Markus Eerola
Interaction In Ceramics - Art, Design and Research, A. Valkonen (ed.), ISBN 951-9384-56-1, ISSN 0782-1778, UIAH, Helsinki 1993

The flame spraying method is conventionally used to cover metallic articles with metallic, plastic or ceramic coatings. In this study we have broadened the possibilities of the flame spraying process by using it to make metallic coatings on hot, blown glass. In principle, various metals can be used, but copper gives the most interesting results, because of its various oxidation reactions and reactions with the glass substrate. Thus this paper only deals with spraying copper on glass. However, the use of other metals or nonmetals with the flame spraying apparatus may be interesting in some special cases.  The principle of the flame spraying apparatus is shown in figure 1. The coating material is in powder form. The powder is first fed into the spray gun and then sprayed with a carrier gas to the flame. Oxygen is usually used as carrier gas and either acetylene or propane as fuel gas. The powder-s temperature will be raised in the flame and the coating material is projected in particulate form to strike the article to be coated. In other words, it melts in the flame forming tiny droplets. The flame accelerates the droplets and they impact on the material to be coated with a typical speed of 10-20 m/s. The particles have enough energy to form small plates when hitting the surface. As the coating proceeds so many plates are formed that a whole coating will be formed on the surface. 

However, there is always some porosity left in the coating due to the voids between the plates. If a totally nonporous coating is required, the coating should either be sintered after the spraying process or a different type of spraying gun. with a higher particle velocity or higher flame temperature, should be used. Adhesion of the coating to the substrate depends on the materials used, the temperature of the droplets impacting the surface and the droplets' kinetic energy. The surface of the substrate must be clean and in many cases also roughened. 

In this study we did not need a fully sintered coating and consequently a rather non-expensive, low-speed spraying gun could be used. One must note that the price of the apparatus easily gets one order of magnitude higher. if one wants to increase the particle speed or flame temperature considerably. 

The advantages of the flame spraying method are that the process itself is fast (typically spraying an article takes less than one minute), the coating is firmly adhered on the substrate, the method can be applied to materials that normally cannot be joined, the thickness of the coating is easy to control, etc. The greatest disadvantage is that the method is critical on the particle size, distribution and the shape of the particles of the sprayed material. For this reason it is both time-consuming and expensive to develop new materials for spraying. 

The flame spraying apparatus used in this study was a Metco 5P-11 equipped with a vibration unit to ensure stable powder flow. Oxygen was used as carrier gas and acetylene as fuel gas. The spraying was carried out on free blown articles the temperature of which was approximately 700-1000šC during the spraying operation. Conventionally, flame spraying is performed on cold metal surfaces. However, this is not possible with glass as the thermal shock would break the piece. It is essential that the coating is sprayed on hot glass and the spraying can easily be joined to blowing. 

During the spraying the distance between the article and the spraying nozzle was 20-30 cm. The coating was normally formed by spraying the same area two or three times. Spraying time was about 40 seconds at the most. The thickness of the coating was in all cases 0,1-0,4 mm. Adequate eye protection is required and when spraying copper, either suffficient ventilation hood or respiratory masks have to be used. 

The copper coating on glass can be an electrical conductor and some special effects can be achieved in this way. However, in this study the coating was merely used for decoration. 

The copper coating was found to have a very good adhesion on the hot glass surface. However, it was found that the elastic properties of metallic copper differ considerably from those of glass in its working temperature. The article to be sprayed should be in as final shape as possible during the spraying process. Any additional glass working with the article tends to break the copper coating. It was almost impossible to benifit from this feature because it could not be predicted, how the copper coating would break.
The sprayed copper oxidis eseasily during the spraying process to copper (I) oxide, Cu20 and further on to copper (11) oxide, CuO. As one can see from figure 2, it is almost impossible to prevent copper from oxidising to Cu20 as the equilibrium of the reaction 2Cu+1/202->Cu20 is on the right side at the temperatures used. However, it is possible to prevent the reaction Cu20 + 1/2O2->2CuO as this reaction stays on the left at temperatures above 1500 K. Thus by preventing Cu20 from oxidising, e.g. by coating the copper layer with hot glass, it is possible to get a red Cu20 layer instead of black CuO layer. 

In this study the different oxidation forms of copper and their different colours were used as an advantage. Copper(ll)oxide has deep black colour, copper(l)oxide has copper red colour. When copper has been sprayed on hot glass, the surface of the article, that will be seen through glass after spraying, has bright metal copper colour. The other side of the article, the one to which the flame has been directed, oxidizes after spraying, and turns to black copper(ll)oxide. Provided that red copper(l)oxide is wanted, the sprayed surface must be prevented from having more oxygen. In practice this is possible by gathering more glass on the article, shortly after spraying the copper. In that case the sprayed copper layer is situated inside the glass piece. 

It is possible to use this method only with fairly small handblown articles because massive and thickglass pieces tend to bend during the hot stage of working, before annealing. This in turns breaks the copper layer easily. If no more glass gathers are taken over the sprayed surface, the article can be larger because the spraying operation can then take place at the end of the hot stage of the working process when the article is not too soft any more. The area to be sprayed could easily be masked e.g. by using shield plates with a required shape for the designed pattern. 

The surface of the sprayed glass article can be manipulated further. Nitrogen acid dissolves copper and its oxide. After acid etching the article has a clean copper surface. It is also possible to engrave through the coating, but diamond tools are not recommended as copper might damage the diamond layer. The copper surface can also be patinated. In the pieces in this study coloured glass powder is in many cases used together with copper on background. 

Info:

Markus Eerola, @
Research Scientist

University of Art and Design Helsinki UIAH
Department of Ceramics and Glass
Hämeentie 135 C 
FIN-00560 Helsinki, Finland 
phone: +358 9 75630273, fax: +358 9 75630275