Tuesday, September 28, 2010

The Chemistry of Silver Stain

Most of us have had experiences with silver stain which have left us baffled. Sometimes the stain is too strong or inexplicably it doesn’t take at all. I’ve heard questions like: Do you fire it up or down? Can you fire it with the other paints or does it need its own schedule?  What causes metaling? Why can’t I match an historic orange or red stain? The very fact that there are so many stains on the market is an indication that something is going on here.

One has to look at chemistry to understand the process by which silver changes the color of glass. In fact a host of variables effect how the stain will take. These include: the chemistry of the stain, the chemistry of the glass, the strength of application, the rate and the temperature of the firing. A very specific series of chemical reactions must take place in order for the stain to develop. 

I’ll try to explain it briefly. Silver ions have the same molecular weight as sodium ions. When applied to the bare surface of a piece of glass they can be coaxed to exchange places by the addition of heat. The silver ions permeate into the body of the glass and react with iron, antimony or arsenic present in the formula of the glass to become silver atoms. As the reaction proceeds the silver atoms clump together to form crystals within the glass.  The resulting crystals transmit yellow light which is the hallmark of this technique.

1 comment:

  1. Silver and sodium do NOT have the same molecular weight. Their molar masses are 107.9 and 22.99 g/mol, respectively. They ARE approximately the same size. Silver +1 ions are 115 picometers (pm), or 1.15x10(-10) meters, while sodium +1 ions are 102 pm. That same size and charge is what allows the exchange to occur.