The word “frit” is Italian in origin and refers to ceramic mixtures that have been melted to form a glass and then crushed into a powder. Glass frit is a type of pre-reacted raw material that can be used as-is or combined with other materials in a custom blend. These frits are used in making such diverse products as decorating enamels, sanitaryware, refractories, electronic assemblies, specialty coatings, vitrified abrasives and traditional pottery glazes.

Confusingly, you might encounter the terms ceramic frit, glass frit, ceramic ink, and glass colors to describe enamels for decorating glass and ceramic surfaces. This article will refer to glass frits as important components in decorative enamels and will review the role of frit in enamels, how glass frit is made, different decorating frits, and how frit properties influence the performance of those enamels.

Decorating Enamels

Properly fired enamels form a permanent coating that provides scratch resistance, chemical durability, gloss and color, all at the desired levels. Figure 1 shows some examples of fired-on enamel decorations that use glass frit. Each enamel component plays a specific role in enabling proper application and firing.

Decorating enamels are composed of glass frit, ceramic pigments and an organic medium, which have been mixed together under high shear to attain a uniform dispersion. Figure 2 shows a three-roll mill, which is one type of machine used to accomplish this mixing and dispersion step. Enamels are carefully formulated to obtain the properties required for successful application, firing and in-service performance. Application methods range from manual to fully automated, and common techniques include screen printing, spraying, rolling or using pre-printed decals.

Enamels typically contain 10-15% of ceramic pigments by weight. These pigments belong to a class of complex oxide colorants, used as fine particles typically less than 8 microns in diameter. Pigments lend color to enamels and, barring unwanted reactions during firing, are inert particles suspended in a layer of glass formed by the frit component. For most decorations, this glass layer is transparent, and the enamel color results from the reflection of incident light from the surfaces of the opaque, colored pigment particles.

The medium is a liquid carrier into which the frit and pigments particles are dispersed, and enamels are typically 15-25% by weight medium. The medium enables the delivery and retention of the frit and pigment particles onto the substrate in the desired pattern. Once the firing process has started, the medium must completely burn out, leaving only the frit and pigment behind. Important medium properties include viscosity, surface tension, drying behavior, strength or adhesion, and burn-out temperature. Most mediums are blends themselves, containing additives to precisely tailor the properties listed previously to optimize the blending, application and firing of the enamel mixture.

Glass frit is usually more than 60% by weight in the unfired enamel mixture, comprising about 70-80% after firing. The frit particles soften and fuse during firing, creating a continuous glass layer bonded chemically and mechanically to the substrate. Pigment particles are dispersed inside this glass layer, and both pigment and glass contribute to the color of the decoration. Gloss level is directly related to the glass refractive index and the fired surface smoothness, while the ratio of frit to pigment is a factor too, with high pigment content leading to a matte appearance. Superficially, glass frit is a high-temperature “glue” that holds the pigment in place. In reality, proper frit selection is crucial because the frit properties profoundly influence both the enamel behavior during firing and the fired decoration properties.

Glass Chemistry, Structure and Properties

Understanding glass frit properties starts with a brief introduction to glass chemistry and structure. Unlike crystalline materials, where atoms are periodically arranged in repeating structures, glasses are disordered or amorphous networks of atoms or molecular units that are not periodic. Common glasses are formed by rapidly quenching a molten liquid, and freezing-in the liquid-type structure before the atoms can form a periodic arrangement.

The classic example is the Zachariasen Random Network Model for silicate glasses, shown schematically in Figure 3, where the tetrahedral Si-O₄ molecular building blocks are shown in two dimensions as Si-O₃ units. The crystal network periodicity and symmetry contrasts with the disorder seen for the arrangements of the Si-O₃ molecular units in the glass network. Oxide components forming strong bonds that strengthen this network are termed network formers (e.g., SiO₂, B₂O₃, GeO₂and P₂O₅), while oxides termed modifiers, like alkalis and alkaline earths, disrupt or weaken the network as shown in Figure 3. Oxides like Al₂O₃, ZnO, PbO, Bi₂O₃, ZrO₂ and TiO₂ are termed intermediates because they can act as formers or modifiers based on the particular glass composition.

With this conceptual model, it is easy to understand that chemistry strongly influences the glass structure, which in turn directly impacts the glass properties. If the network is weakened, glass properties such as the softening temperature and hardness are decreased, while the thermal expansion coefficient is increased.

As the largest component, the glass frit strongly influences overall enamel properties like firing temperature, gloss, scratch resistance and chemical durability. Modifying the glass chemistry can significantly alter most of the important frit properties, possibly in opposite or unwanted directions, so the successful enamel frit chemistry is a compromise. For example, adding more alkali may bring a desired decrease in softening temperature but an unwanted increase in expansion coefficient.¹

Glass Frit Production

The production of glass frit starts with the dry blending of the constituent raw materials together to make a uniform batch. Typical raw materials include processed oxide minerals and chemicals such as sand, borax, boric acid, potassium carbonate, and zinc oxide. The blended batch is then conveyed to a furnace and melted into a single, uniform liquid. Electric or gas-fired furnaces are typical, with gas-fired models generally being more economical and common.

