There are a few pre and post solder steps that are required when torch soldering silver. When silver (other than pure fine silver or argentium silver) is exposed to high heat, it gets something that is called "Fire Scale" which totally discolors the silver and it is very difficult to remove because it goes deep into the metal. This happens because of the copper content that is alloyed in sterling silver. For this reason, I work with fine silver only, but sometimes the strength of sterling is necessary.
To avoid fire scale, you can prepare the pieces of silver by soaking them in a prep solution designed specifically to address fire scale. Also, some people lay the silver on a chunk of natural charcoal (not BBQ charcoal!) when soldering to help absorb oxygen which is in part to blame for fire scale.
I found this information in the Art Jewelry Mag forum by "bleugurl" but I have not personally verified it:
"To prevent fire scale, stop oxygen from getting in contact with the metal. Fire scale is formed whenever silver and sterling is heated above about 1000 degrees F. Oxygen combines with copper to form cuprous oxide which exhibits as firescale.
Prips Flux, denatured alcohol/borax solution, Cupronil…any of these products will work. You might try coating the whole piece in flux before soldering. Be generous with the flux. To properly apply anti-firescale-fluxes, the metal must be heated slightly. Paint, dip the piece or spray on the anti-fs-flux. A mist atomizer bottle works well here. If the metal temperature is correct a white grainy coating will form. Quickly play a flame across the wet fluxed metal to dry it. This will evaporate the alcohol, leaving a white coating of tiny borax crystals."
After soldering, the piece must be pickled to clean all of the residue and flux from it.
This video created by http://www.cooltools.us/ demonstrates how to coat your pieces with prep solution and some handy tricks for soldering small parts like jump rings with a torch. Check it out. I hope it helps you as much as it helped me.
Here is another video demonstrating another method for coating with flux & torch soldering on a piece of charcoal: