End user focus: It pays to glaze

By Jessica Roberts
Published: Monday, 23 August 2010

The final addition to a ceramic body before firing, glaze controls the physical appearance and structural resilience of the ceramic piece. Here some of the key glaze minerals are reviewed

Ceramics is one of the primary markets for industrial minerals. Constituting about 90% of the ceramic body is the clay support, with the remainder of the body taken up by glaze the final stage in the formulation of a ceramic piece.

Ceramic glazes are a critical component of ceramic technology and are designed to impart all of the desired benefits of a ceramic piece to the final product. Glaze, essentially a surface coating of glass, is applied before the firing process and matures during firing to bond with the ceramic surface.

While the appearance of the glaze is an important aspect of the ceramic body (ie. imparting visual effects such as transparency, opaqueness, gloss, matte, smoothness, texture) its structural contributions to the ceramic formulation are abundant: given the correct mixes, glazes can provide enhanced mechanical strength, resistance to abrasion, chemical durability, and resistance to liquids and gases.

To achieve this desired combination of appearance and physical strength, a number of minerals and chemicals are used in the manufacture of glaze formulations one aspect of the formulation being frits.

Frit properties

Frits are essentially pre-batched glass, homogenous in nature, with defined chemical and physical properties most of these determined by the final bulk chemistry. Raw materials such as alumina, boron, silica and zircon are batched together in specific proportions to yield the desired bulk chemistry after the fritting process is complete.

Frit minerals are milled together to produce a homogeneous batch, which is introduced to the frit smelter where the temperature is raised to 1,200ºC 1,450ºC. The batch melts and forms a homogenous glass, which flows from the smelter to quenching rollers. Here, the batch solidifies and shatters into small fragments. These are drained and dried and may be supplied for glaze manufacture in this form or as a milled powder.

The advantages of using frits in glaze formulations include greater homogeneity than the simple mixing of the raw components alone, the ability to use certain soluble compounds (namely borax and soda ash), the removal of gaseous constituents such as CO2 and water, and the maturation of glaze formulations at lower temperatures.

The three essential frit minerals are refined borates (or colemanite), zinc oxide and zircon. Borates initiate glass formation and reduce its viscosity, promoting the creation of a smooth surface. Borates also reduce thermal expansion, enhance glaze resistance, and help to dissolve pigment agents.

Premium grade zircon is milled into either a flour or an opacifier, and in this form it is consumed in ceramics. Around 55% of premium grade zircon production is directed to the ceramics market for use as an opacifier. A smaller market is chemical grade zircon, which is used in advanced ceramics and coloured ceramic pigments.

Zinc oxide has only recently gained popularity in the ceramics market following the development of the single-firing process in the manufacture of tiles. In the mid-1980s approximately 26% of ceramic tiles (floor and wall) were produced via the single-firing process, while traditional twice-fired wall tiles accounted for 64% of production (see Figure 1).

By the late 1990s the firing process had evolved dramatically; single-fired wall and floor tiles had grown to 81% of the tile market, vastly increasing the use of zinc oxide.

Zinc oxide is related to two important characteristics of frits: the maturing temperature and the sealing temperature. During the single-fired wall tile process, CO2 gas is emitted owing to the breakdown of carbonates within the ceramic body. This means that complete outgassing of the body must be achieved before the sealing point of glaze is reached otherwise pinhole defects can appear on the glaze surface after the firing process.

The amount of zinc oxide used in this process is 8-12 wt%, making it one of the principal minerals in glaze formulations (see Tables 1 and 2 for the main glaze minerals).

Figure 1: Evolution of firing processing

Source: Jose Luis Amoros, 2006

Market outlook

The tile industry is the primary driver of glaze and frit consumption, accounting for around 85% of demand. It has traditionally been a strong market performer and has benefitted from its ties to construction and housing industries; however, these same market connections proved to be detrimental to ceramics throughout 2008 and 2009 as construction projects were postponed in developed countries and slowed elsewhere.

