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Past Articles from Glass on Metal

Particle Article
by Bill Helwig
from Volume 1, Number 2, March 1982

     Most artists, craftsmen and hobbyists are accustomed to vitreous enamel as supplied by the various manufacturers and suppliers.  It is common to supply materials in standardized forms and sizes as set by tradition and practicalities.  It would be hard to determine just when the responsibility of enamel preparation moved away from the individual or studio to the manufacturer.  It must be understood that the manufacturer is seldom the user and that if they are users, they then prepare the enamel to their specific needs.

     The specific needs of each person or company using the enamel today is not the same.  It would be unlikely that 80 mesh, lump, threads, liquid slip and painting enamels as commercially produced, could serve the needs of all.  Enamel preparation is best left in the hands of the individual, where compromise can be controlled and used as a positive aspect of the process.

     By definition, 80 mesh enamel consists of special formula glass ground to a particle size which will pass through a standard 80 mesh screen (.0070 of an inch opening).

     Liquid slip enamel consists of glass, wet ground to a particle size which will pass through a standard 200 mesh screen (.0029 of an inch opening) with 2% or less particles left on the screen.  Liquid slip enamels contain additives to control flow, drain and drying.

     Painting enamels consist of glass, ground dry or with oil (paste) to a particle size, which will completely pass through a standard 325 mesh screen (.0017 of an inch opening).  These enamels may or may not have additional non vitreous metal oxide colorants added to intensify or tone color.

     Lump and thread enamels look like their name.  They vary in size, according to method of manufacture.  These two styles of enamel are not considered within the text of this article, except to state that lump and threads when used as bulk material are ready for individual preparation by grinding to a desired particle size.

     The mesh size is determined by the number of openings in a running inch of woven metal screen.  The screening size is further determined by the diameter of the wire used in the woven screen.  Further definition is not necessary at this point.  See Table 1, which illustrates the U.S. standard sieve series of woven metal screens.
TABLE 1
U.S. Standard Sieve Series
Meshes per lineal inch Sieve number Sieve Opening (inches) Sieve Opening (millimeters) Wire Diameter (inches) Wire Diameter (millimeters)
2.58 2-1/2 .315 8.00 .073 1.85
3.03 3 .265 6.73 .065 1.65
3.57 3-1/2 .223 5.66 .057 1.45
4.22 4 .187 4.76 .050 1.27
4.98 5 .157 4.00 .044 1.12
5.81 6 .132 3.36 .040 1.02
6.80 7 .111 2.83 .036 .92
7.89 8 .0937 2.38 .0331 .84
9.21 10 .0787 2.00 .0299 .76
10.72 12 .0661 1.68 .0272 .69
12.58 14 .0555 1.41 .0240 .61
14.66 16 .0469 1.19 .0213 .54
17.15 18 .0394 1.00 .0189 .48
20.16 20 .0331 .84 .0165 .42
23.47 25 .0280 .71 .0146 .37
27.62 30 .0232 .59 .0130 .33
32.15 35 .0197 .50 .0114 .29
38.02 40 .0165 .42 .0098 .25
44.44 45 .0138 .35 .0087 .22
52.36 50 .0117 .297 .0074 .188
61.93 60 .0098 .250 .0064 .162
72.46 70 .0083 .210 .0055 .140
85.47 80 .0070 .177 .0047 .119
101.01 100 .0059 .149 .0040 .102
120.48 120 .0049 .125 .0034 .086
142.86 140 .0041 .105 .0029 .074
166.67 170 .0035 .088 .0025 .063
200. 200 .0029 .074 .0021 .053
238.10 230 .0024 .062 .0018 .046
270.26 270 .0021 .053 .0016 .041
323. 325 .0017 .044 .0014 .036

     Today, enamelists generally work with 80 mesh enamel until a desire for better control is needed.  However, 100 and 150 mesh enamels have been used by commercial producers of enameled objects since well before the turn of the 20th Century.  Expediency as a matter of production control has been ignored, since it has strong commercial overtones to the artist.  As the artist moves away from reality toward individuality, they tend to sacrifice skill for expression, which in turn confuses creativity with struggle, as control moves out of their own persona.

