A Glow In The Dark: Borate glazes proven a thousand years old
By Nigel Wood
February 1999

Borates go back a very long way. Some of the properties so valued today were being exploited thousands of years ago. Legend says the Babylonian goldsmiths of 2,000 BC used borax from the East as a metalworking flux.

One of their most common modern uses in ceramic glazes dates back only a century or two: or so it has been thought. Until then, it was a rarely and little used ingredient because of its scarcity and high cost. Ceramics scholars were fairly certain that the use of borates in glazes went no further back than their use as a supplementary flux in 18th century Europe.

It was a huge surprise, then, when excavations at an eleventh century site near the Great Wall of China turned up two small fragments of pottery which had borate-containing glazes. Dull green and broken, the shards nevertheless displayed the glaze technology of the then far distant future.

The site of Longquanwu (pronounced lung-chwann-woo) is about 20km north of Beijing. It was where some of China's earliest white porcelains were made, at a time when the area was inhabited by the Liao people, whose empire to the north of the Great Wall stretched from the borders of Russia and North Korea in the east to Gansu in the west, encompassing most of present day Mongolia.

The empire lasted from 916 to 1126, when it was overrun by Jurchen Tartars from the north, near where Vladivostok now stands. The Liao, the conquering Tartars and then their conquerors, the Mongols, all made nearby Beijing their capital. Their palaces, temples, pagodas and other major structures all needed thousands of glazed roof tiles: hence the local kilns, and maybe the experiments with borates of which the small scraps are examples.

Most of the Longquanwu pottery has the high lead glazes typical of the period, but the two oddities are a sharp break with this technology and exhibit a remarkable similarity to modern tile glazes in their formulations.

One of the big problems with any archaeological exploration is being able to prove the dates of found items. There is no absolute guarantee that one piece, although dug up next to a neighbor of confirmable date, is of that age too. The only answer is to use an unarguable scientific dating technique, an example of which, particularly suitable for pottery shards, is 'thermoluminescence' or TL.

Borax sponsored a thermoluminescence test on the Longquanwu fragments at the Shanghai Museum in the People's Republic of China.

The test showed the Longquanwu shards are from tiles that were made some 1000 years ago. But the question that remains - and in spite of technology, one that will probably never be resolved - is why, having discovered 'modern' glaze technology, did the Liao potters abandon it?

Nigel Wood is an expert on ancient and modern glaze technology. He researches the subject at Oxford University, England and teaches at London's Royal College of Art and the University of Westminster, as well as internationally.

Thermoluminescence dating

The English chemist Robert Boyle (1627-1691) first noticed thermoluminescence - although he could not explain it - when, for some unknown reason, he took a diamond to bed with him: it glimmered in the dark when warmed by his body.

The defects in natural mineral crystals, such as are present in clays, attract natural ionizing radiation which knocks off and traps loose electrons within the crystal. If heated later, the electrons are freed and when they attach to nearby atoms, photons of light are generated.

Over time, the potential for thermoluminescence builds up as natural radiation is absorbed. When pottery is fired, the clock is set back to zero: so when heated again in the laboratory, in company with a control sample that has been given a known dose of radiation, a calculation accurate to about ten percent of the date can be made by measurements from a sensitive photomultiplier tube.

Borates play an important part in modern ceramic glazes. Up to 12 percent of the formulation, they act as a glass former, flux, viscosity and surface tension modifier, and most importantly are able to adjust the thermal expansion and contraction of the glaze, so preventing crazing. Borates also improve durability against chemical and aqueous attack, and increase scratch resistance.

Liao boron-glaze, and modern double-fast fired glaze ingredient percentages

Oxide	Si	Al	Ca	K	Na	B
Liao	58.1	9.3	4.6	3.1	5.1	10.4 (also 6+% Cu)
Modern	64.5	9.5	5	2.5	5.5	12