Liquid Flame Spraying for Glass Coloring K.A. Gross, J. Tikkanen, J. Keskinen, V. Pitkänen, M. Eerola, R. Siikamaki, and M. Rajala (Submitted 15 January 1999; in revised form 19 May 1999) The liquid flame spraying process has been developed to uniformly color hot glass objects. A solution consisting of a metal nitrate dissolved in alcohol or water is fed to an oxyfuel torch and atomized in the flame. The liquid evaporates from the droplet, and subsequent reactions produce metals or metallic ox- ides that impact the hot glass surface. Flame spraying of Co, Cu, and Ag solutions onto soda-lime silica glass at 900 to 1000 °C have produced blue, blue-green, and yellow colors. Typical spraying times are 5 to 20 s. Other colors have been produced by using a combination of transition metal ions. This method has found application in studio production and in volume manufacturing of glassware. 1. Introduction Color in glass has industrial significance in producing an item with the desired appearance. Crown glass, typically used for window panes and bottles, exhibits a slight green color. This is attributed to the iron impurity in the glass, which, when pres- ent as Fe 2+ , preferentially transmits green light. The addition of manganese oxide can remove this effect by partially undergoing reduction while forcing an oxidation of the ferrous iron to a fer- ric state. The coloring effect of the remaining ferrous irons is re- moved as the manganese produces a purple color; thus providing a complementary color resulting in a uniform absorp- tion across the spectrum (Ref 1). This example of removing a certain color is important in the glass industry when working with impure source materials. Significant effort, however, is di- rected at producing coloring effects. Materials that produce color in glasses can be broadly classi- fied into two groups. The first group includes transition metallic oxides that enter glasses in true solution form. The resulting color depends on how the nature of the glass and coordination of the metallic constituents affects the energy level of the electrons (Ref 2, 3). A blue color, thus, is obtained by adding cobalt to a soda-lime-silica glass; however, pink is observed in phosphate glasses. Light with a wavelength corresponding to the electronic transition of the metal ion is absorbed, imparting a color that is made up of the remaining wavelengths in the visible spectrum (Ref 4). The intensity of the color is dictated partly by the intrin- sic absorption of each transition metal (Table 1) and the concen- tration of the metal in the glass. Colloidal or striking glasses typically produce colors with a longer wavelength such as red, orange, and yellow. The striking process refers to the necessity to reheat the glass for nucleation of crystals. Diffraction from the colloidal particles then pro- duces the coloring effect (Ref 2, 4). The colored glasses pro- duced in the work for this article employed both coloring mechanisms. Colored glasses are usually made in devoted glass furnaces where the composition is fixed. The number of colors available is, therefore, dictated by the number of glass furnaces. Further- more, a change of glass color requires the furnace to be cooled and, possibly, the replacement of the refractory lining. A long homogenization time is typically required to establish a uniform color throughout the glass tank. This leads to a delay in the pro- duction cycle associated with glass color changes. Coatings represent a faster means of producing a colored ar- ticle. In the Middle Ages, copper (Cu) and silver (Ag) were dif- fused into the surface of a colorless glass to produce a red or yellow stained glass window (Ref 1). Today, many processes are available for modifying the surface. For a glass artist, simple techniques exist such as the addition of a transition metal oxide powder or the melting of a colored glass rod onto the surface of a hot glass article. Several colors can be applied to a single glass article in this fashion. Uniformity of coloring a surface, how- ever, cannot be achieved easily using these methods. Tikkanen et al. (Ref 5) thermally sprayed colored glass particles onto hot glass objects and achieved deeply colored surfaces (Fig. 1; see cover of journal for color version). The uniformity of the color was in part dictated by the size of the glass particles, 20 to 120 μm. Periods of a day at high temperatures (between 500 and 600 °C) were required to allow diffusion to produce a more even concentration on the surface while the residual stress was re- Keywords atomization, coloring, glass technology, impactor, nanoparticles, pyrolysis K.A. Gross, J. Tikkanen, J. Keskinen, and V. Pitkänen, Department of Physics, Tampere University of Technology, Box 692, FIN 33101 Tampere, Finland; and M. Eerola, R. Siikamaki, and M. Rajala, Uni- versity of Art and Design, Faculty of Ceramics and Glass Design, Hämeentie 135C, FIN 00560, Helsinki, Finland. K.A. Gross is pres- ently at Department of Materials Engineering, Monash University, Clay- ton, VIC 3168, Australia. Contact e-mail: karlis.gross@eng.monash. edu.au. Table 1 Intrinsic coloring ability of different transition metals based on Ligand theory (Ref 4). Coloring Absorption ion coefficient Coordination Color Co 2+ 30-45 Tetrahedral Intense violet blue Ni 2+ 14 Octahedral Yellow, brown Fe 2+ 9 Octahedral Blue green Cr 3+ 6 Octahedral Green Cu 2+ 3 Octahedral Blue, green Fe 3+ 0.3-1 Tetrahedral Pale yellow-green The corresponding colors produced by transition metals in soda-lime-silicate glasses is given to show the range of colors that is possible. Peer Reviewed JTTEE5 8:583-589 ASM International Journal of Thermal Spray Technology Volume 8(4) December 1999583