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Past Articles from Glass on Metal

Impasto - A New Technique
or The Characteristics of Glass in Use
by Bill Helwig
from Volume 3, No. 2, April 1984

Photo 1     The term impasto has been borrowed from the Italian term- inology for a painting technique, meaning, a thick application of pigments to a canvas or panel, although in this enameling tech- nique it is not the use of a thick enamel paste, rather the appli- cation of wet enamel with the appearance of having been pasted, built up in surfaces of low relief as dictated by the design or image.  The thick-thin application of finely-ground enamel directly to the metal surface in modeled relief allows an image to be physically sculpted.  When such an image or pattern is fired the relief is retained, providing it is not over-fired.  A subsequent application of a light to medium tone transparent enamel evenly applied and fired will give the full effect of modeling the image.  The transparent color over white creates the tonal values of that color.  Thus, with two firings a great many shadings can be obtained without a tedious number of mixings, applications and firings. 
Photo 2     This method uses glass as glass and takes full advantage of the optical property called opacity.  The most opaque enamel applied thinly enough will have, if not transparency, at least translucency; the most transparent enamel applied thickly enough develops opacity.  Impasto as a technique takes advantage of both the transparency of opaque enamel and the opacity of trans- parent enamel.  
Photo 3     The method described uses an opaque white, 325 mesh, lead free, acid resistant enamel, AW 66.  The acid resistant quality is only important if exposed metal areas are to be cleaned chemically after firing.  Any enamel could be used if the entire surface is covered or if the oxidized metal surface is desired.  The thinnest application of enamel can be almost 90% transparent with a greater degree of opacity developed in the thicker applications.  Figure 1 illustrates a cross section of such an application.  The thicker areas are white and the thinner areas less white.  

Figure 1
Figure 2

Photo 4     Figure 2 illustrates a cross section covered with an application of transparent enamel.  It is obvious that the lower areas will have more enamel in them than the higher areas will have on them.  The thickness of the transparent enamel develops darkness and the thickness of the opaque white develops whiteness or lightness of the covering color.  Such a technique allows for the tonal development of a transparent color over the white and reduces the complexities of trying to find various enamels in tones, tints, or hues to create the illusion of dimension for the imagery.  It is necessary to note that there must be a sufficient amount of transparent enamel covering the thicker under enamel areas to insure little or no breakup of the transparent color overlay.  Control can be achieved in several ways.  The application of wet techniques by either brush (photo 8) or spatula gives the most control since the amount being applied is observed closely.  Dry sifting (photo 4) allows the enamel particles to seek the lowest areas and fill them ultimately covering the higher ones.  Wetting out a layer of sifted enamel can insure complete coverage, but it can also draw enamel off the higher areas thus care must be taken.  A holding agent may be applied to catch the sifted particles.  This is usually necessary on shaped dimensional forms. 
Photo 5     The firing of the covering coat is just as important as the application.  I tend to fire to orange peel, remove the piece and check the coverage, resift hot if necessary and return the piece to the furnace until the surface is smooth.  It is important to check the surface just at the orange peel stage to insure visual confirmation as to the correctness and completeness of the covering coat.  The two part firing allows the piece to cool slightly and also permits a longer lower firing cycle.  It is after the coverage has been checked and/or corrected if necessary that a longer firing can be considered to insure the transparency and give sufficient time and temperature for the surface oxides to be absorbed into the enamel. 
Photo 6     The viscosity and the softening temperature also play important factors with the firing of the cover coat.  Flux is usually not used since clear colorless enamel over white only creates white and the tonal modulation of white on white.  This frequently is too subtle to be of value.  
     However the whole process can be used with flux as the impasto layer (photos 6 thru 10).  Since the fines of an enamel also create opacity and 325 mesh material contains a large portion of 400 mesh or smaller material the thinner areas will be more transparent and the thicker areas will be more translucent.  The overlay of transparent enamel to cover the flux image can achieve a fine tonal value.  An image in this case can appear to be very elusive since a diffusion of light is necessary to define the image. 
Photo 7     It is obvious at this point that any enamel can be used for impasto which does not have an overriding chemical reaction with the metal, i.e. opaque reds and oranges, many transparent reds and pinks.  Acid resistance is necessary only as previously described. 
Photo 8     The value of this technique is that subtle shading of mass shapes can be easily achieved.  The technique is not one which is used for details. 
     This technique cannot be used on a previously enameled surface.  The particle size is too fine and would strain line due to the thermal shock and metal expansion giving a crackle effect which is usually disruptive to the design. 
     The impasto technique illustrates and uses the inherent property of glass, opacity.  

Photo 9


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