You can't.  :-)  But, I have a customer that raves about duco cement.  It fills in the cracks, and helps even out the rough edges where pieces of the porcelain have been lost or pulverized, and with a touch up with a marks a lot brand magic marker, they can look like new.  The markers now come in many different colors, and if you buy one of each, you will most likely be able to match the color around the edges of the repair.  Also,  per this customer:  I use acrylic paint (so I can actually custom mix colors) and then top the dogs with a shiny finish with some type of varnish or clear nail polish. By the way, just to be more specific, I use WET N WILD WILD SHINE Clear Nail Protector.  It's a very thin lacquer and you can use it on a very small area without seeing where you started and stopped. I started using it a long time ago.  AND Actually, if you do it well, repairs can't be found."   I am too lazy to go to that much trouble, but she fixed a dog of mine as a favor,  and it looked brand new.  I would reccomend practicing with a few inexpensive pieces to get your experience level up, and then graduate to pieces that are more special to you, as the cememt is very permanent.  It works much better than super glue, since it has a thickness to it that fills in the spaces, and holds the color of the markers.  I have since tried this process myself, and it is amazing how well the repairs come out.  Give it a try and let me know how it works for you!!!  You really have nothing to lose if a piece is badly broken, and it certainly is worth a try!!!  Jay Kerch,  a major seller of vintage dog figurines on ebay..........