A good way to find out if the piece isn’t sintered is to take a sharp object like a needle and scrape the broken inside area. If it falls apart then it isn’t sintered. If it becomes shinny from the scratching then it’s sintered.The chalky white color is just the metal not reflecting light because it is rough and porous.
Due to its particle makeup, metal clay is not as strong as sheet no matter how long it’s fired in the kiln at 1650 degrees F. On a molecular level it is porous and those tiny holes reduce the fired metal clay’s tensile strength.
With that in mind, design your pieces so that stronger in areas that will receive the most stress. For example, if you have a bail that attaches to the main body of the piece then make it thicker where it attaches instead of a thin connection. If your design requires that it be thin in that area then re-design it so that the bail is hidden behind the piece.
I hope this answers your questions. Please email me or post a comment if you have any questions about your clay.