The correspondence between the logogram i₃.sag and the Akkadian word rūštum is clear from the etymology (Sumerian sag, Akkadian rēšum means “head”) even though the lexical list ur₅-ra ḫubullû does not offer an Akkadian equivalent to the logogram. It refers to a “first-class (sesame) oil”.1The distinction between “(sesame) oil” (i₃.giš, ellum[glossary=ellum]) and “first-class (sesame) oil” (i₃.sag, rūštum) is clear from the fact that the two oils appear in a same text, see for example ARM 23 352.
Although the process to obtain this oil is not known, it could be oil taken from the top of the container after pressing: a purer oil, without the residues which, heavier, fall to the bottom of the container. It would be similar to the process still employed in Africa twenty years ago, and filtration only happened at the end with the rest of the oil full of residues; see Bedigian 2010. Sometimes Akkadian term rūštum was translated as “oil of first (pressing)” (CAD E p. 103 s.v. ellum), but there were no successive pressings.
This kind of oil is not attested in the Ur III period. In the Old Babylonian period, logographic writing is predominant. But the syllabic writing is also attested (letter A.1246, for example). This first-class oil could also be scented (TCL 10 81).
In the Old Babylonian period, the price of this oil is higher in comparison with the average one (i₃.giš, ellum): 1 shekel of silver allowed to buy between 2 and 8 litres of first-class (sesame) oil while 4 to 20 litres of average sesame oil; see A.1.1.29.