Commentaire sur L’Exode 39:3

וירקעו AND THEY DID EXTEND — וירקעו is of the same root and meaning as, (Psalms 136:6) “To Him that stretcheth out (לרקע) the earth”. Render וירקעו as the Targum does: ורדידו, they extended by beating. Plates did they beat out of the gold; in old French estendre — to extend into thin plates. Here it informs you how they wove the gold together with the woollen threads. They beat out the thin plates and from them they cut threads along the length of the plates to make of these threads a mixture (to mix them) with each of the different kind of woolen threads in the ephod and the breast-plate in regard to which gold is mentioned in this connection. One thread of gold they intertwined with six threads of blue purple, and so with each kind of wool. For all the materials had their threads six-fold, and the gold formed the seventh with each of them (cf. Yoma 72a).

AND THEY BEAT THE GOLD INTO THIN PLATES. In all the sacred work that was done, Scripture did not add an explanation as to the method of execution except for here. It should rather have said: “and they made the ephod of gold, and blue-purple etc., the work of a skilful workman; they made shoulder-pieces for it etc.,” for such is the way it describes all of them. It is possible that Scripture relates here [how it was made] because of the original thought that they had to put into making the gold threads, for they were greatly astonished [to be told] that the gold should be spun and twined as is done with wool or linen, for no-one ever heard of doing such a thing until that day.

וירקעו את פחי הזהב, the donours of the gold for the priestly garment already flattened the gold into thin stripes so they could easily be converted and cut into “threads” to be embroidered in these garments.