Bodywork
November 1999

Ceramics are certainly no strangers to borates. As a vital ingredient in the glaze, boric oxide (B2O3) has been valued by ceramists for centuries. In modern floor and wall tiles, boric oxide is produced during firing from Neobor® borax pentahydrate, OptiborTM TG boric acid, or mineral borates. Boric oxide brings an array of unique properties to glaze formation.

Underneath the glaze, where the borate is working its magic, the chemical changes in the clay during firing are in fact not very different to what is going on in the glaze. The tile body is a mixture of clay, silica and feldspar. During firing, the kaolinite of the clay, an aluminum silicate, decomposes to silica, water and another aluminum silicate chemical called mullite. The feldspar vitrifies - forming a glassy framework - and bonds into itself crystals of mullite and silica. This is the hard, durable body of the tile.

But what if the glass framework were a borosilicate glass? Knowing borates' properties, it seems likely that they could be useful inside the clay itself too. But would the tile be any better, or cheaper to produce? And if so, what would be the best way to incorporate boric oxide into the clay?

Borax started to tackle these questions in 1995, first investigating the theoretical and laboratory groundwork. Positive results swiftly led to plant trials with a major Spanish producer, and later to collaboration with Spain's Instituto de Technología Cerámica and Centro Ceramico in Italy. All these results were highly encouraging.

In top-of-the-range porcelanic floor tiles, the only non water-absorbent type, able to withstand frost for outside use, it was found that borates made a great difference. Vitrification is better, making the tiles stronger, and the porosity becomes even less. Some evidence was found that the cleanability improves. There are fewer pores to trap dirt. Significantly, the firing temperature could also be lowered by 20 or 30 degrees C, so reducing energy usage and cost. All good news, but was there a catch?

There was. Virtually all tiles for outside use are made using the so-called 'wet' process. Clay and other solids are mixed in water and ground up, the slurry spray dried and the granulated result fed into the tile molds. Unfortunately some, but not all, of the borates tested caused the slurry to thicken unacceptably and led to processing difficulties. Indeed, with a calcium borate like colemanite, the slurry became almost solid.

Tests showed that most borates incorporating soluble metal cations, such as zinc, sodium or calcium made this happen to a greater or lesser extent. But when they are absent, as is the case with Optibor TG, no thickening occurs - and weight for weight - Optibor TG delivers a much higher percentage of boric oxide. An alternative is to use an alkali-borosilicate frit, where the active borate has been turned into a water-insoluble glassy material.

In the two other main tile types, borate incorporation could well revolutionize the industry. The lower quality 'white-body' tiles, which always have to be glazed, have a small but still significant water absorption and they are not as strong as porcelanic tiles. With borate? They are stronger, with a water absorption equivalent to their porcelanic cousins. The white-body tile is higher quality, and can be made with a shorter firing cycle and less energy consumption.

Cheapest of all to produce, the ordinary 'red-body' tile is relatively weak and readily absorbs water. In a freeze it could break apart. With borate? It is the same story of better strength, water absorption almost up to porcelanic standards, and a lower firing temperature.

Most of the Borax work so far has concentrated on the 'wet' process - the predominant process for porcelanic and white-body tiles. For manufacturers employing the 'dry' process for red-body tiles, where all the clay ingredients are milled, mixed and only then made into a slurry, laboratory and plant tests are showing equally good results. Benefits identified so far, in addition to those already described, are that there could be a reduction in the grinding of the mix, meaning increased output, and a cost saving from a small borate addition replacing a significant amount of relatively expensive feldspar.

Wet, dry, porcelanic, red, white? Bodies are better with borates.