Most of the ingots of this type are of pure copper (except from those of pure tin found on the Ulu Burun wreck).

Representations of such ingots occur quite commonly in Egyptian art, mostly in tomb paintings from the reign of Thutmosis III (1490-1436 B.C.) onwards. They also appear to be represented on several Linear B tablets from Knossos and also possibly on a few Minoan and Cypriot seals of the Late Bronze Age. Ingots of this kind are also represented on two major works of Cypriot 12th century B.C. art, a bronze stand from Kourion and the famous "Dieu au Lingot" (= god standing on an ingot) from Enkomi.

Their distribution suggests that they were produced to facilitate the transportation of copper as a raw material. Because of the paintings in the Egyptian tomb of Rekhmire where the carriers of such ingots are labelled as "men of Keftiu", it has been assumed that the principal carriers of such ingots were Minoans or possibly Mycenaeans. However, there is considerable debate over the precise location of "Keftiu" and, in any case, we have no firm basis for believing that the "men of Keftiu" had a monopoly of the trade in oxhide ingots. George Bass (the excavator of Ulu Burun) feels that the Levantines, whether Syrians or Canaanites, could equally well have trafficked in such ingots. The balance of our evidence suggests that the copper of the ingots themselves was mined on Cyprus.

Sourse: http://projectsx.dartmouth.edu/classics/history/bronze_age/lessons/les/22.html#5