Cracks in a Toilet Bowl
When you first notice a dark line on your toilet bowl's surface, whether it is on the inside or outside of the bowl, you need to determine if you are looking at a crack or a scratch, hair or other object. Wiping the dark line with a rag and some bleach will remove other objects and clean the dirt out of a scratch, removing any doubt.
A cracking toilet bowl will start to leak water. How quickly the bowl leaks water depends on the number, size and severity of the cracks in the bowl as well as the location of the cracks. A cracked toilet bowl may drain the water level to the point you must flush multiple times because of the reduced water level in the bowl. The leaking water can lead to water damage on the bathroom floor as well as damage in the subfloor and the room below if the leak is big enough. Water damage can also lead to mold growth, creating a health hazard and further deteriorating the bathroom and subfloor.
A toilet bowl normally holds cold water, not hot or even lukewarm water. Porcelain is delicate by nature and also reacts to temperature changes. Don't try to eliminate a clog in your toilet by pouring warm or boiling hot water into the bowl because you risk heating up the toilet bowl's porcelain so suddenly that the porcelain cracks. Heating porcelain will cause it to expand and eventually form cracks.
Because toilet bowls are made of porcelain, you must exercise caution around the bowl. Hard blows to the toilet bowl, especially with hard or sharp objects, will result in the porcelain cracking. If you are trying to unseat a toilet and the nuts are stuck on the toilet's bolts, do not loosen the nuts using a hammer since you can slip and hit the bowl. Instead, trim the bolts just below the nuts using a close-quarters hacksaw. Over-tightening the nuts on the bolts, turning them past the point you feel resistance from the nuts, will result in the bowl cracking.
Removal and Installation
Once you have cracked your toilet's bowl you cannot completely repair the damage. Before you have a problem with flooding, remove the bowl and install a new one. Removing a toilet and installing a new one does not require you to be an expert plumber, and the job requires common tools. You only need a wax ring, bolts, washers and nuts to install the new toilet as well as about an hour of your time.