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% alcohol.

     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 1500F.

     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 occurs.

     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 1500F 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 is better.

     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 during firing.

     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.

Startling Results
     Since the area of enamel held by water did not release from the surface, even though fired upside down, further testing was conducted.

     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 the surface.

     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.   


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