There are times when a sink or other porcelain fixture is worth saving. Sometimes it is part of a color suite, and the whole set would need to be changed out (exact color matches in porcelain replacements are extremely hard to find except in white). The durability and design of such a piece may also be irreplaceable, as there was a time when plumbing fixtures were made to last as long as possible.
This sink had two problems – there was a piece missing around the drain, and a long crack extended from this, up into the bowl. First we removed the drainpipe and temporarily sealed the hole in the bottom. Then we filled the sink with concentrated hydrogen peroxide. We covered it and let is sit for almost a week to bleach the crack (bleaching can take even much longer). Then we reconstructed pieces around the drain hole and filled in the missing areas with colored, non-yellowing optical epoxy. We avoided using paint or coatings to make the repair as durable as possible.