There was no time when the state of Yan started to build the Great wall,so the stones and bricks of the wall were laid and plastered with clay.According to legend,the king of the state of Yan ordered not to stop the construction even in winter in order to race against time so as to accomplish the task ahead of schedule.Owing to severe cold weather,the artisans needed hot water to mix it with the mud.They carried a large wok to the construction site,propped it up with three pieces of stone,adding firewood to boil hot water.As time went by,finally the wok was unexpectedly burned with a big hole,and the full wok of boiling water drained down and extinguished the fire under the wok.The artisans accidentally discovered that when water spilled on the supporting piece of stone,the hot stones exploded and there left a lot of white powder.One artisans had white powder mixed with water and found that it was moister than the mud and sticky,so he plastered it on the stone bars and cracks between the blocks.As a result,the joints of the stones and bricks plastered with the white powder were much more solid as compared with that of the mud.Since then,the people of Yan began burning lime and used it to plaster the wall seam.

Later,Emperor Qin Shihuang unified China,he followed the example of state of Yan in building the Great Wall.When the construction started,he ordered the people originally from Yan to do the work of the burning lime.Wherever the Great Wall was built,where the lime would be burnt on the hillside and it turned out that the lime quality was very good,later known as “Lime of Ten-Thousand Years”.meaning that the lime will never degenerate over the years.After the construction of the Great Wall was completed,Qin Shihuang had a town built for Yan people to live and named it “Yanjing”,which is now Beijing due to the contribution made by the Yan people in burning the lime.