A borax solution can act as a devitrification spray. That is its usual application in kiln forming. But it can be used in other ways too.
Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.
Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use.
If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.
You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container. Be careful to clean the spray container immediately, as borax crystals form quickly.