If there is a crack in your toilet bowl, water will leak out and potentially cause irreparable damage to the surrounding flooring. As such, a damaged toilet bowl should be repaired as soon as possible. This doesn't have to be a job for a professional; in fact, it's fairly easy to undertake the task yourself.
There are two options available to fix the crack. The first option will be to use glue and epoxy while the second is simply to replace the damaged bowl.
You can pick from any one of several glues available in the market to seal the crack. However, you need to be aware that applying these sealants will offer temporary relief only as the glue/epoxy will wear off after some time.
Step 1 - Turn Off Water Supply
Shut off the water supply to the toilet and empty the tank by flushing. Both the toilet tank and toilet bowl should be completely empty.
Step 2 - Dry the Inside
Take a clean towel and thoroughly dry the porcelain. Moisture can compromise glue adhesion so you may still wait a little while after drying the bowl with a towel to make sure anything you might have missed has a chance to evaporate.
Step 3 - Apply Glue to Crack
As mentioned previously, there are a few types of adhesive you can use for this patch job. Polyurethane glue is strong glue and should be applied in a thin coat. Any excess should be scraped or sanded off when you're done.
Another option is silicone sealant. Using a caulking gun, fill the crack with the sealant before you allow it to dry for at least 24 hours. Wipe away any excess with a wet rag before it has a chance to cure.
The last type of glue you can use is a two-part epoxy, such as JB Weld. This adhesive is quite strong and is used for bonding porcelain, metal, and other non porous materials. Apply it carefully to the crack, filling it before you scrape away any excess and allow the adhesive to dry
Step 4 - Refill and Test
Turn the water supply back on at this time and allow the bowl to fill up with water again. Inspect the crack carefully to see if any water is still leaking through. Either repeat the previous steps to apply more glue or continue to replacing the bowl if your patch was unsuccessful.
The second, more permanent, solution to a cracked toilet bowl will be to simply replace it. No matter what you will need to do this eventually as glue will not hold wet porcelain and clay for a long period of time. The clay although baked and glazed to a smooth finish is susceptible to ever-widening cracks once a hairline fracture has been introduced. Even if it works with the glue, it will eventually crack wide-open and create larger problems.
To replace the toilet bowl, take the following steps.
Step 1 - Turn Off Water Supply
Once again, turn off the water supply and empty the tank by flushing the toilet. The water must be completely drained from both the bowl and the tank.
Step 2 - Remove Old Toilet Bowl
If the toilet tank and toilet bowl are connected with an elbow, this elbow should be removed. Then, unscrew the nuts and loosen the bolts that are holding the bowl to the floor. It can then be removed after breaking the wax seal that secures it to the floor and toilet flange. The flange opening should be stuffed with old rags in an attempt to contain the sewer gases until the new bowl is installed. Clean the wax ring off the floor completely as well.
Step 3 - Install New Toilet Bowl
Insert a wax ring in place of the original. Lower the new toilet bowl onto the ring so that it is properly aligned, leaving no damage and securing the bowl tightly.
Spread a new bead of caulking around the base of the bowl to seal it at the floor level. Smooth it out by coating your finger in water and running it over the sealant.
Step 4 - Connect
Tighten the bolts and washers on the new bowl. Cut any extra length of bolts off just above the nuts. Then, seal the base of the toilet as above before you connect the shut off valve and supply stub to the tubing.
Step 5 - Test New Toilet
The water supply should be turned on and the system tested to see that it is working properly after the replacement in complete. If at any point of time, you feel that things are not going as planned, you should call for professional help and let the water supply remain in the off position.