Description and usage
The mirror was a flat piece of highly polished metal used to reflect an image. Mirrors were usually made of copper, bronze, or brass, sometimes of silver.
Translation
Even though the ancient world did not have mirrors made of glass, in 1CO 13:12 and JAS 1:23 the function of the mirror to give a reflection is more important than the material from which it is made. However, in EXO 38:8 it will be necessary to employ an expression (or a footnote) that shows that the mirrors in question were made of metal.
In JOB 37:18 it is the hard and polished appearance of the mirror which is in focus. It is not easy to convey both the hardness and the shininess without considerable expansion, and translations tend to emphasize one aspect or the other; for example, ITCL says “As shiny as a mirror,” and NIV has “hard as a mirror of cast bronze.” Attempts to combine the two without much expansion may leave the reader with questions; for example, GNT has “as hard as polished metal,” but is polished metal harder than metal that is not polished? An expanded translation for the whole verse might be “Can you join God in spreading out the sky so that it looks as hard as a shiny mirror made of cast bronze?”