Faster Firing Frits
Borax think tank to track and facilitate change in world ceramic industry
January 1997

Pottery is one of man's most ancient technologies, almost certainly pre-dating the invention of the wheel. The earliest pots were formed from muddy Chaldean clays, shaped by neolithic hands, and baked hard by the Sumerian sun. Change came gradually - firing in ovens or kilns, the use of molds, glazing, coloring, eventually mass production. Progress was inexorable but slow - until the 1980s and 90s when the pace of change accelerated rapidly. Technological innovation was driven primarily by the Italian and Spanish ceramic industries, most notably in the production of floor and wall tiles.

However comparable developments in raw material formulations were perhaps less radical, and the important ceramic industries of Asia and Latin America modernized less rapidly. A multi-skilled, specialist team has now been established by Borax to track technical changes and requirements, relate them to borate capabilities, and help maximize benefits of the new whiteware technologies across the world.

Whiteware? The term covers most categories of ceramics apart from heavy clays (bricks, blocks, and roof tiles), concrete and cement, refractories, and technical ceramics. Today the huge whitewares market - annual sales in excess of $8.5 billion worldwide - mainly comprises tiles (36 percent), sanitary ware (32 percent), and tableware (20 percent). Nearly all whiteware products are glazed.

Glazes are the thin, impervious, glassy coatings fused onto ceramic substrates to protect and beautify them. Historically borates have been crucial glaze constituents for floor and wall tiles and for certain forms of tableware and pottery. Although they tend to account for less than five percent of whiteware compositions, glazes provide most of any article's aesthetic appeal and technical characteristics, in particular its protection from physical or chemical damage. In the past, glaze formulations have often changed less than the production processes in which they are used. Some formulations have responded more successfully than others to the new processing environments.

Glazes are usually applied as slip - a thick aqueous slurry. Water soluble components would diffuse into the article being glazed, thereby altering the glaze composition and possibly damaging the substrate. Soluble components can also upset the rheology (flow properties) of the slip, and ingredients can sometimes react violently on being heated, with evolution of water or gases which can produce blemishes in the glaze. Therefore water soluble or reactive glaze components, such as borates, are rendered insoluble by being fused with silica into an unreactive glass - a process known as fritting. The frit is quenched rapidly and ground.

The final glaze composition is achieved by mixing the frit with other insoluble ingredients and with coloring oxides, and milling them to produce a fine powder. The powder can be applied directly onto the surface to be glazed or - more typically - as slip. In traditional firing, the ceramic piece will have been biscuit fired prior to glazing; in modern one-firing processes, body and glaze are fired simultaneously.

The introduction of advanced whiteware process technology, in particular the move to faster firing, together with exacting worldwide quality standards and increasing legislation relative to health and emissions has imposed complex new demands on frit-makers and glazing practice. Certain tried and tested frits perform very well in the new regimes; others may seem adequate for the job but in practice fall short of optimizing the advanced technology; while some are less able to cope technically with the new firing speeds, quality standards, or emissions controls. Correct borate use remains a key factor - crazing and bubbling ('pinholing') being two of the glaze defects it prevents. In short, new technology often calls for new frits.

The choice of borate depends on many factors, including the processing conditions of the final ceramic articles, the composition of the final glaze and its requirement for secondary oxides such as sodium and calcium, and finally the cost and local availability of borates and other raw material ingredients. Anhydrous borax is usually the preferred sodium borate on technical grounds, since it removes the necessity of driving off water of crystallization; Neobor® pentahydrate borax is the lower-cost alternative, and is widely used.

In the last few years, some very successful frit formulations have been developed. Borax Technology has been associated with many of these, and through its worldwide capability has actively contributed to some. Its laboratories assess the performance of borates in fritting and glazing, and cooperate in new product development. Much work has been undertaken for, or in conjunction with, other sections of the ceramic industry, and Borax now commands a unique corpus of (non-proprietary) data and experience which is at the service of frit and whiteware manufacturers globally.

Particular expertise has been developed in the use of borates for controlling the melt, and fine-tuning the final properties of the glaze. Borates have a number of effects including initiation of glaze formation at the required stage, increasing the refractive index, improving surface durability, and reducing the thermal expansion coefficient of the glaze coating. The latter can ensure a perfect thermal fit between glaze and body.

However, the move to higher temperatures for faster tile firing necessitates changes in the boric oxide (B2O3) content of frits, and the near elimination of alkali and lead oxides. Originally used as the principal fluxes in standard frit bases, those oxides are now being replaced by increasing amounts of calcium, magnesium, and zinc oxides. In formulations where sodium is undesirable, boric acid, boric oxide, and hydroboracite are viable sources of B2O3.

Such, typically, are implications of the new technology. They are causing chain reactions throughout the industry, and form the essential reason why Borax has established its multi-disciplinary ceramics team. Consisting of chemists, materials scientists, chemical engineers, and market experts in Europe and America, the team is on call across the world to work with different sectors of the ceramic industries. It helps to improve product and manufacturing performance, to maximize borate benefits, and to develop frit recipes which take full advantage technically, economically, and aesthetically of the new possibilities.