Melt temperatures vary based on the composition; however, temperatures of 2,400-2,800°F (1,350-1,550°C) are common. The molten glass is held at temperature to ensure complete melting, to increase chemical uniformity, and to remove bubbles trapped in the molten glass. These bubbles originate as air entrained in the dry batch or from gases (e.g., water, CO_x, NO_x, SO_x) evolved by decomposition and melting reactions.

Once the glass is fully melted, it is poured from the furnace and quenched using water, air or contact with cooled surfaces (e.g., roller quenching). Figure 4 shows a stream of molten glass exiting a furnace and being water quenched. The water quenching process freezes-in the amorphous structure and fractures the glass stream into granules (~ 1 cm or less) that can be collected and handled. The rapid cooling also leaves behind residual stresses in the glass granules that assist in subsequent processing steps used to reduce the frit particle size.

Reducing the frit particle size is important so that the enamel mixture can be blended, applied and fired properly. A variety of methods and machines are used to reduce particle size. A common method is ball milling, where the granular frit particles are rotated inside a ceramic-lined drum along with much larger and harder ceramic balls, called media. Collisions between the glass particles and the media break the glass into increasingly smaller pieces. Specific particle size ranges can then be selected from the milled material using screens or air classification methods.

Figure 5 shows different size range frits along with some ball mill media. Typical enamel frit particles (far right in Figure 5) are all less than 20 microns, with 90% of the particles less than 12-15 microns. This size range enables ready dispersion in the medium and smooth application of the mixed enamel to form the decoration pattern. Over-grinding can generate excessive amounts of very fine particles (< 1 micron), which in turn greatly increases the frit surface area, thereby increasing the amount of medium needed to disperse the frit. Since the medium must be burned out, normally one would try to minimize the amount of medium required for dispersion and application. Larger frit particles (center in Figure 5), selected by screening the milled frit between an upper and lower screen size, may be applied to an article to get a decoration with texture, as shown in the Figure 6 close-up of a decorated glass.

Glass Frit Systems

Traditional glasses used for decorative or sealing applications belonged to the PbO-B₂O₃-SiO₂ system. This family of glasses also contains minor quantities of alkalis and alkaline earth components and is known for low firing temperatures, wide and forgiving firing ranges, high gloss, and high durability under certain use conditions. Due to health and safety concerns related to the manufacture, labeling, and use of products containing lead, these traditional frit systems have seen declining use over the last decade. Despite this overall trend, these products are still produced and used for certain decorative applications where their appearance and performance warrants it.

Lead-free glasses in the ZnO-B₂O₃-SiO₂ family were developed for decorating and sealing applications and are in widespread use today. This glass family offers an acceptable range of firing temperatures, gloss and durability for many applications. As is also true for many traditional frits, when the composition is adjusted to reduce the firing temperature and/or increase gloss in this glass family, the chemical durability and mechanical hardness generally also decrease.

Compositions in the Bi₂O₃-B₂O₃-SiO₂ system provide increased chemical durability vs. ZnO-B₂O₃-SiO₂ glasses with comparable peak firing temperatures and are used in many applications. One trade-off is that the range of acceptable firing temperatures is somewhat reduced, and bismuth-containing frits can react with certain pigments, particularly a family of vibrant red, yellow and orange pigments containing cadmium. Such reactions inhibit proper color development in the fired enamel and limit the spectrum of available colors for this system. Bismuth oxide prices have also shown significant price volatility over the last several years, increasing the potential base costs for frits from this glass family.

All of the previously mentioned base glass systems contain other minor components (e.g., TiO₂, ZrO₂, Al₂O₃, alkalis, alkaline earths, fluorine) melted into the glass that are used to tailor the properties to optimize processing and performance. Sometimes other minor additives are included with the already-prepared frit in order to fine-tune the enamel properties upon firing. One example is b-spodumene, a ceramic material with a thermal expansion coefficient (TEC) of nearly zero, which is used to lower the overall TEC for a fired enamel. Other additives are used to cause the frit to partially or fully crystallize during the firing cycle (i.e., devitrifying or glass-ceramic frits) so that a new range of properties and performance are obtained in the fired enamel.

Requirements for Enamel Frits

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 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 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 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. 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.

Finding the Best Choice

Glass frit is a crucial component in decorating enamels, determining the firing behavior, appearance and in-service performance for the decoration. Glass frit selection for particular application and firing conditions, as well as use environment, is an engineering compromise. The best choice must simultaneously optimize several properties that depend on the glass chemistry and structure in subtle and often opposing ways. Decorators are encouraged to discuss their specific needs with their enamel supplier to reach the best solution, rather than assume that a general-formulation enamel will be able to do multiple jobs.   

Reference

1.  The interested reader is referred to introductory texts on glass science by A.K. Varshneya or J.E. Shelby for more in-depth explorations on the topic.

 Editor’s Note: This article is based on a presentation given at the Deco ’14 meeting of the Society of Glass and Ceramic Decorated Products (SGCDpro). For more information, visit www.sgcd.org.