     Control is a relative matter.  Just how much relative control a person wants is determined by the person.  The amount of control he or she may obtain is determined by the process and the materials as an interrelationship.

     The control of the enamel particle size and/or mixture of particle sizes, lies in the hands of the one applying the enamel.  The desired result is that which the glass particles produce during the fusing to a metal or glass surface.  It is the glass particles which have control of what they will do naturally under the influence of holding agents and heat.  The person working with the material has alternatives which the glass does not.  These alternatives are the choice of the individual by selection of particle size, method of application, firing temperature-time, and removal from the heat.  These selections are relevant to the amount of integration needed for the desired result (technique and design), with the natural properties of glass.

     It is the natural properties of glass under heat, which determine the importance of selection and control of the particle size.

     80 mesh enamel is a mixture of particles ranging in size from .0070 of an inch to infinity.  The amount of each particle size contained therein, varies with method of grinding or crushing.  The smaller particles are always in the greater proportion to the whole.  The process of grinding itself determines this, unless a size screening is also used to separate the material by size.  Hand grinding and selected screening from lumps, gives the greatest control of particle size and freshness of material to the individual.

     Enamel particles fall through a screen according to the degree of friction exerted by the screen against the particle.  The particles do not pass through the screen in accordance to their weight.  The smaller particles pass through first, followed by the larger particles.  When sifting a piece, it is the smaller particles which hold the larger particles in place.  As the particle size increases, the sifting control decreases.  The larger particles of glass will bounce on the surface, unless their hitting of the surface is either softened with previously sifted smaller particles or wet with a holding agent.  Something must catch the particles in either case and hold them in place, to keep them from knocking about and dislodging previously sifted material or bouncing into an unwanted area.  This becomes of greater importance, as the shape of the surface moves from flat to vertical.

     An increase in particle size with the finer portions removed, can be used to an advantage when a specific texture is desired or when the larger particles are to remove themselves from an edge or surface during sifting.  The latter is used most frequently in production work, where the enamel is caught in a depression, yet falls from a narrow edge.

     As the particle size decreases, the ease of sifting will decrease, especially with materials under 150 mesh.  Humidity, as well as the surface silica gel, causes the material to clump as it falls and/or clogs the screen.  This is true for some enamels more than others.  Both actions reduce control and effectiveness of the procedure, unless that is the effect desired.

     The use of liquid with the enamel particles as an holding agent, suspending agent or controlling agent during application, should be considered where specifics are necessary.

     As the need for control increases, it is natural that the number of restrictions would also increase as the particle size decreases.  If the particle size decreases and the application thickness were to remain the same, the volume of enamel would increase, since the volume is greater with finer particles.  However, if both particle size and volume decrease for thinner coverage and control of details, the control of application then increases.  It is when the particles do not touch each other that an application would be considered too thin.  Caution must be taken as to whether the firing of such a coat would give a desirable result.

     A distinction will be made here, between wet enamel mixed with a liquid and liquid slip enamel.  Wet enamel for 'wet packing' has a liquid added for control and holding of the application by hand, while liquid slip enamels are applied by spraying or dipping.  The liquid slip enamels are generally used for greater surface coverage, even coatings and speed.  Liquid slip enamels will not be individually considered within this text, due to their specialization, although in general principle the same technology does apply.  Wet enamels, when applied to a surface of metal or glass, have within the surface tension of the liquid, the ability to wet each other and draw closer, more compact than in a dry, sifted state.  When such materials are applied, the tool movement also aids in settling the individual particles in to a more completely compact mass.

     Other than for dry sifting and wet application, specific technical data has been omitted.  These subjects will be expanded as full topic articles in future issues of Glass on Metal.  I have restricted this article to the understanding of particle size and the influence of heat as basic concepts necessary for advancing enameling practices.

Effect of Heat
     The smaller the particle size, the less heat (temperature - time) required to soften the particle or mass of particles.  Conversely, the larger the particle size, the greater the heat (temperature - time) required to soften the particle or mass.  Variation of particle size alone does not give assurance of result, unless the temperature - time has also been given equal importance.  The degree of integration (coverage) of alteration (texture, color) depends on particle size, amount and method of application under the conditions of heat for a considered length of time.

