- Publications -
Interaction In Ceramics - Art, Design and Research, A. Valkonen (ed.),
ISBN 951-9384-56-1, ISSN 0782-1778, UIAH, Helsinki 1993
COLOUR SURFACES ON GLASS BY FLAME SPRAYING
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
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.
Markus Eerola, @
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