An enamel frit must soften and adhere to the substrate without damaging it. This requires a relatively low softening temperature, typically ranging from 1,050-1,500°F (565-815°C). A decorated ceramic object might fire at 1,500°F, while thin-stemmed glasses might deform at 1,100°F. Clearly the same frit will not work equally well in both cases.
Once softened, the frit particles should flow together to eliminate porosity and form a coherent glass layer. In addition to the softening temperature, this overall firing behavior is related to the viscosity and surface tension, both of which vary with temperature. The glass frit should not soften too much before the organic medium is burned away, or combustion gases or partially burned medium can damage or become trapped in the glass layer. Escaping gases can disrupt the decoration and its bond to the substrate, causing pinholes or flaking. Carbon or bubbles trapped in the glass can result in unwanted light scattering or discoloration that will alter the color and gloss. Optimal appearance requires first removing the volatile medium and then allowing the frit to soften and flow sufficiently to obtain a coherent, pore- and defect-free glass layer with a smooth surface.
The enamel frit and substrate must be thermally compatible, meaning that once they are bonded during firing and start to cool down, their TEC must match sufficiently to avoid excessive stresses, or the decoration could crack or peel; the substrate could even fracture. Pigment type and amount can influence the enamel frit TEC, but the glass frit plays the dominant role. Most enamels have a slightly higher TEC than the substrate, so that on cooling a residual tension and compression exists in the enamel frit and the substrate, respectively. Type I borosilicate glass TEC is around 3.3 x 10-6/°C, while a soda lime silica container TEC is about 9.0 x 10-6/°C, so the same frit is unlikely to work well for both substrates. Ceramic products from different manufacturers can have significantly different TEC values. This can lead to quality issues when decorating blended lots from multiple sources, so it is recommended to run compatibility tests whenever possible when lots or sources are changed.
Decorating enamels are often used because of their mechanical and chemical durability, properties largely dependent on these same characteristics for the glass frit. Within a glass family, hardness and chemical durability generally decrease as the softening temperature is decreased. The common desire to lower firing temperature or reduce firing time pushes decorators to use lower temperature enamels or to slightly under-fire their current enamel frit. Either approach can reduce the mechanical and chemical durability. This can happen if the decoration and substrate are poorly matched, particularly if cracks are present. Cracks or poorly bonded areas increase the probability of damage from chemical attack or mechanical stresses.
Note that “chemical durability” is a somewhat generic term, and that durability for a decoration can vary widely under different conditions (e.g., acids, bases, temperature cycles, etc.). Standard testing methods, where samples have been generated under conditions simulating production firing, are recommended in order to make relative comparisons between candidate enamels.
This article comes from ceramicindustry edit released