     The effect of heat on a particle of glass changes as the particle size decreases, and its exposed surfaces increase.  As the surface increases, the amount of heat, related to time, required to soften that particle decreases.  This variable is a control and related to all vitreous enameling processes.

     This variable will differ not only with particle size, but also with each glass formula.  It has been noted in the laboratory that the same material, but of different mesh sizes fired at the same temperature will have a differential of as much as 45 seconds between, when 80 mesh and 325 mesh will mature and clear.  If time were to remain the constant rather than temperature, the temperature range could well be over 100F.  Time should be the second controlling factor, dependent on temperature first.  This especially with enamel direct on metal, due to the oxidization of the metal and the ability for the glass to absorb that oxide without discoloration.

     As heat effects the glass particle, that particle's surface energy increases, as does the particle's internal friction.  These mechanical forces oppose each other.  This opposition is between the internal friction, spreading of the drop and the surface energy, contraction of the film into a drop.  This fundamental is called Hysteresis, where activity is made to vary through a cycle of values, one of which lags behind.

     The particle, as it softens (drop), wants to both spread and contract at the same time, Figure 1.  One of these processes lags slightly behind the other, depending on the type of glass and supporting surface.  The internal friction (viscosity), surface tension and surface energy, resistance to shear (tear or spread), constitutes the coverage of a surface called wetting.  Wetting is the special form of the interactions of a liquid with a solid.  (It is important to understand that glass is a liquid.  It is considered a solid only in it's super cooled state.)  Wetting precedes solution and diffusion, and in practice appears either in the form of the spreading of a drop of liquid or a solid, or on the contrary, in the form of the contractions of liquid films into drops.

     Thin layers of enamel applied as powder (dry particles), preliminarily spread on the solid surface, as the heat increases, the particles merely fuse together, but do not spread, Figure 4.

     The relationship of a layer of enamel on the solid surface of metal and the solid, but softening surface of glass, must take into account that the metal at the temperatures we are discussing (1500F) remains solid, while the glass has a transition point of becoming liquid.

     Thus, the occurrence of flaws in the covering coat of an enamel on metal may form bare patches, open pores, etc., due to the process of contraction, i.e. the contraction of thin layers of liquid into drops, Figure 5.  While the occurrence of flaws in the covering coat of an enamel on glass may form open patches, color spots, etc., due not only to the process of contraction of the new surface being pulled by its own internal friction and surface energy, but because the same forces are becoming more equally active in the under layer of glass, Figure 6.

     It is also a factor that with small thin volumes and layers, the force of surface tension (capillary force) can begin to play a decisive part.  Much of this depends upon radical thick-thin areas juxtaposed with each other.

     The visual effect of the surface energy and internal friction, is clearly illustrated by the meniscus properties, which create the curvature, caused by surface tension.  The unwetted surface appears convex and the wetted surface appears concave.  This relates to the typical 'orange peel' surface, which appears during the firing of sifted enamel.

     Since the enamel particles fuse together, but do not spread, it should be clarified that gravity may move the enamel on non flat surfaces.  This, however, should not be confused with enamel spreading as it relates to softening.

     During the heating of an object, which has had enamel applied to it, consideration must be given to the fact that the metal is expanding at a faster and greater rate than the glass.  The glass before softening is expanding, but not at the same rate and not to the same degree.  It is at this point, the bisque surface of the enamel tears (Figures 7 & 7A) when the tear exposes the surface of the metal, it may cause a bare patch.  As the glass continues to soften, open pores or closed pores occur depending on the amount of enamel applied.  The closed pores show discoloration of the enamel (black or green on copper).  This discoloration has a metal oxide center with color dispersing from it.  When a tear occurs exposing the surface of an undercoating of glass, it is at this point that a 'pull through' can occur, which may create a texture or color pattern to the covering enamel.  The tear of the surface becomes accented, as the two masses become more liquid, Figure 8.

