Boron Facilitates Lead-free Tableware
By Philip Jackson, Ph.D.
December 1995

Almost everyone will realize that ceramic tableware encompasses plates, cups, saucers, cereal bowls, jugs, and so on. It is perhaps less well known that the seven billion dollar world tableware industry is divided into many sectors - bone china, stoneware, earthenware, and hotelware to name just a few.

Tableware usually comprises three distinct components. As depicted in the cross-section of a plate (figure 1), these are the body, the glaze, and the decoration. The position of the decoration is not always as shown in figure 1: decoration can be on-glaze, under-glaze, or even in-glaze. Body, glaze, and decoration used to be fired in three consecutive processes, usually at progressively lower firing temperatures; however, pressure to reduce manufacturing time, increase efficiency/productivity, and comply with 'right first time' policies means that it is increasingly common for two or more of the components to be fired at the same time.

This article focuses on glaze and decoration, since a further little known fact is that these commonly contain lead.

Many glaze ingredients, including lead oxide and borax, are soluble in water. If applied wet to the surface of a clay body they will be absorbed into the very clay for which they are meant to provide a glassy outer surface. The process of fritting, however, renders such ingredients insoluble. This means they cannot be absorbed, but remain on the surface of the body throughout the firing. In the fritting process, several oxides are fused into molten glass. The glass is then quenched and milled. Frit, the main component of the glaze, is mixed with a little clay (and perhaps other minerals), suspended in water, applied to the body by spraying or hand-dipping, and fired somewhere in the 1050°C to 1400°C range.

The colors used for decoration comprise inorganic pigments and a low-melting glass or flux. Normally the flux also contains lead. After color application, firing melts the flux to create a glassy matrix within which the refractory pigment is safely embedded. Temperatures of 800°C to 1250°C are used to mature colors.

Lead has long been used in glazes and fluxes because of the many superior and desirable properties it imparts to the ware. These include: a blemish-free glaze surface, due to efficient release of gases from the glaze layer during firing; glazes with exceptional gloss and brilliance; deep colors, due to the high refractive index imparted by the lead to the flux; and good chemical durability, as regards to the food acids and dishwasher detergents encountered in service. Lead is indeed a boon to the ceramics manufacturer.

However, lead can be toxic. Until the mid-19th century, lead workers and users were often at risk, including those who used lead oxide in the glazing of ceramics. But the hazards of working with lead became well understood more than a century ago, and safe practices were introduced. These virtually eliminated accidental lead poisoning and enabled lead's remarkable properties to continue to be exploited beneficially.

The public has been protected further in recent years through the introduction of lead-release testing for ceramics. This entails soaking representative items of finished ware in a strong acetic acid solution and then measuring the level of lead leached out, using atomic absorption spectroscopy. The object is to find out whether any significant amounts of lead can potentially be extracted from the tableware into food during use. Most foods, of course, are far less aggressive in causing lead leaching than acetic acid. The test is designed to be a worst case scenario. Along with the test procedure, strict limits for lead release have been set by almost every country in the world. These laws have helped to ensure that there is no threat to health whatsoever from leaded ware produced by reputable manufacturers who have formulated and fired their products correctly.

Nevertheless there recently has been a major lead scare in the United States, the world's largest tableware market. Unsurprisingly, perhaps, this scare can be traced to poorly-made craft pottery. The ware in question was purchased in Italy by a American named Don Wallace. After returning to the United States, his wife began drinking from the ware, and became unwell. The doctors failed to make any connection between Fran Wallace's illness and her Italian pottery. They were baffled by her symptoms. Don Wallace (who also became ill) ultimately learned the truth by delving in medical literature. Subsequent studies revealed the lead glaze to be so badly under-fired, that is was literally dissolving into the contained beverages as if it was sugar.

Happily both Wallaces made full recoveries, but their much publicized saga led to considerable alarm among the American public. Once environmental and health advisors had joined the debate, the Food and Drug Administration (FDA) came under great pressure to ban the use of lead altogether and immediately. This was viewed with great dismay throughout the world's ceramics industries. More than a century of research had failed to come up with unleaded glazes and colors that came anywhere near the performance of their leaded counterparts.

However, the Coalition of Safe Ceramics, a representative body of tableware suppliers and producers, was ultimately able to convince the FDA that, in the short term, a lowering of lead release limits was the way forward. This bought the tableware industry a little time since, in general terms, it still can enjoy the benefits of lead while complying with the new and stricter laws. However, it is clearly essential that good lead-free glazes be discovered, so that if and when a total ban on lead - however unjust and unnecessary - comes into force, solutions will exist.

Based in Stoke-on-Trent, England, the historic heart of the modern ceramics industry, Ceram Research has an international membership and probably represents the industry's foremost collective research capability. Since 1990, it has been scouring the Periodic Table for lead replacements. Having considered the implications of cost, availability, and toxicity, just two compositional options initially appeared viable for both glazes and color fluxes: bismuth and advanced borosilicates.

Bismuth undoubtedly gave the better results, but its relatively low availability and high cost raised question marks about the provision of a universal solution. These findings coincided with those of the glaze and color suppliers who had been conducting their own research effort in parallel with Ceram Research.

The advanced borosilicate (ABS) option essentially involves eliminating lead from compositions, increasing the boric oxide content, and then juggling with the remaining oxides (silica, alumina, soda and lime). Low-melting ABS glazes were shown to give consistent if unspectacular performance, while ABS color fluxes were promising.

More recent work by Ceram Research has shown that ABS performance can be very significantly improved by the addition of low bismuth levels. These contain about three percent bismuth by weight as opposed to ten percent for high bismuth glazes. Having optimized the compositions, work has now turned to the improvement of other critical parameters such as glaze application and firing schedule. For bone china these glazes have recently been proven in large-scale factory trials; further trials are planned to demonstrate low bismuth ABS glazes for other tableware types such as earthenware, hotelware, and fine china. It should be pointed out that not all suppliers of glaze see low bismuth ABS as the only way forward. High bismuth and non-bismuth ABS glazes are believed to be viable for at least some scenarios.

The world ceramic industry believes that lead oxide can be used with absolute safety. But should a lead ban come, it is now in a much better position to cope with it, thanks to boron and bismuth.

Dr. Jackson is a Ceram Research scientist.