Influence of Holding Agents on Enamel Clarity (Experimental Notes)
by Bill Helwig
Volume 1, Number 3, May 1982
During continued research involving
particle size, the relationship of particle size and amount of holding agent
used, demonstrated a critical influence on the clarity of transparent enamels
applied directly to the base metal.
Tests were made using a wetting
agent, Klyr-fire, Thompson Enamel Holding Agent, water and alcohol in varying
proportioned amounts in both wet and dry applications. These tests
indicate the following: As the amount of holding agent decreases, the
transparency of the enamel increases. As the particle size decreases, the
amount of holding agent must be decreased if greater transparency is desired.
Liquids are added to enamel for two
major reasons, control for application and holding ability. Glass
particles do not dissolve in the liquids used, but wet each other to form a
workable mass mixture. The consistency of the mass is controlled by the
amount of liquid used in the mix as well as particle size of the
material. Addition of liquid can increase control for application as well
as decrease control when excessive. The amount of control is determined
by both process and the individual indicating that preparation becomes the responsibility
of the enamelist, not the manufacturer.
For the purposes of these tests,
only water soluble liquids were used. The use of oil holding and
suspending agents will be covered at another time. Enamel used was 80
mesh clear flux with and without the fines (-325) removed. Mixtures of
holding agents and water in the following proportions were used: 100%-0;
75%-25%; 50%-50%, 25%-75%; to 0-100%; 50%-50% mixture of water/alcohol and 100%
Method of application was with a
brush to a slightly curved, chemically cleaned, copper surface.
The amount of drain, ease,
evenness, and ability to hold an edge were checked during application as well
as after drying.
Observations - The holding agents
used excluding 100% water, the water/alcohol mixture and 100% alcohol,
increased the drying time. As the drying time increased, the movement of
the mass increased on the slightly curved surface. The fine particles
moved to the lower surface. As the viscosity of the mixture increased,
the ability of the enamel to hold a true edge increased. Also, and of
equal importance, as the viscosity of the mixture increased, the ability to
apply an even application also increased. The individual particles of
glass fell out of suspension more slowly, giving greater control.
Alcohol, 100%, had the least control.
One the applications were dry, it
was difficult to determine the consistency of application except where base
metal was exposed between the grains of enamel.
Conclusion - the mixture of the
enamel and the holding agent is determined by the amount of control in
relationship to the design and the known or desired results.
All the samples ere fired at the
same time for two minutes at 1500°F.
Observations - as the amount of
holding agent decreased and the water increased, the clarity (freedom from
trapped gas bubbles) increases. This was also true with material which
had the fines (-325) removed. Where 100% holding agent was used and
application was the heaviest, the enamel occasionally pulled away from the
copper surface during firing. It was also apparent that in mixtures where
less holding agents were used, the edge of the enamel mass 'burned' less.
Conclusions - Although greater
control occurs with increased holding agent, less clarity of transparency also
The same mixtures were repeated,
but in this test the holding agent was brushed on the surface of the clean
metal and the two 80 mesh enamels were sifted onto the wet surface, which
caught and held the particles. The pieces were then dried and fired upside
down at 1500°F for 2 minutes.
Observations before firing - Speed
of sifting the enamel application must accommodate the speed of drying.
It is easy to apply an excessive coat of enamel when sifting on a wet
surface. This problem of excessive enamel is probably due to the wet
surface color change on the sifted enamel, the eye-hand coordination and the illogical
rationalization that since a holding agent is used, it can hold more and more
Observations after firing - The
edge appears to have a greater degree of 'burn'. There is an overall and consistent
cloudiness to areas which were heavier in both holding agent and enamel
application. Red copper oxide was present in the coats of enamel where
the fines were removed. All areas of enamel held upside down
Conclusions - There is less bounce
and dislodgement of the sifted enamel on a wet surface as opposed to sifting on
a dry surface. This is considerably more obvious when the fines (325
mesh) were removed. Excessive holding agent, fines and/or enamel
decreases the quality of the fired enamel.
Since the area of enamel held by water did not
release from the surface, even though fired upside down, further testing was
80 mesh flux with and without the
-325 mesh was applied to a 6 inch diameter clean copper plate. One third
of the metal surface (wetted with a thin film of liquid soap to reduce surface
tension) received a spray application of water, as a holding agent.
Alcohol applied by spraying was
used to hold the enamel on the opposite third, leaving the center third exposed
so edges of the enamel could be checked. Results after firing upside down
were as before on the water held third. Where the -325 mesh material was
removed from the enamel, on the alcohol held third, the enamel released from
Further, upside down tests
completed on an 11 inch shallow plate; concaved and convex surfaces; surfaces
without a wetting agent; using both Thompson and Vitrearc enamels and on
previously enameled surfaces revealed that using water as a holding agent would
give complete success. However, tests using alcohol were not as
successful, release at the outside edge occurred 50% of the time.
How or why both water or alcohol
work as holding agents defies our knowledge, but it has on repeated testing.
One conclusion is that the action
of absorption and evaporation compacts the enamel particles into a crust having
sufficient strength to hold to the surface, if handled with care, for firing.
It is important to note that the
amount of enamel applied was never excessive, this its own weight did not cause
separation from the surface of the metal.