The ultimate expert on iridescence is Corning Incorporated’s Research Fellow, Greg Merkel. Here’s Greg’s comments on gold and blue Aurene.

It’s good to see people talking about iridescence! I’ve found that iridescent luster glass (i.e., an iridescent surface + a metallic underlayer, such as in Blue and Gold Aurene) has often been misunderstood, or not understood at all, even by the glassmakers themselves (contemporary ones included). So we can be forgiven our own confusion.

From what I’ve found, I would just add the following:

1.Both Blue and Gold Aurene possess a thin layer of silver particles just below the surface. These are formed when the hot glass object, still on the pontil, is subjected to an oxygen-depleted (“reducing”) flame (and before being sprayed with tin chloride). The silver produces the metallic luster, but it is not responsible for the iridescence. At thus point, the glass looks like a silver mirror – dark gray, smooth, and highly reflective.

2.The hot “metallic-surfaced” glass is then sprayed with a tin chloride solution to produce a layer of tin oxide, and reheated again, this time in the regular oxidizing flame. The length of time it is sprayed with tin chloride determines the thickness of the tin oxide layer and the color of the iridescence. As I understand it, the reheating does not change the color of the iridescence, but instead serves to pucker the surface and convert the appearance from a glossy iridescence to a matte iridescence. The spraying could take place on the bench or within a furnace.

3.Blue Aurene appears blue for two reasons: (1) Cobalt in the glass imparts a blue color to any light that is transmitted through the glass. (2) The tin oxide layer on the surface is exactly the thickness necessary to produce a blue “interference” color, that is, a blue iridescence, to light reflected from the surface. Our visual impression of the glassware is the combination of these two contributions.

4.Gold Aurene appears gold for two or three reasons: (1) The silver particles themselves produce an amber color to any light that is transmitted through the glass (the same thing occurs in Blue Aurene, but it is not as evident due to being masked by the blue component from the cobalt – however, some thin pieces of Blue Aurene may look green or even amber in transmitted light, and this is from the silver particles). (2) The tin oxide layer on the surface is exactly the thickness necessary to produce a yellow “interference” color, that is, a yellow iridescence, to light reflected from the surface. Again, our visual impression of the glassware is the combination of these two contributions. (The tin oxide layer is thicker than the layer on Blue Aurene). (3) In later Gold Aurene ware, the hot glass object was also sprayed with an iron chloride solution as well as the tin chloride, and the orange iron oxide particles that formed in the tin oxide layer served as a pigment to impart a deeper gold color to the reflected light.

5.If the tin oxide layer on a piece of Blue Aurene becomes thicker in some areas, the iridescence will change from blue to pale silvery green to pale yellow. Likewise, if the tin oxide layer is thinner on some areas of a Gold Aurene object, the iridescence will change from yellow to pale silvery green to blue (or even purple, if thin enough). One often sees examples of Blue or Gold Aurene with these color variations. The gradations are due entirely to variation in the thickness of the tin oxide, which was subject to the control employed by the person who was spraying the hot glass. Therefore, we can expect to observe the same inhomogeneity on an Aurene-cased acid-etched object, as shown in the photographs in the recent Gazettes.

6.The remaining uncertainty lies with identifying whether the Aurene layer on a given acid-etched vase is Blue Aurene or Gold Aurene, especially since Blue Aurene (cobalt base glass) can exhibit yellow iridescence if sprayed too thick, and Gold Aurene (cobalt-free) can have a blue iridescence if sprayed too thin. (The same uncertainty arises with the identification of the Aurene leaf and vine decorations on some Aurene decorated pieces.) With a solid piece of Aurene, one can look at the glass in transmitted light to determine if there is a blue color to the base glass, and be secure in one’s identification. In the acid-etched pieces, the Aurene layer may be too thin to appear bluish even if it does contain cobalt, plus the light passing through the underlying body of the object will already be colored before it ever passes through the Aurene (presuming the body of the object is, itself, colored).

7.It would be interesting to see whether one could determine the color of the Aurene glass itself (NOT its iridescent surface) on an acid-etched piece by examining the color of the edge of the Aurene design where it has been cut through by the acid etching. The light that one sees on an edge has generally been reflected many times through the length of the thin “sheet” of glass, and its color is thereby deepened by repeated absorption. A magnifying glass would probably be needed (especially given the median age of most Steuben collectors, myself included!). Those of you with a good macro lens may even be able to image the edges and send them along to us. (I don’t have any Aurene ACB pieces to try this myself, but I will accept donations.)

Sorry for the long-winded contribution (and any typos) – when it comes to Aurene, there are usually no short answers!

Any opinions expressed by participants to the Gazette are the opinions of the authors and are not endorsed by or the opinions of the Carder Steuben Club