Regarding the dilution of wine. it appears we are mirroring an ancient debate.
Babylonian Talmud, b. Shabb. 8:1 [link]
MISHNA I.: The prescribed quantities (of victuals and beverages) prohibited to be carried about on the Sabbath (are as follows): Sufficient wine in a goblet, which with the addition of a certain quantity of water would make a full goblet of wine (fit to drink); milk to the quantity of a mouthful, honey sufficient to cover a wound with, oil sufficient to anoint a small limb with, and water in quantities sufficient for a medical bath for the eyes. For all other liquids and also of whatever can be poured out, the prescribed quantity is a quarter of a lug (about a quart). R. Simeon says: The prescribed quantities for the liquids enumerated in this Mishna are also a quarter of a lug, and the various prescribed quantities specified apply only to those who store such liquids.
GEMARA: A Boraitha, in addition to this Mishna, states: "The quantity which suffices for a good goblet of wine." What is to be understood by a good goblet? The goblet used in benediction after meals.
R. Na'hman in the name of R. Abuhu said: "A goblet used at benediction after meals must contain no less than a fourth of a quarter lug (of pure wine), so that when mixed with water the prescribed quantity (a quarter lug) will be made." Said Rabha: We have learned this in our Mishna: "Sufficient wine in a goblet, which with addition of water would make a full goblet"--commented on by the Boraitha to mean "which would make a good goblet." From the close of the Mishna we learn: "For all other liquids the prescribed quantity is a quarter of a lug." [And] he is in accordance with his theory elsewhere, that wine which is not strong enough to be mixed with three parts of water is not considered wine at all. Said Abayi: "There are two objections to this: Firstly, there is a Mishna that wine fit to drink is such as has been mixed with two-thirds water, like the wine of Sharon; secondly, do you think that the water in the pitcher (intended for mixing with the wine) is counted in?" Rejoined Rabha: The first objection does not hold good, as Sharon wine is an exception, which although weak is nevertheless good; or it may be that there the particularity is the color, which is not changed by an addition of two-thirds; but concerning taste, I say that only one which can bear three-fourths of water is considered. As to the second objection, concerning water in the pitcher, it is also nothing as concerning Sabbath. The quality and not the quantity is considered, and the wine in question is of that quality.
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The dilution of wine probably varied depending upon various factors, particularly its strength at the time of its distribution.
In any case, the mixture would not have been of an exact nature, rather the measurement would have been done by eye.
A consideration of the taste was the purpose of the dilution rather than the lowering of the alcohol content.
"Grape juice," isn't a term found in Scripture, rather "
the blood of the grape" (Deuteronomy 32:14) or "
new wine" (Heb. תִּירוֹשׁ, תִּירֹשׁ,
thirosh).
They knew that "
new wine," still had some potential for detrimental effects and were wary about over indulgence (Hosea 4:11b
"...wine, and new wine, which take away the understanding").
Like many things in life, wine has good and bad characteristics.
It was used medicinally: Luke 10:34; 1 Tim. 5:23
It was considered one of God's blessings: Gen 27:28; Deut. 7:13
It is associated with joy and with being merry... Ps 104:14-15; John 2:1-10; (c.f. "
foaming wine" (Deut. 32:14)
Jesus often uses parables to describe the kingdom of God as a wedding feast (Mt 9:14–15; Mk 2:18–20 and Lk 5:33–35; Rev 19:7–9) or simply a feast (Mt 8:11–12). The banquet in John 2 images the coming time of the Messiah's blessing (Is 25:6–8).
The "good wine," therefore is that which the Messiah brings forth.
Yet wine is also used as a symbol of God’s judgment. Israel’s disobedience was punished by wine shortages (Deut. 28:39; Job 24:11; Amos 5:11; Zeph 1:13).
And over-indulgence of wine (both then and now) could be an addiction displayed by an unawareness of the cause of injury, red eyes, delusions, perversions, confusion, and apathy: Proverbs 23:29-35.
Rob