Sulfamic acid comes in a crystal form that is added to water or comes in an already mixed format. When sulfamic acid is mixed with water, it essentially becomes sulfuric acid. Sulfuric acid is a mineral acid that is effective in cleaning hard surfaces and removes hard water deposits such as lime scale.
When using sulfamic acid powder, follow the manufacturer's directions for mixing it with cold water. Use a long-handled cleaning brush to apply the sulfamic acid mixture to the slate tiles and scrub well; then rinse thoroughly with water.
Many hardware or home repair stores sell a sulfamic acid wash solution. No additional water or mixing is needed for this cleaning product. Apply the solution to the slate tiles and scrub well with a long-handled brush. Rinse very carefully with water until all acid is removed.
Sulfuric acid is very toxic and hazardous if ingested or deposited on the skin. Always keep your cleaning area well ventilated to prevent ingesting any fumes and wear cleaning gloves when working with the acid. Eye wear is recommended as well.