Silver stain

Silver stain colours are not used in the manufacture of bulk glass (there are better and less temperamental yellows) but are widely used in surface treatment of leadlight pieces. A paste is painted on to the glass, allowed to dry, and fired (around 600°C) below softening temperature. A yellow, amber or orange stain results where the paint was applied, and extends into the interior of the glass (unlike enamels) so is very durable. Silver stain is widely used in traditional church windows, and has been since 1460.

How does it work?
The paint consists of a silver salt (silver oxide, silver chloride, silver carbonate or silver sulphate), clay or red ochre, and a drying liquid medium (for example turpentine, gum arabic or water). The following description is a summary mainly derived from W.A.Weyl, Coloured Glasses, ISBN 0-900682-06-X, available from the Society of Glass Technology. There are four essential steps: surface exchange, diffusion, reduction, and clumping.

The surface exchange, which is active from 400°C up, causes the silver (Ag) ions in the paint to exchange places with sodium (Na) ions in the surface of the glass. These ions are almost the same size and have the same charge, so the process proceeds easily. However, it would be stopped quickly by an equilibrium amount of silver in the glass and sodium in the paint if it were not for the clay or ochre binder, which acts to capture the resulting sodium salt, thus forcing more of the glass sodium ions to exchange with silver.
In the second step the silver ions diffuse into the body of the glass by further sodium-silver exchanges deeper in the glass, forming a concentration gradient with highest concentration at the surface and extending up to 1mm into the glass. However the glass is still colourless. These steps work with any soda or potassium-based glass, and are active even down to 200°C.
The silver ions are reduced by electron donors in the glass, typically iron (Fe), antimony or arsenic, becoming silver atoms. This step is critically dependent on these constituents and small changes in impurities in clear glass (usually iron) can make a substantial difference to the expression of the silver colour.
Finally the silver atoms clump together to form small crystals. At a size of 10 to 20nm, the affected glass transmits yellow light and reflects blue (a dichroic effect). 1 nanometer (nm) = 1 thousand millionth of a meter. The clumping takes place at high temperatures (600°C) because silver atoms are less mobile than silver ions and need the higher energy to move.

So, what does this mean?
The expression of silver stain may vary from glass to glass depending mainly on minor constituents; it also depends on the concentration of silver in the paint, and less critically the length of time at high temperature. However, basically leadlight ('stained glass') artists rely on a pale yellow through to amber tone.

But wait, there's more!
I happened to insert a silver PMC (Precious Metal Clay) leaf into a cast paperweight I was experimenting with, and the result was a pale mauve colour to the object. What was happening? Normally silver objects cast in glass do not end up this colour… (PS there is a green layer of glass under the leaf.)
The explanation I have come up with is that because it was a casting, it spent a very long time at high temperature, both at process temperature and annealing. Thus the clumping was much larger than normal, and the colour produced different. The surface reaction was supplied by silver oxide on the surface of the tiny grains of PMC, and the rest as normal. Looking from the back, a halo of yellow is visible around the object. The edges of the halo indicate that the silver may have diffused as far as 2mm. Weyl suggests that the colours that may be produced by silver depend on the clump size something like this table (which is for a liquid sol). My next step is to experiment and see how repeatable the colour is (I got the mauve twice) and whether I can go further down the series by long times at high temperature.

Clump size	Colour
nm	Transmitted	Reflected
10-20	yellow	blue
25-35	red	green
35-45	reddish-purple	yellowish-green
50-60	bluish-purple	yellowish-green
70-80	blue	brown
120-130	green	brown