Q: I know that hard water can leave mineral deposits that cause spotting on glassware. Does very hard water also inhibit a detergent's ability to dissolve completely? Is the fine powder-like film residue on dishware actually the remains of undissolved detergent?
A: That white deposit is not undissolved detergent. It is a film residue caused by the hard-water minerals that remain after the water has evaporated – the same culprit that causes spotting! To prevent either from happening, you need to adjust two things – the amount of detergent and the water temperature – and use a rinsing agent. In addition to helping with the mineral-deposit problem, the following should also result in clean, shiny dishes.
Detergent amount: Water hardness plays a role in the amount of detergent that is needed. Too little or too much detergent will affect your dishwasher's ability to clean. Generally, soft water requires less detergent while hard water requires more. Follow the usage recommendations on the detergent package and in the dishwasher's Owner's Manual. Check with your local water utility to find out the hardness rating for your water.
Water temperature: The optimum temperature is 140°F, but it should be no lower than 120°F. For older machines, you may need to run the water in your sink until it is hot before starting your dishwasher.
Rinsing aid: Adding a rinsing aid improves the sheeting action of the dishwasher, which will help wash the minerals away from the dishes.