Calcium deposits -- or lime scale -- come from hard water, which contains high concentrations of calcium carbonate, as well as magnesium and other salts. These minerals produce unsightly stains on the toilet bowl and can block the water inlet holes under the bowl rim, causing the toilet to flush sluggishly. It takes an acidic cleanser to dissolve them; you can't do it with soap. Use a strong acid for severe scale buildup.
Appropriate Cleaning Products
You can use any acidic toilet bowl cleanser to dissolve lime scale, especially one that contains hydrochloric acid, which is particularly strong. You can also clean your bowl with a household acid, such as vinegar, which contains acetic acid, or lemon juice, which contains citric acid. All of these cleaners are safe to use, but they take time to work, and if your toilet is clogged or heavily stained, you may prefer a stronger cleaner. The strongest one available for this purpose is muriatic acid, which dissolves scale quickly but must be handled with extreme care.
Cleaning With Vinegar
If you simply pour vinegar on the scale, it will run off without having much effect, so it's better to make a paste, using borax as a thickening agent. Before spreading the paste inside the bowl and around the rim, flush a 5-gallon bucket of water to empty the bowl. Make the paste by adding full-strength vinegar to borax powder and spread it on all the stains and around the bottom of the rim. After you've given the vinegar several hours to work, brush it off with a toilet brush. Avoid scrubbing porcelain with metal brushes or abrasives; you can easily scratch it.
Muriatic Acid for Heavy Buildup
Muriatic acid, available at hardware stores, handles heavy stains quickly, but it gives off noxious fumes and requires ventilation, as well as the use of a respirator and rubber gloves. Avoid this cleaner if you're on a septic system, and check with your local health department to make sure it's legal to flush it into the waste system. Empty the bowl and pour in about 12 ounces of muriatic acid; then use a toilet brush to spread it around the bowl and under the rim. Leave it in the bowl for 30 minutes, redistributing it occasionally with the brush. When it's time to flush, pour in 1/2 pound of baking soda to neutralize the acid and wait about 30 more minutes; then flush several times.
Help for Blocked Water Inlet Holes
If the holes around the rim of the bowl are encrusted with calcium and the toilet flushes sluggishly, you may have to physically clear scale out of the holes. Strip the plastic from a 1-foot length of 14-gauge electrical wire to make a handy implement for doing this. After cleaning the bowl with muriatic acid and flushing with clear water, empty the tank, pour in a gallon of white vinegar and lift the flapper momentarily to allow some of it to drain. Repeat every 10 minutes until the vinegar is gone; by this time, the toilet inlet holes should be clear and open.