Ramban on Exodus 39:3/רמב"ן על שמות ל״ט:ג׳

1AND 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.