The Khitans of the Liao dynasty employed heavy armoured cavalry as the core of their army. In battle they arrayed light cavalry in the front and two layers of armoured cavalry in the back. Even foragers were armoured. Units of Khitan heavy cavalry were organized in groups of 500 to 700 men. Unlike some other empires originating from nomadic tribes, the Khitans preferred to fight in dense heavy cavalry formations rather than the wide formations of horse archers. During the Song dynasty (960–1279) it became fashionable to create warts on pieces of armour to imitate cold forged steel, a product typically produced by non-Han people in modern Qinghai. Warts created from cold work were actually spots of higher carbon in the original steel, thus aesthetic warts on non-cold forged steel served no purpose. According to Shen Kuo, armour constructed of cold forged steel was impenetrable to arrows shot at a distance of 50 paces. Even if the arrow happened to hit a drill hole, the arrowhead was the one which was ruined. However crossbows were still prized for their ability to penetrate heavy armour. The "History of Song" notes that Song "tools of war were exceedingly effective, never before seen in recent times," and "their weapons and armor were very good", but "their troops weren't always effective." The Western Xia made modest use of heavy cavalry, of which it had 3,000 at its height. The Jurchens had a reputation for making high quality armour and weapons. Both metal and quilted armour were worn by Jurchens. The Jurchen army was organized into units of a thousand and a hundred. Every hundred was composed of two fifty men social and economic units called "punian". Each punian was supposed to have 20 men equipped with armour and lances or halberds. These 20 men formed a standard two rank five deep battle formation while the others formed three ranks of archers. In 1232 the Jurchens used cast iron bombs against the Mongols at the siege of Kaifeng. The "History of Jin" states that the fire created by the blast could penetrate even iron armour. According to Meng Hong, the reason for the Mongols' success was that they possessed more iron than previous steppe peoples. Both Chinese and European sources concur that Mongols wore substantial armour capable of stopping arrows. A Song source notes that one way to pierce heavily clad Mongol warriors was to use small arrows capable of entering the eye slits of their helmet. According to Thomas the Archdeacon, Mongol arrows were capable of penetrating all known types of armour at the time, but their own leather armour could withstand the arrows of their enemies.