbonate in the cooking liquor. Commercial antiscalants are typically expensive and rarely keep the system scale-free. Lignin derivatives have been proposed as antiscalants, because of their ability to chelate calcium, but they have not been adopted commercially. We demonstrate that addition of similar to 1% black liquor to white liquor reduces or prevents calcium scaling, as shown by a simple new test that measures the degree to which scale resists abrasive removal from a metal surface. These findings were validated in a full-scale mill trial, where the black liquor not only inhibited calcium scale formation, but also removed existing scale.