Another important thing to consider is if it is at all worth it to fix the crack. If it will take you longer to fix the piece than it would take you to make a new one, then it will be penny-wise-pound-foolish to fix it. It is also good to learn when a crack is really not worth the effort and when it may make a real difference. Some cracks are more worthy to fix than others.
A piece that is strictly for decoration and will not get confused for a utilitarian piece, is a perfect piece to fix, both artificially and structurally; that is with or without ceramic materials. In fact, a crack that distract from the beauty, should be fixed. However, when the utilitarian strength of a piece, for instance the handle of a teapot, is weakened by the fixing process, it is not worth it to put your reputation as a ceramic artist on the line.
All potters must at least fix one crack in their lifetime, or else they have not had the “full clay experience” - I say that tongue-in-the-cheek. However, there was one potter known, not only for fixing cracks, but she changed the whole perspective on American porcelain as she was recognized as the most important United States ceramic artist in the 20th century (Arts and Antiques magazine - March 2000).
One of very few women in the USA to study and practice pottery at the time, she worked primarily in porcelain, experimenting with American clay to create a true high-fire porcelain.
It took her over 1,000 hours to make, but as it often happens with porcelain, there were small cracks when it came from the kiln. Taxile Doat, her teacher advised her to trash the piece, as it appeared to be irreparable. She did not give it up and spent hours grinding bisque to powder, mixing it with some powdered glaze, and filled the cracks. After re-glazing and re-firing, the piece was pulled from the kiln with no signs of cracks or reparation.
How does Antoinette fixes cracks in her pots?
Here is how I fix cracks: Mix up some paper clay from your clay body. Add a few drops of clear glaze and some finely grounded bisque from the same clay as the mug. Clean any dust away and add some clear glaze on the chip. As it dries, it may open up some cracks again, but keep filling it with more paper clay. Remember paper clay must still shrink to catch up with the rest of the body, so it needs enough filler that will not shrink that much.
When you put it back in the kiln, put some silica on the shelf under the piece to prevent possible sticking. If it is a foot rim, you may have to sand it when it comes out.