Vol 1 No 1 Enamel Color Change Due to Chemical or Phyiscal Relationships During Firing

Glass On Metal
The Enamelist Newsletter
Vol. 1 No. 1 January 1982
By: Bill Helwig
It is frequently expressed that certain enamel frits “do not work” on a specific metal or that they change color, turn black or green, etc. It is not that they “do not work,” it is that they “work” differently than what was desired. What may have occurred should not be looked at as wrong or of lesser quality, for in most cases the glass still has the same physical and chemical characteristics after the color change. Color was the only change. The color change should be looked at as part of the character of that glass and used as such, rather than being discarded as a poor result without merit. This is a clear case of not using glass as glass and in such cases the potential has limitations rather than freedom.
All enamels change some of their characteristics when applied and fired on different base metals. It is apparent that some transparent enamels change color characteristics more than others, when compared to the color change occurring with enamels which are more opaque. The color change may take place in several different ways. Two are quite startling – due to the fact that a chemical reaction occurs totally changing the color with no apparent relationship. The other changes are more- easily expected or understood, since they occur due to a physical change rather than a chemical change. The physical changes are due to glass and color density, opacity and the subtractive and additive nature of light and pigment mix. Not all enamels have an obvious color change, however, all enamel colors are affected by both the base metal and the time-temperature relationship during firing. The color change is most apparent with transparent colored frits. It should be understood from the outset of this information, that the manner in which glass fuses to the surface of the metal is the basis for both chemical color change, as well as its holding ability. Under the conditions of heat, the base metal oxidizes more readily during firing than at room temperature, prior to the applied enamel softening and melting into a continuous mass. This closes off the metal surface to the action of oxygen in the furnace, which oxidizes the surface under the applied enamel frit. Time and temperature determine the amount of oxidization which occurs. This time-temperature relationship also affects the rate of softening of the enamel frit. If the temperature is not sufficient to quickly fuse the glass, the surface oxidation increases to such an extent that when the piece cools, metal and glass could part at the interface, especially true with hard enamels underfired. If metal and glass are fused together, the transparent enamel may ap- pear very dark, due to metal oxide. This can, if in excess, totally block the reflected light from the metal surface. If time is not sufficient for the glass to absorb the metal oxide present (take it into solution), the color of the transparent glass is affected by the color of that oxide physically. This is most apparent on a first firing of a light transparent color which shows a reddish – brownish non-metalic opacity at the interface of the glass and metal. If both time and temperature are sufficient, the metal oxide to the eye has been absorbed into the glass, allowing light to reflect from the cleaned of oxide metal surface, To illustrate this color change on a simple basis – when copper is covered with a transparent colorless enamel (flux) and fired sufficiently, only the pink color of the copper will affect the physical color of the glass. If, however, there are thin areas in the flux coat, these areas may appear to be a light blue to green in color. What occurs, is that the amount of copper oxide, taken into the glass has chemically colored the glass. This coloration occurs because.of two factors:
(1) The amount of oxide in relationship to the amount of glass (the greater the proportion of oxide to glass, the darker the color will be).
(2) The time – temperature relationship.
If the chemical action were carried to extremes, the amount of glass would reduce proportionally to the amount of oxide until a metalic black glass-like surface occurs. This extreme, to the general enamelist, is known as “burned up” or “burned out”. Technically, this is a form of devitrification. These circumstances can occur with any enamel; transparent, translucent or opaque. Chemical action changes become more complex. This is true when the metal oxide from the base metal reacts with “metal oxides” of the colored glass itself. This is particularly true in gold bearing colors; cadmium and/ or selenium bearing colors; and to some extent, colors containing chromium.
