Glass; Looking Glass

Concise Bible Dictionary:

Though glass was known to the Egyptians (the monuments showing their mode of glass blowing), it does not appear to be mentioned in the Old Testament. In Isaiah 3:2323The glasses, and the fine linen, and the hoods, and the vails. (Isaiah 3:23) the word “glasses” (gillayon) may signify small tablets of metal to serve as mirrors, such as the women used. The LXX translates it their “transparent garments.” In Ecclesiastes 38:8 it distinctly says that the laver was made of brass out of the women’s looking glasses, showing that brazen mirrors were then used. The root of the Hebrew word marah is raah, to see. In Job 37:1818Hast thou with him spread out the sky, which is strong, and as a molten looking glass? (Job 37:18) it is from the same root, where the sky is compared to a molten mirror.
The MIRROR is referred to by the word ἔσοπτρον, translated “glass” (James 1:2323For if any be a hearer of the word, and not a doer, he is like unto a man beholding his natural face in a glass: (James 1:23)), but the same word is applied to “glass” or a dim window through, δία, which we see obscurely, as a semi-transparent substance (1 Cor. 13:1212For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known. (1 Corinthians 13:12)). In the Revelation the word is ὔαός, and is called “clear,” “transparent,” and “like crystal,” which evidently refers to glass (Rev. 4:66And before the throne there was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four beasts full of eyes before and behind. (Revelation 4:6); Rev. 15:22And I saw as it were a sea of glass mingled with fire: and them that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, stand on the sea of glass, having the harps of God. (Revelation 15:2); Rev. 21:18,2118And the building of the wall of it was of jasper: and the city was pure gold, like unto clear glass. (Revelation 21:18)
21And the twelve gates were twelve pearls; every several gate was of one pearl: and the street of the city was pure gold, as it were transparent glass. (Revelation 21:21)
). The sea of glass signifies fixed purity. Many specimens of glass have been discovered in the explorations at Jerusalem.

From Manners and Customs of the Bible:

Critics differ as to the meaning of the word rendered “glass” in this verse. Many suppose it means a metallic mirror, as it evidently does in James 1:2323For if any be a hearer of the word, and not a doer, he is like unto a man beholding his natural face in a glass: (James 1:23). (For an account of ancient mirrors, see note on Exodus 38:88And he made the laver of brass, and the foot of it of brass, of the lookingglasses of the women assembling, which assembled at the door of the tabernacle of the congregation. (Exodus 38:8), #139.) Such a mirror, covered with a thin veil, as was often done to protect from dust and dampness, would present a dim, shadowy reflection, causing the beholder to see “darkly,” or more literally, enigmatically. Others think that the “glass” in this text was the lapis specularis, a kind of talc of which the ancients sometimes made their Windows. Through this the indistinct outlines of an object could be seen, but the beholder was left to guess what the object might be. He was looking at an enigma; he saw “darkly.”
We have thus a beautiful illustration of the difference in clearness of vision between the present life and the future. The veil will be taken from the mirror, so that the reflection will be clear; or, the semi-transparent window will be removed, so that nothing shall obstruct the sight.

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