can use any jeweler's silver solder on metal clay. There are a few basic "rules":
1. Burnish the surface to be soldered VERY well. This will help minimize the solder wicking into the small crevices left by the sintering process.
2. Maximize the surface area being soldered. In other words, if you are soldering an earpost onto metal clay, you are much better off using one with a large pad vs. one with a small or no pad.
3. Minimize the time you spend heating the metal. This can mean lower solder temperature or adjusting the way you heat the metal. Remember that lower temp solders are a poor color match for silver and especially fine silver.
4. Pickling mc can be tricky. I never recommend doing it while the piece is still hot as it will allow even more of the acid to enter the porous places in the clay. MC should always be rinsed in a baking soda and water bath after pickling. Water is not sufficient to rinse the pickle out.
Another note: Pickle does not remove firescale. It only hides it by etching the copper out of the surface and leaving a fine silver surface. If you polish it much at all, it will break through the fine silver and you will figure out where that firescale went !! Burnishing (such as a tumbler) is not abrasive enough (unless you use a cutting medium or leave a piece in too long) to remove the fine silver.