In gold bearing colors, those known as gold rubies, trans- parent pink and purple enamels, the color changes can be considered radical on copper and silver base metals. This is due to the effect of copper and/or silver oxides’ effect on the gold particle suspended in the enamel itself. The gold particle size, as well as the amount of crystals per mass, determines the depth of color. Other colorants may determine hue and tone. Gold rubies direct over copper tend to appear opaque. They are more transparent and/or less opaque (liver color) when cooled slowly after firing. In theory, it is felt that the gold crystals have grown so large and so com- pact as to have created a color density, which transforms the transparent glass opaque in appearance by blocking reflected light from the metal surface. Thus, the crystal growth causes a physical blocking of light, thus a color change. The action of said enamels on silver is more chemical. When silver oxide is taken into solution, the oxide tends to act as a colorant first, then as an opacifier. This causes the color of the glass to change from red-pink, to green-yellow, to opaque green or opaque yellow-tan. The color is green in the case of transparent purples. This action can be reduced in part or controlled, if the amount of enamel is sufficient to equalize the chemical activity during the initial and subsequent firings. The amount of coloring components entering the volume of glass, must not be in excess to the proportion of what that glass can absorb and retain its initial color integrity. The gold rubies on gilding metal (Tombac), – 95 parts copper and 5 parts zinc, – retain their colors and transparency. It is my theory that the oxidizing zinc reduces the amount of oxygen available to oxidize the copper during firing. Zinc oxide is a glass modifying agent which does not impart color. Copper oxide, CuO, is a colorforming agent in glass.
The cadmium/selenium enamel colors – transparent or opaque – react with the base metals of copper and/or silver with radical color changes due to chemical activity between the base metal and the glass.
The chemical activity is increased with extended time at normal firing temperatures. Decreased temperatures are advised to reduce the proportion of interface chemical activity.
The color stability of red, red-orange, orange enamel colored by cadmium sulfo selenium is extremely limited, unless layered between stable enamels which exert limited chemical activity. At present, I do not have a reliable theory except surface oxidation with which to explain the color changes.
Enamels containing potasium dichromate as a colorant for green, sometimes appear grayed – less transparent on copper. The reflected metal surface will not appear bright or metalic. This chemical action with the base copper is similar to my theory expressed about the use of gilding metal. The chemical activity of chrome, in its need for oxygen, alters the surface with its copper oxides. This produces a physically etched metal surface, exposing a crystal pattern which reflects light by diffraction, causing the color to gray, due to light. If the surface of the copper is slightly oxidized prior to applying and firing the enamel, this gray effect can be reduced or eliminated.
Besides the now obvious chemical reactions between different base metals and enamels, there are physical color changes produced due to the color of the metal. Consideration clearly illustrates why the same transparent color will appear a different color on different metals. Gold should be considered as orange or a strong yellow, silver as white, sterling silver as light gray, copper as pink, and gilding metal as yellow. The physical color of a base metal, when physically colored by a transparent glass, becomes an additive color mix to create variation in hue and tone. Example – transparent purple enamel on gold relates as a mix of purple and orange thus creating a monochromatic color of complements.
Density of the coloration, as well as the thickness of the glass on metal, compose another color change to be considered. This is a physical condition which alters the value of color.   This lightness or darkness of color is due to the amount of light which enters and exits the glass by reflection from the metal surface. Thickness of the glass and the amount of colorants in the glass affect the visual appearance of the color. (Example, Basse Taille). The thicker an amount of glass on the metal surface, the less light will be reflected and emitted back to the viewer. The greater the amount of colorants, the more light will be absorbed in the glass. Excess of colorant and thickness quickly creates a density which causes the transparent enamel to appear darker or opaque. (Example – dark opaques, like blue and black, are actually a transparent base frit, with a high density of colorant.)
Opacifiers are frequently added to create whiteness. The opacifiers reflect the passage of light into the glass (opaque), and defuse the light in a simi-translucent or opalescent glass, depending on the amount and type of opacifier. This is a physical color change produced in manufacture. It is possible to exceed the temperature of some opacifiers and through firing render them less opaque (white) and more transparent.
An example of this would be when a light opaque enamel is overfired causing the glass to be more transparent and appear darker. This is due to the absence of white reflecting particles which were taken into solution. Particle density is why a light transparent, having fewer color particles, has little effect when fired over a dark color, which has more color particles. An exception to this is when a transparent enamel is used as a color filter for a subtractive color mix. Subtractive color theory pertains to light rather than pigment. Mixing colored lights allows one to subtract color through colored filters, i.e. color photography. This topic will be dealt with in a future issue. There is the possibility of a chemical action between two layers of enamels due to reaction of the color oxides. It is also possible for the base metal oxides to be dispersed through a layer of glass to the extent that they affect the color of a second coat of an enamel. These chemical and physical color changes are, or can be, all interrelated on a single piece. The freedom to work with enamel as glass should be determined by the dynamic potential of the glass, not the individual.
– Harold B. Helwig