In 1433, the people of Paro forged 7,000 iron chain links for Drupthob Thangtong Gyalpo. The Tibetan adept had travelled to Paro to mine iron ore and to forge iron chains, which he used to build numerous suspension bridges in Tibet. Even after 585 years, most of these iron chains have not rusted.

Revered as Drupthob Chagzampa or the “Great Iron Bridge Builder,” he was an early and unequalled master of engineering skills. Some sources say that in his lifetime, he built at least 60 chain suspension bridges; 50 in Tibet and 10 in Bhutan.  

In 1979, Metallurgists from the Confederate Technical College of the ETH University in Zurich conducted a chemical analysis of a chain link from one of the bridges in Bhutan. The investigation report states that the iron chains of the Bhutanese suspension bridges were produced of forged iron using a fire welding technology that has slipped into obscurity.

Metallurgists discovered that the surface of the chain links had a thin film of iron that contained arsenic. The Romans were the only others who used these technique to forge their swords.

The technique of using arsenic was noticed when a link of chain was analyzed during a research. Blanche C. Olschak, Augusto Gansser and E. M. Buhrer, in their book Himalayas, state: “The section where the link had been welded showed an arsenic content of 2.6%.” 

The European scholars found that arsenic lowered the melting point of iron and the technique was rediscovered in Bhutan in 1433. The melting point of wrought iron is 1,482 degree Celsius while that of Tungsten is 3,399 degree Celsius. 

By adding two to three percent of the poison to the iron, the melting point can be decreased. It is then possible to join the links more easily, increasing the durability of the weld and keeping it free from rust.

Arsenic is poison and occurs naturally in the earth’s crust and is found in Bhutan. It is commonly used in wood preservation and as insecticides. While the natural colour of the semi- metallic substance is gray, in its crystalline form it is an unstable yellow.  Iron ore is rich in iron oxides and the colour can vary from red to bright yellow.

Mural of the, “Great Bridge Builder,” graces the walls of the Dumtseg lhakhang in Paro where a white bowl with the yellow substance is laid out in front of him. A similar mural of his heart son, Dewa Zangpo is depicted in the same monastery holding in his hand, a white bowl with the yellow substance filled to the brim.

Cyrus Stearns is the leading author of Drupthob Chagzampa. In his book, The King of Empty Plains, the author is fascinated with the iron welding technique. Stearns states that according to one scientific opinion, the 15th century chains forged in Bhutan were manufactured by oxidizing the iron in a dough-like state. This was the only steel-manufacturing method known at that time, when high temperatures capable of smelting steel were still unattainable. 

The research of Olschak, Gansser, and Buhrer mentions that the great adept was particularly pleased with eight blacksmiths at Ouchu Gadrak, present day Woochu village opposite the Paro airport. This is not surprising, as the village continues to produce some of the country’s finest black and sword smiths.  

According to a research by the three Europeans, the great adept gave each of the eight blacksmiths an anvil stone that could not be broken, by any strong man who pounds hot or cold iron on it with a hammer. The chains were forged link-by-link on the spot, out of local iron supplies. 

While there is no mention of arsenic, “a very special vessel filled with beer was said to have been provided,” to Thangtong Gyalpo’s apprentices. In three months the blacksmiths forged seven thousand links, using strong stone hammers, stone chisels and iron tools.

After the people of Paro forged 7,000 iron chain links, the Great Bridge Builder tried to persuade his folks from Tibet to help him transport it back. When he could not persuade them, the people of Paro volunteered to carry all the 7,000 chain links equivalent to 14,000 loads of iron as far north as the Tibetan town of Phari. 

Bhutanese had enlightened self vested interest. In return for the favour, the Drupthob agreed to build an iron bridge at Chuwori, on the Tsangpo River in Tibet, which would make it easier for the Bhutanese pilgrims to visit the holy city of Lhasa. 

In his lifetime, the Tibetan master is said to have built at least 50 bridges in Tibet and 10 in Bhutan.  The 10 chazams in Bhutan are in Tamchog, Doksum, Dangme, Khoma, Trashigang, Chaze, Changchi, Wangdi, Chuzom and in Chukha. The first four on the list still exist.

In 1783, Samuel Turner visited Bhutan and maintained a detailed report of his journey.  He described the Chukha bridge in detail and was full of praise for the architect. 

In his writings he said, he crossed the chain bridge called Chuka-Chaz-zum that stretched over the Thimpchu river, a short distance above the castle of Chukha.

Awed by the engineering marvel, the English officer lavishly praised the architect, Thangtong Gyalpo and credited the genius, who deservedly ranks high upon the rolls of fame.

 “In a nation where no records are kept to perpetuate the memory of the achievement of genius, and in which the minds of the people are remarkably prone to superstition, perhaps more than a century may not be necessary, to defy the author of a great work. Thus it is, said that the bridge of Chukha is reckoned to be of more than mortal production. No less a being than Dewta Tehuptehup could possible have contrived so curious a piece of mechanism.”

Turner described the Chukha Chakzam in great details. He described it as a very curious and simple bridge, for the accommodation of single passengers. The bridge consisted of two large ropes made of twisted creepers, stretched parallel to each other, and encircled with a hoop. The traveller, who wished to cross over from hence had only to place himself between the ropes and sit down on the hoop, seizing one rope in each hand by means of which he had to slide himself along, and cross an abyss.

Turner said that only one horse was admitted to go over the bridge at a time.  “The five chains that support the platform, are placed several layers of strong coarse mats of bamboo, loosely put down, so as to play with the swing of the bridge; and that a fence on each side, of the same materials, contributes to the security of the passenger.”

The bridges both in Tibet and in Bhutan were a game changer as the technical innovation made it possible to cross the perilous deep gorges with fast flowing icy rivers. Construction of numerous pathways followed. Much later, the British even organized a postal system along these routes.

Before the time of Thangtong Gyalpo, bridges were rare in Tibet. This was mainly because of the scarcity of iron ore. According to Kunpang Chodrak, the biographer of Kunkhyen Dolpopa, each iron link was worth one twentieth of an ounce of gold in the 14th century.  

The Great Bridge Builder imported 7,000 chain iron links from Bhutan. This is equivalent in value to 350 ounces of gold. The present value of the iron chains would have been almost half a million USD, but the intrinsic value is lot more. The chains had 15 links and even today they are highly sought after as material for swords and religious objects.


In his life -time, the, “Great Bridge Builder,” built at least 60 iron chain suspension bridges spanning over central Asia and deep into eastern Bhutan. By using arsenic, the forerunner was able to weld together single iron chain links, making the iron bridges free of corrosion.  He enhanced the art of blacksmithery to the highest level and gained immortality as the “Great Iron Bridge Builder.”

Contributed by 

Tshering Tashi