The industry suffered a sharp decline in production in 2008 following years of undisturbed growth. The most affected tile producers in the top 15 producing countries were largely clustered in Europe, and included Italy, Spain and Turkey. The drop in demand was felt worldwide, however, and in 2008 tile exports grew just 0.47% (IM May 2010: Tile boom cracks).

Worryingly, 2009 did not bring much respite for tile producers, and the slow recovery of the ceramics industry has only gained momentum this year.

“2009 was the worst year in the history of the ceramics industry and the quantities produced worldwide may have fallen slightly for the first time ever,” Pietro Cassani, chairman of the Association of Italian Manufacturers of Machinery and Equipment for Ceramics (Acimac), commented.

European producers appeared to lose out in the exports business, with customers ranging from Estonia to the USA reducing tile imports by as much as 37%.

China is the world’s largest tile producer and also the sector’s largest consumer. As Gavin Diener, senior consultant at TZMI told IM: “The Chinese ceramics industry is the key growth driver for supply [of ceramic minerals], which in turn is driven by urbanisation and the emerging middle class in the country.”

Naturally, this has affected traditional ceramic hubs as frit and glaze manufacturing has gradually moved to facilities within high market growth regions namely China, India, and the Middle East. This shift in manufacturing is also linked to cost reduction through labour and freight prices, and usually lower raw material costs.

In the ceramics industry, as in almost every sector, the buzz words are “low-cost” and “energy-saving”. Of the top 15 tile producers, eight of these countries are located in Asia and the Middle East; ie., countries where growth potential and cost-effectiveness is the greatest. Unless the construction industries fail in these developing hubs, this trend looks set to continue.

Table 1: Main raw materials for some principal oxides

Principal Oxide Mineral Source Also Adds
Li2O Spodumene Al2O3, SiO2
Na2O Feldspar K2O, Al2O3, SiO2
Nepheline syenite K2O, Al2O3, SiO2
K2O Feldspar Na2O, Al2O3, SiO2
MgO Dolomite CaO
Talc Al2O3, SiO2
CaO Calcite (whiting) -
Wollastonite SiO2
Al2O3 Kaolin SiO2
Pyrophyllite SiO2
Feldspars Na2O, K2O, SiO2
Nepheline Syenite Na2O, K2O, SiO2
SiO2 Quartz -
Feldspars Na2O, K2O, Al2O3
Nepheline Syenite Na2O, K2O, Al2O3
Kaolin Al2O3
Pyrophyllite Al2O3
Wollastonite CaO
Talc MgO, Al2O3
Zircon ZrO2
ZrO2 Zircon SiO2

Source: Martin Stentiford, ceramics consultant

Table 2: Role of certain oxides in glass formation

Important oxides Function Comments
SiO2, ZrO2, B2O3 Network formers Silica is the most important oxide and its proportion is largely responsible for the determination of the maturing temperature of the resultant glaze. The higher the silica content, the higher the maturing temperature and the lower the thermal expansion coefficient.
ZrO2, Al2O3, PbO, ZnO  Intermediates Al2O3 prevents recrystallisation of the glaze on cooling. It also increases the melt viscosity and improves the hardness and durability of the glaze. An excess of alumina gives rise to matte effects.
ZrO2 is primarily used as an opacifier (in the form of zircon), but it is also slightly soluble in glaze formulations and thus improves durability. Combined with other oxides, it can also function as a network former.
Li2O, Na2O, K2O, MgO, CaO, SrO, BaO Modifiers Fluxing oxides are added to reduce the maturing temperature, and can be divided into groups of alkali metal oxides, alkaline earth oxides, lead oxide and boric oxide. Alkali metal oxides are the most potent (particularly Li2O); alkaline earth oxides are most active as fluxes at temperatures above 1100°C; lead oxide is an active flux up to about 1150°C; and boric oxide, although strictly a network former, allows more fluxing oxides to be introduced without destroying the silicate lattice.

Source: Martin Stentiford, ceramics consultant