The Gaya used iron ingots to forge ironware. Such ingots are among the most important relics excavated from Gaya tombs. Typical Gaya ingots date from the fourth century A.D. and have largely been found in areas near the base of the Nakdong River. Ingots from the fifth century have been excavated at Changwon, Masan, Haman, Uiryeong, Goseong, and other locations.
Ingots have usually been found in bundles of ten. At the Yangdong-ri site in Gimhae the following bundles were found: three bundles in Wooden Chamber Tomb No. 235; four bundles in Wooden Chamber Tomb No. 162; and one bundle in Wooden Chamber Tomb No. 280. The ingots in these tombs date from the second to third century A.D.
From the beginning of the first to the end of the third century, broad iron axes were used in ways similar to the ingots. Broad iron axes have been excavated at sites in Moosan Hogok (Hamgyeongbuk-do), Galhyeon-ri (Hwanghaebuk-do), Ipsil-ri, and Gujeong-dong (Gyeongju). But such axes were discovered in large quantities at the Daho-ri site in Changwon in the lower Nakdong River region. That the metal objects were actually used as axes was determined after some were found with two different kinds of wooden handles attached to them.
Iron materials found at the Daho-ri site are somewhat different from those at the Yangdong-ri. Iron axes at the Yangdong-ri site can be classified into two types : broad iron axes and stick-shaped iron axes. These axes look similar to axes found at the Daho-ri, but the latter metal objects appear to have lost their original function as axes. The form of broad iron axes changed completely during the fourth century. The axes became broader and eventually lost their blades. Thus, rather being broad iron axes, the new objects were what might be called "broad axe-like ironware."
During the fourth century ingots were in wide use in Gaya. The ingots were basically metal plates that were narrow in the middle and broad at both sides. Typically, the ingots were 4-5 mm thick, but ingots 1 cm thick have also been excavated at Daeseong-dong Tomb in Gimhae. Most of the unearthed ingots are 5-8 cm wide. Iron rods and broad ingots were excavated together in Tomb No. 54 in Bokcheon-dong, revealing a variety in ingot types from the same period. Ingots from the fifth century excavated at Bokcheon-dong in Dongnae show some degree of standardization, but it appears that such standardization was limited to a relatively small geographical area. In the early fifth century, ingots in general were over 40 cm long, but they grew smaller after the middle of the fifth century. At Dugu-dong Limsuk Tomb No. 6 in Busan, ingots from the sixth century were only 4.5 cm long and 1.5㎝ wide.
The use of ingots is associated with various theories, such as raw material theory, currency theory, land purchase theory, and prestige theory. Scientific analysis of ingots' metallic properties indicates that ingots were used to manufacture iron implements. But the ingot's function as currency was also important, as indicated by the uniformity and standardization of ingots and the tendency toward the use of smaller ingots during the fifth century. Samguk-ji mentions that ingots were used as money in China and that Byunhan supplied ingots to Naknang and Daebang. Thus, it seems that iron ingots were not only used for manufacturing iron implements, but they also functioned as a form of currency. However, ingots buried in tombs also seemed to represent prestige relics and land purchases (for the tombs), it is not easy to describe ingots' exact uses, or offer one satisfactory theory.
In addition to ingots, cast-iron axes and iron rods also were used in the manufacture of iron products. Although the cast-iron axe was a tool, a cast iron axe excavated from Okjeon Tomb No. M3 at Hapcheon had likely been used to manufacture other iron implements. The core left over from casting was not removed, thereby rendering the axe useless as a practical tool. Using these axes to manufacture iron products required more work than ingots, as the axes had to be re-melted and the process of removing carbon to manufacture ironware produced pig iron. Therefore, the cast iron axe may have been used more often as currency than raw material for the manufacture of ironware. Iron rods from Okjeon Tomb No. M3 at Hapcheon measured 5.4-22 cm long, 0.8-3.cm wide, and 0.7-3 cm thick, making them more suitable as raw material than ingots because of their variable sizes.