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Though the thought of "acid" may make you cringe, once it exits your home and enters the sewage highway, it becomes so dilute as to be inconsequential, even in a septic tank. The problem in using a very weak, "natural" acid such as vinegar or lemon juice, is that it can take forever to get any results, especially if the calcium deposits are heavy. I must comment, though, that the acid they used was not extremely strong. Really strong acids, such as muriatic acid, can dissolve calcium deposits in seconds. However, it also will eat away at the porcelain!
Most toilet bowls can be restored unless they are severely scratched or cracked. The inside surface of a toilet bowl is extremely hard, smooth and resistant to most chemicals. Toilets are formed from a ceramic clay similar to wall or floor tile, and are specially glazed to have a glass-like or "vitreous" surface... hence the name "vitreous" china. This silky-smoothness inhibits the growth of germs by making it difficult for waste to stick to the surface after flushing.
Over time, constant exposure to water-borne minerals and chemicals can cause discoloration and staining, even on vitreous china, requiring more-than-superficial cleaning. Effective heavy-duty cleaning products fall into two general categories, acid-based or chlorine-based.
NOTE: Please... NEVER MIX DIFFERENT CLEANING PRODUCTS... the resulting chemical reaction can be very dangerous!!
1) Commercial toilet bowl cleaners are acid-based, allowing them to chemically dissolve mineral deposits and stains caused by hard and iron-rich water. They are also very strong disinfectants.
Because of their high acidity, though, these cleaners are highly corrosive to the skin and eyes and can damage a wide variety of surfaces. Always follow the label directions carefully and wear gloves and eye protection.
There are other products designed strictly for stain removal. These are also acid-based and come in powder or liquid form.
2) Cleaning products utilizing chlorine bleach may be used for cleaning and disinfection, but chlorine will not have any significant effect on mineral stains. In fact, a good way to know if the stains are from minerals is if they are resistant to chlorine!
Just add a half-cup or so of liquid household bleach to the toilet water. Swish it around with a toilet brush to get it under the rim and let it sit for a few minutes before flushing.
Try not to slosh it out of the bowl, as the chlorine is also dangerous to skin and some materials. It can give your bathroom carpet leprosy in a New York minute!
Using pumice stone is an accepted way to clean deposits from toilets. It is abrasive enough to do the job, works well when wet and will "usually" not damage the toilet surface as long as it is used wet... usually because pumice stone is one of the most abrasive products you can use for cleaning aside from sandpaper and can cause permanant damage to all but the hardest surfaces.
Abrasive cleaners work quite well for many cleaning tasks, but I have learned from experience to use them as the last and not first resort. I have seen too many surfaces permanently damaged by overuse of commercial abrasive powders, so I tend to perhaps "err" on the side of caution.
Right on! Commercial toilet bowl cleaners are quite strong, but they are most powerful when used undiluted. Nice trick to drain the bowl, too! Nothing nastier than using a sponge and elbow-length neoprene gloves!