by Bill Helwig
from Volume 1, Number 2, March 1982
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
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
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.
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
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
U.S. Standard Sieve Series
per lineal inch
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
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 100°F. 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
(1500°F) remains solid, while the glass has a transition point of
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
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 1500°F 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.
Measurement, Figure 10.
|Thickness at the Top
|Thickness at the Bottom
|Thickness at the Top
|Thickness at the Bottom
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