Iridized Sheet Glass
Wonder how iridized glass is made? Basically, we apply a metal coating to the surface of the sheet glass by a process known as spray pyrolysis. As metal salts are applied to the very hot glass surface they burn, or pyrolyse, and the metal ions bond to the glass surface.
When the resulting coating approaches 1 wavelength of light in thickness (about 1/3 micron), colors appear. These coatings are known as 'thick' coatings, and have no transmitted color. They are less intense and shiny than dichroic coatings, which are much thinner, and are known as 'thin layer' coatings.
Color variations from gold and blue, to pink, purple and green swirling throughout Rainbow irids result from minor variations in thickness of the coating. Specific colors such as Gold or Silver irids are thinner and require even more specific thicknesses. Tin based irid coatings add substantial strength and resistance to chipping to the glass surface.
How to use irids in glass work? Iridized glass can by used in myriad ways to add rich color, crackly patterns, and glittery highlights to glass. Minimize firing or reheating temperatures to retain the most color.
Try placing the irid side down against the shelf surface or mold wall for a solid, durable, and colorful surface. Conversely, imbedded layers of irid, when allowed to stretch with glass flow in the kiln, will create crackle patterns and glittery highlights.
Or, use irids as a shelf release agent. Place clear or black glass with the irid coating down or against the mold face to clean up and simplify your mold release. This can be worthwhile even if the color won't be seen in the final work.