This week in the workshop I had the hassle of dealing with fire stain. If you have tried silver soldering you may be familiar with the scourge that is fire stain. Just when you think you’re nearly finished and you begin the final stages of polishing on the lathe those nasty little blotches appear. For those of you lucky enough not to what this looks like it is a pink or purple stain that can develop in sterling silver during soldering.
Here’s the science bit… The oxygen in the air penetrates the outer surface of the object oxidizing the copper content in the sterling silver. The silver is then pickled in sulfuric acid which removes the oxidized copper on the surface. This oxidisation is known as fire scale. The difference is that fire stain can’t be removed in the pickeling process. The problem with fire stain is that it discolours the reflective nature of the polished silver and either needs to be covered by plating or filed away.
Mostly you don’t see the extent of the fire stain until you have the piece polished and finished. The best way to see it is to hold the silver against a piece of white paper. This way you have a uniform white reflection on the silver that highlights the pinky purple stain.
Here’s a photo of a cuff link that I made recently that had some fire stain. You can see the pink blotches on either side of the cut away circle design. There are lots of things you do to remove fire stain which involve chemical solutions or silver plating. Unfortunately this can be expensive and because it doesn’t happen me that often I haven’t seen the need to invest in them. The other method for getting rid of the fire stain is by abrasive removal using either a polishing lathe or other abrasive techniques. I think the best way to remove it is to use some wet emery paper on the stained area. It can be slow work but it allows you to concentrate on the area as opposed to a polishing lathe which can soften the sharp edges and detail of a piece. Once you have removed the patches of fire stain you can then finish the piece on the polishing lathe.
The ideal solution to fire stain is to prevent it from happening in the first place. As the oxygen is the cause of the stain many silversmiths recommend creating oxygen free conditions when soldering. Ganoskin have a good piece on how to go about achieving this, to see it click here. I find the best way to prevent fire stain is to make sure you are using the correct size flame for the piece you are soldering. A very hot flame can over heat the piece but also a flame that is too soft means it takes much longer to melt the solder. Both these problems can cause fire stain. The other thing is to make sure the piece is clean so you don’t need to prolong the soldering process due to dirty metal.
Hope this helps in the fight against fire stain!