Scale

As water travels through the earth, it picks up minerals, like calcium and magnesium. Calcium and magnesium are found in most water supplies; when the water eventually enters your home and is heated, it forms a rock-like scale build-up.
Scale is obvious on shower doors and fixtures, but it will also form in places you can’t see. Common problem areas include water pipes and hot water-using appliances, such as your water heater and dishwasher. Homeowners frequently purchase expensive cleaners and chemicals to remove the scale, or chip away at it with sharp objects; however, they are often disappointed with the difficulty, expense and effectiveness of their efforts.