Silver stain is prone to an effect called “metaling” which occurs as an oxidation on the surface of the glass. It is characterized by a milky opalescence which can have a blue, green or brown cast. It is visible in reflected light. In transmitted light the stain will still appear yellow but with a reduction in transparency. Metaling can be unsightly and can only be removed from the surface of the glass by abrasion or with hydrofluoric acid. The cause of metaling is linked to temperature; the higher the stain is fired the more likely it is to occur. The chemistry of the stain, the chemistry of the glass and the thickness of application also play a role. Some stains are formulated with the addition of copper sulfate to intensify their potency. These are more likely to metal. Of the test samples the amber stains exhibited metaling, although not in the range of their thinner application.