     Particle size also determines the clarity of transparent glass.  With the remelt of glass at the temperatures generally used, fining cannot take place.  Fining is the removal of bubbles from a glass melt by either holding the glass for some time at a temperature below its melting temperature, or by adding a fining agent to the original glass.  Without fining, gas opacification occurs at the interface of the individual particles, causing the transparent enamel to appear opaque or whiter.  These gas bubbles caught in the glass, reduce the transparent effect, Figure 9.  Any surface decomposition will also add to the overall effect.

     It has long been felt, that larger particles of enamel give greater transparency.  This is true; however, the glass will never be more transparent than when it was originally smelted under the conditions of remelt.  As the size of the particle increases, the potential of trapping gas bubbles decreases; however, the size of each bubble trapped will increase.  As the amount of enamel applied increases, regardless of particle size, the potential of the entrapment of gas bubbles also increases.

     The increased volume and the decreased particle size, increase the number of traps.  These traps are literally the small spaces between each particle of glass.  As the glass particles soften and wet each other, the surface, on applications of other than thin, may close before gas within the thickness can escape.

     There are two additional factors which relate and can help reduce the possible number of trapped gas bubbles.  The reduction of organic holding materials, which produce carbon monoxide, and the reduction of moisture which produces excess hydrogen gas bubbles and oxygen for excessive oxidation of the base metal.

     The second method to reduce the possible number of gas bubbles, is to use a mixture of two particle sizes.  The smaller particle filling the gaps left between the larger particles.  The smaller particle would reduce possible traps as well as insure that the particles touch each other.

     Early texts on enameling suggest stoning between each application and firing.  Although this is contemporarily overlooked, such an action would open the surface, allowing more gas bubbles to escape before subsequent applications and firings.  This is of great importance between the first and second thin applications, since they become the ground for all other applications.  If a gas bubble does move through the thin layer of glass and escape, the under enamel color is exposed, which will cause a closed pin hole spot if two different colors of enamel have been used. 

     When larger particle sizes are used, there is an increase in the temperature - time factor.  Larger particles require a greater amount of heat to soften.  The amount of surface oxidation to the support metal would increase with lower temperature and increased time, if the smaller particles of glass have been removed.  The gaps between each particle can expose more of the metal surface to oxygen.  As the amount of oxide increases, discoloration occurs to a greater extent.  This problem is as disconcerting as gas bubbles.

     It is important to seal off the surface of the metal as quickly as possible.  If excess oxidation does occur, a network of red copper oxide outlines the particle size and forms a red, red-brown color.  This color may also be green, depending on whether the enamel is opaque or transparent.  This discoloration is another factor which indicates the necessity of a mixture of particle sizes when applying a thin layer of enamel.

     In all cases, it is suggested that particles below 325 mesh be removed, unless some opacity is desired.

     Figure 10 illustrates a variety of particle sizes and volumes of application.  Although it may be hard to read the photograph, in actuality, it demonstrates the relationship between particle size and volume necessary to produce maximum clarity with reduced oxide and gas bubbles.  The sample was fired at 1500F twice for two minutes consecutively.  It should be noted that 'burn out' occurred on the second firing, not the first, and then only when the enamel was less thank .005 of an inch.

Thickness Measurement, Figure 10.
  A B C D E
Particle Size .0098 .0070 .0059 .0029 .0017
Thickness at the Top .028 .021 .019 .016 .013
Thickness at the Bottom .013 .013 .008 .004 .003
  F G H I J
Thickness at the Top .020 .018 .015 .014 .018
Thickness at the Bottom .008 .005 .004 .003 .004

     The thickness measurements of figures 10 illustrate the necessity of thin applications.  Just how thin an enamel can be applied is determined by relative particle size and the control exerted by the person doing the application.  It is logical, based on the figures presented, that a sufficient application need be no more than two particles high in thickness and touching each other on all sides.  Once such a thin application has been established by firing, consideration must be given to the effect future firings may have on that coat, if it remains exposed to heat on a second firing.

     When the applications of small particle size materials are used, the firing temperature or time may be reduced to insure no 'burn out'.  After a second application, the temperature or time may be increased to clear the enamel.  Although it may increase work time, it will decrease the number and quality of the disrupting gas opacification bubbles.

     In conclusion and in conjunction with Mr. Carpenter's article in this issue, the particle size, its preparation, application and firing, must be controlled by the individual if maximum effect is desired.   

 

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